UNIT 2 • Genetic and genomic research can have social and environmental implications. • Variability and diversity of living organisms result from the distribution of genetic materials during the process of meiosis. Overall Expectations In this unit, you will... • evaluate the importance of some recent contributions to our knowledge of genetic processes, and analyze social and ethical implications of genetic and genomic research • investigate genetic processes, including those that occur during meiosis, and analyze data to solve basic genetics problems involving monohybrid and dihybrid crosses • demonstrate an understanding of concepts, processes, and technologies related to the transmission of hereditary characteristics Unit Contents Chapter 4 Cell Division and Reproduction How do the processes of mitosis and meiosis explain heredity and genetic variation? Chapter 5 Patterns of Inheritance How are traits inherited, and how can inheritance be predicted? Chapter 6 Complex Patterns of Inheritance How have recent discoveries in genetics improved our understanding of inheritance, and of how to treat and prevent genetic disorders? 152 Genetic Processes W hat comes to mind when you think of genetics research? Producing crops with a higher yield? Curing diseases? What about growing new tissues and organs that are genetically matched to the people receiving them? One goal of stem cell research is to create new tissues, nerves, blood vessels, and even organs to repair or replace those lost due to disease or damage. Stem cells have the ability to renew themselves and the potential to become any of a number of specialized cells. By using a patient’s own stem cells to generate the new tissue or organ, the body’s rejection of the transplant is minimized. Although regenerative medicine is still in its infancy, researchers have been able to grow several different organs and tissues outside of the body. For example, the ear shown here is being grown in the laboratory using a person’s stem cells and a mould as a scaffold. Medical researchers hope that this technology will allow replacement of tissue in people who are victims of severe burns and other traumas. Many of the research topics in the field of genetics are controversial. Genetics research may challenge, or be challenged by, people with concerns related to ethics, personal privacy, and social justice. Determining how best to balance these concerns with the potential for improved quality of life for the world’s citizens is, and will likely remain, a challenge for many years to come. As you study this unit, look ahead to the Unit 2 Project on pages 278 to 279, which gives you an opportunity to demonstrate and apply your new knowledge and skills. Keep a planning folder so you can complete the project in stages as you progress through the unit. 153 UNIT 2 Preparation The Structure of Cells • Developments in microscopy have made it possible to look at the internal structures of cells. • Some organelles are found in all cells, while others are found only in plant cells or animal cells. • Cells contain a variety of organelles, each of which has its own structure and function. • The cell is the basic organizational unit of life. All cells come from pre-existing cells, and all organisms are made of one or more cells. 1. Which of the following structures are visible with a light microscope? a. cell membrane b. nucleus c. nucleolus d. mitochondria e. endoplasmic reticulum 3. Why is the size of a cell limited? 4. What is the function of the nucleus? a. It sorts and packages proteins and other molecules for transport outside the cell. b. It separates the inside of the cell from the external environment, and controls the flow of materials into and out of the cell. c. It is where energy is released from glucose to fuel cell activities. d. It controls all cell activities. e. It helps to produce mitochondria. 2. Plant and animal cells contain a variety of structures. Provide the names of the structures that are labelled in the animal and plant cells shown below. In your answer, be sure to indiciate which ones are in the animal cell and which ones are in the plant cell. H E D B C G I A F L K J 154 MHR • Unit 2 Genetic Processes Genetic Material • The nucleus of a cell contains chromosomes. • Chromosomes contain deoxyribonucleic acid (DNA), which encodes all of an organism’s genetic information. • Each gene is a code, or blueprint, for making a protein. Proteins are essential for the activities in a cell and determine how an organism looks and functions. • DNA is divided into segments, called genes. Genes carry the instructions necessary for the growth and maintenance of an organism. 5. Draw a concept map to connect the following terms: nucleus, chromosome, gene, DNA, protein. 7. Draw a simple diagram of a portion of a DNA molecule, and indicate the location of a gene. 6. What is a change in the usual order of a DNA sequence called? a. a mutagen d. a mutation b. a protein e. a mitotic spindle c. a clone 8. Which of the following are letters that represent the four types of building-block molecules in DNA? a. A, T, C, G d. G, A, P, K b. A, T, P, R e. C, A, G, P c. C, T, A, S Cell Division • When plant and animal cells divide by mitosis, they form two identical daughter cells. • A cell’s DNA is copied before cell division so that each daughter cell gets the same genetic material as the parent. • In multicellular organisms, cell division allows individuals to grow and to replace lost or damaged cells. • Cell division is a continuous process that involves two stages: mitosis, to divide the nucleus, and cytokinesis, to divide the cytoplasm. 9. Which of the following are achieved through mitosis? I – growth of the organism II – replacement of cells III – repair of damaged tissues IV – growth of the cell V – sexual reproduction Choose the letter of the correct answer. a. I only b. I and II c. I, II, and III d. I, II, III, and IV e. All of I through V. 12. Mitosis occurs in a number of phases. In animal cells, it involves two organelles called centrosomes, which organize spindle fibres. These fibres attach to the centromeres of the chromosomes. Indicate which labels in the diagram correspond with the italicized terms. D B A C 10. Cell division consists of two steps. Name the two steps, and describe the main process that occurs in each step. 11. Describe the change in appearance of DNA during cell division. 13. How do the daughter cells formed during cell division compare genetically to the parent cell? 14. How does cytokinesis differ between plant and animal cells? Unit 2 Preparation • MHR 155 The Cell Cycle • Some cells live a long time, while others have a short life span. The length of a cell’s life depends on where in the body it is found and what conditions it must endure. • Cell division is carefully controlled so that cells are produced only when they are needed. There are molecules at checkpoints in the cell cycle that can exert this control. • The life of a cell, called the cell cycle, can be divided into two main stages: interphase and cell division. • In some cells, the control over cell division can be lost. This may lead to diseases such as cancer. • Interphase consists of two growth stages and a DNA replication stage. 15. All cells go through a cell cycle. The two main phases of the cell cycle are cell division and interphase, which is divided into three stages. Match each of the following to the labels in the diagram shown below. Do not write in this textbook. • cytokinesis • DNA replication • growth and preparation (G1) • growth and preparation (G2) • interphase • mitosis 16. Cell division is carefully controlled. What are two reasons why cells may stop dividing? 17. Provide an example of a type of cell that has a relatively short life span and an example of a type of cell that has a relatively long life span. Relate these differences in cell cycle duration to the functions of the cells. 18. How does cancer relate to the cell cycle? B F A E D 156 MHR • Unit 2 Genetic Processes C Cells, Tissues, and Organs • There are many different types of cells in the human body. Cell specialization is influenced by the contents of an individual cell’s cytoplasm, environmental factors, and secretions from neighbouring cells. • Cells work together to form tissue. Tissues form organs, and organs co-ordinate functions to form an organ system. 19. One example of an organ system is the circulatory system, shown below. Describe another example of an organ system, and the cells, tissues, and organs involved. Draw a diagram to represent the system, tissues, and organs. • Stem cells have the potential to specialize to become any type of cell. • Research into stem cells may lead to the development of new medical treatments for repairing and replacing damaged cells and tissues. 20. In multicellular organisms, most cells are specialized to perform certain tasks. What is cell differentiation? Why is it important for tissue and organ development? 21. What is the difference between stem cells and other types of adult cells? The heart is a major organ of the human circulatory system. Heart tissue made up of muscle cells keeps a heart beating. The circulatory system moves blood throughout the human body. Specialized muscle cells form heart tissue. Unit 2 Preparation • MHR 157 CHAPTER 4 Cell Division and Reproduction Specific Expectations In this chapter, you will learn how to . . . • D1.1 analyze, on the basis of research, some of the social and ethical implications of research in genetics and genomics (4.2, 4.3) • D1.2 evaluate, on the basis of research, the importance of some recent contributions to knowledge, techniques, and technologies related to genetic processes (4.2, 4.3) • D2.1 use appropriate terminology related to genetic processes (4.1, 4.2, 4.3) • D2.2 investigate the process of meiosis, using a microscope or similar instrument, or a computer simulation, and draw biological diagrams to help explain the main phases of the process (4.2) • D3.1 explain the phases in the process of meiosis in terms of cell division, the movement of chromosomes, and crossing over of genetic material (4.2) • D3.4 describe some genetic disorders caused by chromosomal abnormalities or other genetic mutations in terms of chromosomes affected, physical effects, and treatments (4.2) • D3.5 describe some reproductive technologies, and explain how their use can increase the genetic diversity of species (4.3) Traditionally, choosing a new puppy for a pet has involved picking one out of a litter. Thanks to genetics research, a retired Canadian police officer named James Symington took a different approach. Symington won a contest to have his dog Trakr cloned. Trakr was a heroic search and rescue dog that worked with Symington for many years. Symington and Trakr were among the first K9 search and rescue teams at the World Trade Center after the September 11, 2001, terrorist attacks. Symington is now the proud owner of five dogs that not only look like Trakr but also exhibit many of his traits. Advancements in genetics research, such as cloning, have led to many changes in reproductive technologies involving animals, as well as humans. As genetic techniques continue to develop, however, critics worry about ethical issues related to certain practices and wonder just how far new technologies should go. 158 MHR • Unit 2 Genetic Processes Launch Activity To Clone or Not to Clone? Cloning is the process of creating identical copies of a gene, a cell, or an entire organism. Cloning has widespread applications, many of which you will learn about in this chapter. How do your opinions on a variety of cloning applications compare with those of your classmates? How do you think your opinions will change after you learn more about the applications? These cloned piglets were produced in 2002. Their DNA was modified so that their organs could be suitable for transplant into humans. Materials • 5 index cards, labelled Strongly Agree, Agree, Neutral, Disagree, and Strongly Disagree • tape Procedure 1. Your teacher will tape five cards to the walls of your classroom. The cards represent how strongly you agree or disagree with an application of cloning. 2. Each time your teacher reads aloud about a cloning application, stand beneath the card that reflects your opinion on that application. 3. Your teacher will record the voting results on the board. Be sure to copy this information down, as well as any discussion points that arise. Questions 1. Did the number and type of cloning applications surprise you? Explain your answer. 2. Are there any cloning applications that you think should only be used under certain circumstances? If so, what are they, and what are the acceptable circumstances? 3. Are there any applications for which you would have preferred more information before stating an opinion? If so, what are they? Chapter 4 Cell Division and Reproduction • MHR 159 SECTION 4.1 Key Terms genetics somatic cell chromosome sister chromatid centromere spindle fibre centrosome genome sex chromosome autosome homologous chromosome gene allele karyotype Cell Division and Genetic Material Each of us started as one cell. How do we become an organism with more than a trillion cells? Even as adults, our bodies need to continuously regenerate cells. For example, red bloods cells only live for about four months, so new ones must be made. If you scrape your shin, your body produces new skin cells for healing. For new cells and new organisms to carry out their functions, it is essential that they receive the correct genetic information. Genetics is the field of biology that involves the study of how genetic information is passed from one generation of organisms or cells to the next generation. Understanding genetics begins with understanding cellular processes. The cell theory, developed in the mid-1800s, is one of the central ideas in biology. The cell theory states that • all living things are composed of one or more cells • cells are the smallest units of living organisms • new cells come only from pre-existing cells by cell division Since all new cells are the product of existing cells, it follows that traits must be passed from one cell, the parent cell, to new cells, the daughter cells. When scientists were developing the cell theory, they did not know how this occurred. Today, we know that traits are passed on through genetic material in the form of deoxyribonucleic acid (DNA). When a cell divides, as shown in Figure 4.1, each new cell receives genetic information from the parent cell. Figure 4.1 New daughter cells that form from cell division contain the genetic information, DNA, from the parent cell. This scanning electron micrograph shows new cells of the central nervous system forming. Magnification: 1500x The Cell Cycle genetics the study of heredity and variation of living organisms and how genetic information is passed from one generation to the next somatic cell a plant or animal cell that forms the body of the organism; excludes reproductive cells You have learned in previous science courses that cells reproduce through controlled growth and division in a process called the cell cycle. All somatic cells—body cells of plants and animals—go through cell cycles. Each time a cell goes through one complete cycle, it becomes two cells. When the cell cycle is repeated continuously, the result is a continuous production of new cells. In multicellular organisms, there are three functions of cell division: growth of the organism, repair of tissues and organs that have been damaged, and maintenance to replace dying or dead cells. The duration of the cell cycle depends on the type of cell. Some cells, such as certain cells in flies, complete the cycle in as few as eight minutes. Other cells, such as some liver cells, take over a year. For most healthy, actively dividing animal cells, the cell cycle takes about 12 to 24 hours. 160 MHR • Unit 2 Genetic Processes Stages of the Cell Cycle As shown in Figure 4.2, there are three main stages of the cell cycle. • Interphase is the stage during which a cell carries out its normal functions, grows, and makes copies of its genetic material in preparation for the next stage of the cycle. • Mitosis is the stage during which a cell’s nucleus and genetic material divide. • Cytokinesis begins near the end of mitosis and involves the division of the cell cytoplasm and creation of a new cell. Interphase S phase: DNA synthesis and replication G1 phase: rapid growth and cell activity G2 phase: cell prepares for division Mitosis and cytokinesis Figure 4.2 Interphase is the stage of growth and intense cell activity. Mitosis and cytokinesis involve the division of genetic material and cell contents. Proper functioning of the cell cycle is essential for an organism to develop normally and to remain healthy. Specific points in the cell cycle, called cell cycle checkpoints, monitor growth to ensure the cycle continues when it should. Regulation of the cell cycle occurs through a complex network of signals in the cell. Something that interferes with these signals could result in uncontrolled growth. For example, cancer is the result of uncontrolled, rapid cell division. Cancerous cells progress quickly from one cell division to the next, resulting in a mass of cells, called a tumour. Interphase During interphase, the cell grows, develops into a mature, functioning cell, copies its DNA, and prepares for division. Biologists divide interphase into three phases, called G1, S, and G2. The G1, or Growth 1, phase is the major period of growth for a cell. During this phase in the cell cycle, the cell is synthesizing many new molecules in preparation for the next phase in the cell cycle. The next phase is called the S, or Synthesis, phase because the cellular DNA is copied, or replicated. During this phase, the DNA exists as uncondensed fibres called chromatin. Cells that complete the S phase then enter the G2, or Growth 2, phase. In this final phase of interphase, the cell synthesizes more molecules prior to mitosis and cell division. Figure 4.3 shows a cell in interphase. Figure 4.3 During interphase, the cell prepares for cell division. When a cell in interphase is viewed under a microscope, the nucleus and chromatin are clearly visible. Chapter 4 Cell Division and Reproduction • MHR 161 chromosome a structure in the nucleus that contains DNA sister chromatid one of two chromosomes that are genetically identical and held together at the centromere centromere the region where two sister chromatids are held together in a chromosome spindle fibre a microtubule structure that facilitates the movement of chromosomes within a cell Mitosis During mitosis the cell’s copied genetic material separates and the cell prepares to split into two cells. The key activity of mitosis is the accurate separation of the cell’s replicated DNA. This enables the cell’s genetic information to pass into the new cells intact, resulting in two cells that are genetically identical. Figure 4.4 summarizes interphase and the four stages in mitosis: prophase, metaphase, anaphase, and telophase. Prophase During prophase the cell’s chromatin condenses into chromosomes, which contain the DNA. Because the DNA was copied during interphase, each chromosome in prophase exists as two copies of one chromosome. As shown in Figure 4.5, the two chromosome arms are called sister chromatids and the chromosomes are joined at the middle, called the centromere. Other structures in the cell also change during this phase. The nuclear membrane breaks down, and the nucleolus disappears. Spindle fibres, made of hollow tube-like structures called microtubules, are formed from the centrosomes as they move apart to opposite poles of the cell. Together, the fibres and centrosomes are called the spindle apparatus, which moves and organizes the chromosomes during mitosis. centrosome a structure that helps to form the spindle fibres centrosomes nuclear membrane nucleolus A nucleus chromatin Figure 4.4 These illustrations and light micrographs show what happens during interphase and mitosis. Interphase Predict What would be the result if there was not an equal distribution of chromosomes in the cell at the end of anaphase? E nuclear membrane reappears two daughter cells form Telophase 162 MHR • Unit 2 Genetic Processes Metaphase During metaphase the spindle fibres guide the chromosomes to the equator (centre line) of the cell. The spindle fibres from opposite poles attach to the centromere of each chromosome. Biologists consider each pair of sister chromatids to be a single chromosome as long as the chromatids remain joined at the centromere. centromere Anaphase During anaphase each centromere splits apart, and the sister chromatids separate from each other. The separated sister chromatids are now referred to as chromosomes. The spindle fibres shorten, pulling the chromosomes to opposite poles of the cell. At the end of anaphase, one complete set of chromosomes has been gathered at each pole of the cell. sister chromatids Telophase Telophase begins when the chromosomes have reached the opposite poles of the cell. The chromosomes start to unwind into strands of less-visible chromatin. The spindle fibres break down, the nuclear membrane forms around the new set of chromosomes, and a nucleolus forms within each new nucleus. centrosomes migrate toward poles growing spindle fibres B Figure 4.5 Chromosomes in prophase are actually pairs of sister chromatids that are attached at the centromere. Magnification: 25 000× disappearing nuclear membrane replicated chromosome centrosomes now at poles C Prophase D sister chromatids centromere Metaphase Anaphase chromosome Chapter 4 Cell Division and Reproduction • MHR 163 Cytokinesis Mitosis is the process of nuclear division. It is followed by cytokinesis, which is division of the cytoplasm to complete the creation of two new daughter cells. During cytokinesis in animal cells, an indentation forms in the cell membrane along the equator of the cell. This indentation deepens until the cell is pinched in two. The cytoplasm divides equally between the two halves of the cell. Cytokinesis ends with the separation of the two genetically identical daughter cells. The daughter cells are now in G1 of interphase. An animal cell undergoing cytokinesis is shown in Figure 4.6. In animal cells, cytokinesis is accomplished by means of microfilaments that constrict, or pinch, the cytoplasm. Other types of cells complete cell division in different ways. • Structural differences between plant cells and animal cells lead to differences in how these two types of cells undergo cell division. For example, a plant cell has a rigid cell wall covering its cell membrane. This cell wall is much stronger than the membrane of an animal cell. The cell wall does not pinch in and furrow during cytokinesis. Instead, a new structure called a cell plate forms between the two daughter nuclei. Cell walls then form on either side of the cell plate. Once the new cell wall is complete, two genetically identical plant cells have formed. • Prokaryotic cells do not have a nucleus—they complete cell division with a process called binary fission. When prokaryotic DNA is duplicated, both copies attach to the cell membrane. As the cell membrane grows, the attached DNA molecules are pulled apart. The cell completes fission, producing two new prokaryotic cells. Figure 4.6 Cytokinesis begins with a furrow that pinches the cell and eventually splits the two cells apart. This transmission electron micrograph shows two identical kidney cells forming. Magnification: 1700× Learning Check 1. What are the three stages in the cell cycle? 2. According to Figure 4.2, what is the longest stage of the cell cycle for the average cell? 3. Describe the appearance of a cell’s genetic material at prophase of mitosis. 4. Some drugs that combat cancer inhibit mitosis. What effect might this have on healing times? 164 MHR • Unit 2 Genetic Processes 5. You learned that the length of the cell cycle varies between cell types. Predict which of the three phases of the cell cycle varies, and provide an explanation for your answer. 6. Describe the daughter cells that would be formed if, during mitosis, all of the chromosomes lined up on one side of the equator instead of along the equator. Do you think either daughter cell would be viable? Explain your answer. The Structures of Genetic Material Figure 4.7 shows the relationship between DNA, chromatin, and chromosomes. DNA is made up of two long strands that form a spiral shape called a double helix. During most of the cell cycle, DNA exists as strands of chromatin fibre. Once mitosis begins, the chromatin condenses into distinct chromosomes. The individual units of each strand of DNA are called nucleotides. Nucleotides are composed of a phosphate group, a sugar group, and a base. The sugar and phosphate groups form the backbones of the two nucleotide strands. The bases protrude inward at regular intervals. The four bases in DNA are adenine (A), guanine (G), thymine (T), and cytosine (C). Nucleotides are often identified by their bases. Each base is paired in a particular manner. Adenine on one strand pairs with thymine on the opposite strand. Similarly, guanine pairs with cytosine. The A-T and the G-C pairs are called complementary base pairs. A DNA mutation, or genetic mutation, is a change in the nucleotide sequence of DNA. The complete DNA sequence in every cell of an organism is called the organism’s genome. DNA Key A adenine C cytosine G guanine C S T thymine S sugar P P P P P P phosphate P P A S S P P P T A G C S P S S C P S T S S G T A T S S G G A C S S S S P S genome the complete DNA sequence of an organism chromosome P chromatin fibre P interactions to form base pairs P Figure 4.7 DNA is part of chromatin fibre, which condenses to form chromosomes. Making Exact Copies of DNA When DNA is replicated during interphase, the double helix unwinds and each strand of DNA serves as a template for a new strand. As shown in Figure 4.8, when DNA is copied, each of the new double-stranded DNA molecules contains one original strand of DNA and one new strand of DNA. This method of replication is called semi-conservative because each new DNA molecule conserves half of the original DNA. DNA replication Figure 4.8 A new DNA molecule has one original strand. Chapter 4 Cell Division and Reproduction • MHR 165 Activity 4.1 Modelling DNA Step 3: Repeat step 2 with the other pipe cleaner, using a piece of tape that represents a complementary base. In this case, however, leave some of the sticky side of the tape exposed. Connect the two pipe cleaners (DNA strands) by overlapping this exposed sticky side of the tape with the piece of tape on the opposite pipe cleaner. Step 4: Continue steps 2 and 3, adding “bases” along the length of each pipe cleaner. Make sure to use all of the four different colours of tape. DNA consists of strands of nucleotides bonded together, with the phosphate and sugar groups of each nucleotide linked together to form the outer backbones. The bases of each nucleotide form base pairs between strands to form a helical structure. In this activity you will assemble your own DNA model. Materials • 4 pipe cleaners, 2 of one colour and 2 of a different colour • 2.5 cm pieces of tape, 4 different colours Procedure 1. Place two pipe cleaners of the same colour on the table, parallel to each other. These represent the sugar-phosphate backbones of your DNA model. 2. Each colour of tape represents a certain base of the nucleotides. Your teacher will tell you which colour represents which base. Wrap a piece of tape to one of the pipe cleaners, according to the following instructions and the illustration below. Step 1: Start at one end of the pipe cleaner, 2 to 3 cm from the end. Step 2: Centre a piece of tape on the pipe cleaner and fold the tape around the pipe cleaner. Press the sticky surfaces together. 3. Holding both ends of the double-stranded DNA model, twist the two ends in opposite directions to form a helical structure. 4. Using the other, different-coloured pipe cleaners and your DNA model, simulate DNA replication by making two new double-stranded DNA models. Questions 1. What determined the bases you added in step 3? 2. Why do you think it is important when reporting the DNA sequence of a gene to designate what end of the DNA the sequence is read from? 3. How did your original model act as a template for the new DNA molecule? Chromosomes Are Paired sex chromosome an X or Y chromosome, which determines the genetic sex of an organism autosome a chromosome that is not involved in determining the sex of an organism The number of individual chromosomes each cell contains varies from one species to another. The number of chromosomes that a cell has does not necessarily reflect the complexity of the organism. For example, amoebas are unicellular organisms but have many more chromosomes than humans. Human somatic cells have 46 chromosomes. These can be organized into 23 pairs of chromosomes. For each pair, one chromosome is from the father and the other chromosome is from the mother. One chromosome pair is the sex chromosomes. The sex chromosomes, called X and Y, determine the sex of an individual. A human female has two X chromosomes, and a human male has one X chromosome and one Y chromosome. The sex chromosomes are always counted as a pair, even though X and Y are not similar. The remaining 22 pairs of chromosomes are called autosomes, a term used to refer to all the chromosomes except the sex chromosomes. Chromosomes are paired based on sharing similar characteristics. 166 MHR • Unit 2 Genetic Processes Homologous Chromosomes Contain Alleles As shown in Figure 4.9, homologous chromosomes are pairs of chromosomes that appear similar, in terms of their length, centromere location, and banding pattern when stained with certain dyes. However, homologous pairs are not identical to each other. As you know, chromosomes contain the cell’s DNA. Genes are sections of DNA that contain genetic information for the inheritance of specific traits. Homologous chromosomes carry genes for the same traits, such as hair colour, at the same location. However, they can carry different forms of the same gene. Different forms of the same gene are called alleles. These different forms account for the differences in specific traits, such as brown hair versus blond hair. Homologous Chromosomes Homolo gene a part of a chromosome that governs the expression of a trait and is passed on to offspring; it has a specific DNA sequence allele a different form of the same gene banding pattern allele of gene A homologous chromosome a chromosome that contains the same sequence of genes as another chromosome allele of gene A karyotype a photograph of pairs of homologous chromosomes in a cell length centromere position Figure 4.9 Homologous chromosomes have several characteristics in common. However, they are not identical to one another. For example, they can carry different forms of the same gene, called alleles. Examining Chromosomes: The Karyotype The particular set of chromosomes that an individual has is called the person’s karyotype [KAER-ee-oh-tihp]. To prepare a karyotype, a cell sample is collected and treated to stop cell division during metaphase of mitosis. The sample is stained, which produces a banding pattern on the chromosomes that is clearly visible under a microscope. Then, the chromosomes are sorted and paired. The autosomes are numbered 1 through 22 and the sex chromosomes are labelled as X or Y. Figure 4.10 shows an example of a karyotype. This karyotype is of a female because there are two X chromosomes. Males have one X chromosome and one Y chromosome. As you can see in Figure 4.11, a Y chromosome is much smaller than an X chromosome. Figure 4.10 This is a human karyotype. The chromosome pairs are arranged and numbered in order of their length, from longest to shortest. The sex chromosomes are placed last in a karyotype. Note that the banding patterns between homologous chromosomes are different in this image because of the type of dye that was used. Figure 4.11 In males, the sex chromosomes do not match. The Y chromosome is much smaller than the X chromosome. Explain why the sex chromosomes in this karyotype are or are not homologous chromosomes. Chapter 4 Cell Division and Reproduction • MHR 167 Section 4.1 RE V IE W Section Summary • Somatic (body) cells divide to allow for the growth of the organism, to repair tissues and organs that have been damaged, and to replace dead or dying cells. • Genes are sections of DNA that contain genetic information for the inheritance of specific traits. Different forms of the same gene are called alleles. • The cell cycle is divided into three phases: interphase, mitosis, and cytokinesis. Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. • Chromosomes in human somatic cells are organized into 23 pairs. One pair is the sex chromosomes, which determine the sex of the individual. The other 22 pairs are the autosomes. A karyotype is used to analyze chromosomes in a cell. Review Questions 1. K/U List the three foundational statements of the cell theory. 2. A When you cut the tip of a finger, a scab forms and new skin appears underneath the scab. If you cut your finger deeply enough, however, it may take a long time to regain the feeling in the tip of your finger. Use your understanding of the cell cycle to explain these observations. 12. C Using a diagram or flowchart, illustrate the relationships among nucleotide, DNA, gene, allele, chromatin, and chromosome. 13. T/I Why is the word homologous used to describe chromosome pairs, rather than the word identical? 14. Sketch a pair of homologous chromosomes as they would appear during metaphase of mitosis. Label the following: sister chromatids, centromere, gene, and allele. C 3. A Do you expect the rate of cell division to be higher in an adult or a child? Explain your answer. 15. 4. Describe the three stages of interphase, and explain their importance. C Draw and label a karyotype for an organism that has three pairs of homologous chromosomes. 16. Why are the X and Y chromosomes commonly referred to as the sex chromosomes? 17. T/I The image below shows chromosomes in a human cell. a. What is this representation called and how is it prepared? b. Identify the sex of the individual. c. Does this individual have the correct number of chromosomes? 18. How would a karyotype help doctors to diagnose a genetic disorder that results from the partial deletion of one end of a chromosome? 5. K/U K/U Using Figure 4.4, identify the phase of mitosis that each of the following events occur in. a. migration of sister chromatids to the poles b. condensation of chromatin into chromosomes c. formation of a nuclear membrane 6. Sketch the four phases of mitosis. Include labels to explain what is happening in each phase. 7. A Imagine you are a chromosome in a cell undergoing mitosis. Describe the key events that you experience. 8. K/U State the important functions of mitosis and cytokinesis. 9. How do daughter cells compare genetically to the parent cell? C K/U K/U 10. T/I Scientists in a lab have isolated a substance that prevents cells from synthesizing spindle fibres. How would this substance affect cell division? Explain. 11. T/I A scientist studying a group of somatic cells notices that when the cell cycle is complete, half of the daughter cells have no chromosomes and the other half have 92 chromosomes. In what phase of mitosis did an error most likely occur? Explain. 168 MHR • Unit 2 Genetic Processes A SECTION Sexual Reproduction 4.2 When somatic cells reproduce by mitosis, the new daughter cells have the same genetic information as the parent cells. Reproduction that requires only one parent and leads to the production of genetically identical offspring is called asexual reproduction. Bacteria are an example of an organism that reproduces by asexual reproduction. If mitosis were the only strategy for reproducing cells, we would produce exact clones of ourselves during reproduction. However, except for identical twins, no person is an exact genetic copy of another. That is because humans reproduce through sexual reproduction, which involves two parents and leads to the production of genetically distinct offspring. Key Terms asexual reproduction sexual reproduction gamete zygote fertilization haploid diploid meiosis Haploid and Diploid Cells in Sexual Reproduction synapsis Sexual reproduction involves the fusion of a male reproductive cell with a female reproductive cell. These reproductive cells are called gametes, and the cell that results from this fusion is called a zygote. The process of combining gametes to form a zygote is called fertilization. In humans, the male gamete is the sperm cell and the female gamete is the egg cell or ovum. Figure 4.12 shows that when gamete cells fuse during fertilization, the resulting zygote has the same number of chromosomes as the somatic cells for that organism. Gametes must, therefore, have half the number of chromosomes as the parent cells. Gametes, which contain single, unpaired chromosomes, are said to be haploid (from a Greek word meaning single). The haploid number of chromosomes in a species is designated as n. Cells that contain pairs of chromosomes, which includes all somatic cells, are said to be diploid (from a Greek word meaning double). Each human gamete is haploid, with n = 23. After fertilization, the zygote cell is diploid with a total of 2n chromosomes—that is, n chromosomes from the female parent plus n chromosomes from the male parent. The diploid number in humans, therefore, is 46. Notice that n also describes the number of pairs of chromosomes in an organism. When two human gametes combine, 23 pairs of homologous chromosomes are formed. spermatogenesis Haploid and Diploid Cells in Fertilization zygote (diploid) 2n grows into adult male or adult female female (diploid) 2n crossing over non-disjunction monosomy trisomy asexual reproduction reproduction that requires only one parent and produces genetically identical offspring sexual reproduction reproduction that requires two parents and produces genetically distinct offspring gamete a male or female reproductive cell zygote a cell formed by the fusion of two gametes male (diploid) 2n meiosis oogenesis fertilization in humans, the joining of male and female haploid gametes male gamete (haploid) n fertilization haploid a cell that contains half the number of chromosomes as the parent cell diploid a cell that contains pairs of homologous chromosomes female gamete (haploid) n Figure 4.12 When gametes combine in fertilization, the resulting cell is diploid. Describe In this example, how many chromosomes does the diploid cell have? Chapter 4 Cell Division and Reproduction • MHR 169 Meiosis—Producing Haploid Gametes meiosis the cellular process that produces cells containing half the number of chromosomes as the parent cell synapsis the aligning of homologous chromosomes during prophase I in meiosis I The process that produces gametes with a haploid number of chromosomes is called meiosis. Meiosis has two key outcomes: • Genetic Reduction: Meiosis is a form of cell division that produces daughter cells with half the number of chromosomes of the parent cell. • Genetic Recombination: The products of meiosis have different combinations of alleles. Genetic recombination gives rise to offspring that are genetically different from one another and their parents. This greatly increases the genetic variation in a population. Interphase Cells that will divide by meiosis proceed through the growth and synthesis phase of interphase before dividing. This includes replication of chromosomes. Thus, at the start of meiosis, a cell contains duplicated chromosomes. Each chromosome is made up of a pair of identical sister chromatids held together at the centromere. Phases of Meiosis Like mitosis, meiosis involves a precise sequence of events that can be grouped into four distinct phases: prophase, metaphase, anaphase, and telophase. Meiosis, however, involves two complete cycles of the four phases, called meiosis I and meiosis II. Refer to Figure 4.13 as you read through the descriptions of the phases. Meiosis I Prophase I In prophase I, each pair of homologous chromosomes (one chromosome from each parent) lines up side by side. This aligning of homologous chromosomes is called synapsis. At synapsis, the homologous chromosomes are held tightly together along their lengths. While they are lined up, segments of the chromosomes may be exchanged. This process of exchange of genetic information is an important mechanism for providing genetic diversity, and is discussed later in this section. As prophase I continues, the centrosomes move to the poles of the cell and the spindle apparatus forms. Metaphase I In metaphase I, the pairs of homologous chromosomes line up along the equator of the cell. The spindle fibres attach to the centromere of each homologous chromosome. Anaphase I In anaphase I, the homologous chromosomes separate and move to opposite poles of the cell. Because the sister chromatids are still held together, the centromeres do not split as they do in mitosis. As a result, a single chromosome (made up of two sister chromatids) from each homologous pair moves to each pole of the cell. The chromosome number is reduced from 2n (diploid) to n (haploid). Telophase I In telophase I, the homologous chromosomes begin to uncoil and the spindle fibres disappear. Cytokinesis takes place, a nuclear membrane forms around each group of homologous chromosomes, and two cells form. Each of these new cells is now haploid. 170 MHR • Unit 2 Genetic Processes Meiosis II The phases of meiosis II are similar to the phases of mitosis. The key difference is that the cell that undergoes division during meiosis II is haploid instead of diploid. A haploid number of chromosomes line up at the equator during metaphase II. During anaphase II, the sister chromatids are pulled apart at the centromere by the spindle fibres. The chromosomes move toward the opposite poles of the cell. The chromosomes reach the poles during telophase II, and the nuclear membrane and nuclei reform. At the end of meiosis II, cytokinesis occurs, resulting in four haploid cells, each with n number of chromosomes. SuggestedInvestigation Inquiry Investigation 4-A, The Phases of Meiosis Interphase Prophase I Metaphase I Telophase II Anaphase I Meiosis I Meiosis II Anaphase II Metaphase II Telophase I Prophase II Figure 4.13 Meiosis involves two complete cycles of four phases. Notice that each cell contains some chromosomes from the mother (yellow), some chromosomes from the father (blue), and some chromosomes with segments that have been exchanged (yellow and blue). Magnification: 200× Predict What would be the result if there was no exchange of genetic material at prophase I? Chapter 4 Cell Division and Reproduction • MHR 171 A Comparison of Mitosis and Meiosis Study Figure 4.14, which compares mitosis and meiosis. Recall that mitosis consists of only one set of division phases and produces two diploid daughter cells that are identical. Meiosis, however, consists of two sets of divisions and produces four haploid daughter cells that are not identical. Meiosis is important for organisms such as humans because it results in genetic variation. This allows for genetic diversity within a population. Figure 4.14 Comparing mitosis and meiosis can help to understand key differences between the two processes. parent cell (before chromosome replication) Mitosis Meiosis Meiosis I prophase I prophase chromosome replication chromosome replication duplicated chromosome (two sister chromatids) synapsis and exchange of genetic material 2n = 4 homologous pairs line up at the equator chromosomes line up at the equator metaphase metaphase I anaphase I telophase I anaphase telophase homologous chromosomes separate during anaphase I; sister chromatids remain together sister chromatids separate during anaphase daughter cells of meiosis I haploid n=2 Meiosis II 2n 2n daughter cells of mitosis n n n n daughter cells of meiosis II chromosomes do not replicate again; sister chromatids separate during anaphase II Learning Check 7. What is the difference between a gamete and a zygote? 8. The diploid number of chromosomes for dogs is 78. How many chromosomes are in the gamete cell of a dog? Explain your answer. 9. Draw a sketch of a cell at anaphase I. What is the key difference between this phase and anaphase of mitosis? 172 MHR • Unit 2 Genetic Processes 10. If the number of chromosomes were not reduced during meiosis, how many chromosomes would a human gamete have? How many chromosomes would result after fertilization? 11. What phases of meiosis are most like the phases of mitosis? Explain your answer. 12. Occasionally, errors occur during meiosis that result in an incorrect number of chromosomes in the daughter cells. During which phase(s) of meiosis are these errors likely to occur? Explain. Activity 4.2 Modelling Chromosomes in Meiosis The process of meiosis results in the production of haploid cells. Use model chromosomes to follow what happens to the chromosomes during the phases of meiosis. 3. Draw a sketch of your chromosome arrangements as you simulate each phase. Also, make note of steps that represent haploid cells and steps that represent diploid cells. Materials • models of homologous chromosomes Questions Procedure 1. Which step represents the formation of a gamete? How many gametes were produced? 1. Working in groups of four, obtain models of two pairs of homologous chromosomes from your teacher. 2. Use your models to simulate what happens to two pairs of homologous chromosomes during meiosis. Begin by arranging your models as chromosomes that are lined up at the start of meiosis I. Notice that there is more than one way that the chromosomes can align at meiosis I. 2. How would the different ways of aligning the chromosomes in meiosis I affect the final model cells produced? What is the significance of this? 3. If the model cell produced after meiosis were to undergo mitosis, how would the daughter cell compare to it? Gamete Formation in Animals The products of meiosis are haploid gametes. In humans, the gametes are sperm and eggs. The process of sperm production is called spermatogenesis [spur-MAT-oh-genuh-sis], and the process of egg production is called oogenesis [OH-oh-gen-uh-sis]. Both of these processes involve meiosis, but they take place in slightly different ways. Spermatogenesis In most male animals, meiosis takes place in the testes. As shown in Figure 4.15, the process of spermatogenesis starts with a diploid cell called a spermatogonium. Beginning at puberty, spermatogonia reproduce by mitosis, and the resulting cells undergo meiosis to form four haploid cells. Following meiosis II, the cells undergo a final set of developmental stages to develop into mature sperm. The nucleus and certain molecules required by the cell are organized into a “head” region. The midsection holds many mitochondria, which are an energy resource for the cell. Finally, a long tail-like flagellum develops for locomotion. spermatogenesis the process of producing male gametes (sperm) in mammals oogenesis the process of producing female gametes (eggs) in mammals Spermatogenesis mature sperm cells (n) spermatogonium (2n) Figure 4.15 In spermatogenesis, four haploid sperm cells form from one diploid cell. Chapter 4 Cell Division and Reproduction • MHR 173 Oogenesis In most female animals, meiosis takes place in the ovaries. Oogenesis, shown in Figure 4.16, starts with a diploid cell called an oogonium. Before birth, the oogonia reproduce by mitosis, and they begin meiosis, but stop at prophase I. Meiosis I will continue for one cell each month beginning at puberty. Oogenesis involves an unequal division of cytoplasm. The cell that receives most of the cytoplasm after the first division continues through meiosis I and II to form a viable egg. This cell contains a large quantity of nutrients that will support the zygote after fertilization. The other, smaller cell formed is called a polar body. The polar body will degenerate. The final stages of meiosis II are not completed unless fertilization by a sperm cell occurs. When meiosis II is completed, the mature egg and another polar body are produced. The haploid nucleus of the egg cell then fuses with the haploid nucleus of the sperm cell to complete fertilization and create a diploid zygote. Oogenesis Figure 4.16 In oogenesis, one haploid mature egg cell forms from a diploid cell. polar body (n) polar body (n) mature egg (n) oogonium (2n) Multiple Births Sometimes, a woman gives birth to more than one baby at once. This can happen when more than one egg is released. For example, if two eggs are released and both are fertilized, fraternal twins may be born. On the other hand, if a single zygote divides into two separate bodies in the first few days of development, identical twins may be born. As you can see from Figure 4.17, fraternal twins may be no more alike than any other siblings, while identical twins are genetically identical to one another. Figure 4.17 Fraternal twins (A) are formed from two eggs being fertilized by two sperm cells. Identical twins (B) are formed from the splitting of a single zygote during the first few days of development. A B The Importance of Meiosis for Genetic Variation In contrast to mitosis, the outcome of meiosis is the formation of genetically distinct haploid gametes. What processes create new combinations of genetic material in meiosis? Remember that each diploid cell has two copies of each chromosome. One copy of this homologous pair was contributed by the female gamete (egg), so it is of maternal origin. The other chromosome was contributed by the male gamete (sperm), so it is of paternal origin. During meiosis, genetic variation is ensured in two ways: • by the creation of gametes that carry different combinations of maternal and paternal chromosomes, in a process called independent assortment • by the exchange of genetic material between maternal and paternal chromosomes, in a process called crossing over 174 MHR • Unit 2 Genetic Processes Independent Assortment During metaphase I, chromosomes are arranged in homologous pairs along the equator of the cell. In each pair, the chromosome of maternal origin is oriented toward one pole of the cell, and the chromosome of paternal origin is oriented toward the other pole. This orientation of each pair of chromosomes is independent of the orientation of the other pairs. Depending on how the chromosomes line up, a number of different combinations of chromosomes may be found in the gametes. Figure 4.18 shows the different possible gametes that can be produced from an organism that has diploid cells with three chromosome pairs. The number of genetically distinct gametes that can be produced from a diploid cell is 2n, where n is the number of chromosome pairs in the diploid cell. Each human, with 23 pairs of chromosomes, can therefore produce 223, or 8 388 608, genetically distinct gametes. paternal gamete maternal gamete diploid offspring homologous pairs potential gametes Figure 4.18 The diploid cell for this organism has three chromosome pairs. The potential combinations of chromosomes produce eight genetically different gametes (23 = 8). Crossing Over While homologous chromosomes are lined up during prophase I, non-sister chromatids of homologous chromosomes may exchange pieces of chromosome. This process is called crossing over and is shown in Figure 4.19. Crossing over can occur at several points along non-sister chromatids. A section of chromosome that is crossed over may contain hundreds or even thousands of genes. As a result of crossing over, individual chromosomes contain some genes of maternal origin and some genes of paternal origin. This dramatically increases the genetic diversity of the gametes produced. crossing over the exchange of chromosomal segments between a pair of homologous chromosomes Figure 4.19 During synapsis in prophase I, non-sister chromatids cross over and exchange segments of DNA to produce a new combination of genes on a chromosome. Chapter 4 Cell Division and Reproduction • MHR 175 Activity 4.3 Modelling Crossing Over During prophase I of meiosis, chromosomes line up and an exchange of genetic information between homologous pairs may occur. In this activity, you will model crossing over using strips of clay. The twist tie represents the centromere of the chromosomes, holding the sister chromatids (the strips of clay) together. The letters represent three genes on the chromosomes. a a A Materials • modelling clay (two colours) • twist ties • ruler • marker • masking tape Procedure 1. Roll out two 10 cm lengths of modelling clay. 2. Attach twist ties, and use pieces of tape to label each strip of clay as shown in the diagram. A 3. Perform three or four different crossovers with the chromosome models. Draw or take a picture of the resulting chromosomes. Questions twist tie B d B d b D b D 1. In this activity, you performed different crossovers. Each crossover had a different outcome in terms of the exchange of genetic material between the chromosomes. Explain the differences between the crossovers. 2. Scientists have found that there are exceptions to independent assortment. Genes that are close together on a chromosome tend to be inherited together. How do your models support this? 3. What do you think would happen if a crossover occurred between homologous pairs within a gene? Learning Check 13. How is the outcome of meiosis different from the outcome of mitosis? 14. How many genetically different gametes can be produced from a diploid cell with seven chromosome pairs? Explain your reasoning. 15. The gametes in Figure 4.18 illustrate the concept of independent assortment. How could this diagram be altered to also demonstrate crossing over? 16. Using the concepts you have learned, explain how you can have your grandfather’s eyes and your grandmother’s nose. 17. Although you will inherit a combination of traits from your parents, you may notice that some traits seem to be inherited in combination (for example, hair colour and eye colour). Why may some traits be inherited together? 18. A young boy needs a bone marrow transplant, which requires a closely matching genetic donor. Who would be an ideal donor? Explain your answer. Errors During Meiosis The two processes that produce genetic variation, independent assortment and crossing over, also provide the potential for chromosomal abnormalities. Many of the errors that occur during meiosis produce gametes that cannot survive. However, some gametes do survive. If they are fertilized, they will produce a zygote. Since every cell in an offspring is produced from the one zygote cell, all of the cells in the offspring will contain the error. There are two types of chromosomal errors that can occur during meiosis: changes in chromosome structure and changes to chromosome number. 176 MHR • Unit 2 Genetic Processes Errors Caused by Changes in Chromosome Structure During crossing over, the chemical bonds that hold the DNA together in the chromosome are broken and reformed. Sometimes, the chromosomes do not reform correctly. Also, non-homologous pairs may cross over, producing chromosomes that contain genes not normally on that chromosome. Errors to chromosome structure include • Deletion: a piece of a chromosome is deleted • Duplication: a section of a chromosome appears two or more times in a row • Inversion: a section of a chromosome is inverted • Translocation: a segment of one chromosome becomes attached to a different chromosome These four categories of errors and an example of a genetic disorder associated with each are shown in Table 4.1. Some disorders are associated with more than one type of error. For example, scientists at The Hospital for Sick Children in Toronto have identified duplications, inversions, and translocations in individuals with autism. Autism, also called autism spectrum disorder, is a complex developmental disorder that is found in about 1 in 165 children. Table 4.1 Chromosome Structural Errors Error in Chromosome Structure deletion duplication inversion translocation Example of Genetic Disorder Cri du Chat Cri du Chat (French for “cry of a cat”) syndrome is caused by a deletion in chromosome 5. Many children with this syndrome cry with a high-pitched, catlike sound. Other symptoms include low birth weight, widely spaced eyes, recessed chin, and developmental and cognitive delays. There is no cure for this disorder. Charcot-Marie-Tooth Disease Most cases of Charcot-Marie-Tooth disease are caused by duplication of a gene on chromosome 17. The most common symptoms are muscle weakness and loss of some sensation in the lower legs, feet, and hands. A high foot arch with constantly flexed toes is often present. There is no cure for this disorder. FG Syndrome A form of FG syndrome is caused by the inversion of a section of the X chromosome. This syndrome occurs almost exclusively in males. Symptoms include intellectual disabilities of varying degrees, delayed motor development, low muscle tone, and broad toes and thumbs. There is no cure for this disorder. Chronic Myelogenous Leukemia Most cases of chronic myelogenous leukemia (CML), which is a cancer of the white blood cells, are caused by a translocation between chromosome 9 and 22. This results in the formation of an abnormal gene. Treatment of CML involves using a drug that stops the increased production of white blood cells that the abnormal gene causes. Chapter 4 Cell Division and Reproduction • MHR 177 non-disjunction the failure of homologous chromosome pairs or sister chromatids to separate during meiosis Errors Caused by Changes in Chromosome Number Sometimes homologous chromosome pairs or sister chromatids do not separate as they should during meiosis. This phenomenon is called non-disjunction. Non-disjunction can occur in anaphase I or II of meiosis. In anaphase I, non-disjunction occurs when homologous chromosome pairs do not separate to opposite poles. Instead, one entire pair is pulled toward the same pole. In anaphase II, non-disjunction occurs when sister chromatids do not separate to opposite poles. Instead, both sister chromatids are pulled toward the same pole. As a result, non-disjunction produces gametes that have too few or too many chromosomes, as shown in Figure 4.20. Pair of Homologous Chromosomes A Pair of Homologous Chromosomes B nondisjunction meiosis II two gametes have one extra and two gametes have one fewer chromosome than normal normal meiosis I normal meiosis II gametes have usual number of chromosomes non-disjunction one gamete has one extra and the other has one fewer chromosome Figure 4.20 Non-disjunction results in gametes with too many or too few chromosomes. Non-disjunction may take place during anaphase I (A) or anaphase II (B). Genetic Disorders Associated with Chromosome Number Many of the genetic disorders that have been identified are due to an individual having an incorrect number of chromosomes. One example of such a disorder is Down syndrome. Individuals born with this condition have an extra chromosome or an extra piece of chromosome 21, as shown in Figure 4.21. The incidence of non-disjunction leading to Down syndrome increases with maternal age. For example, the chance of conceiving a child with Down syndrome is 1 in 1490 for women between ages 20 and 24 and increases to 1 in 106 for women at age 40. At age 49, the chance increases to about 1 in 11. Figure 4.21 Many cases of Down syndrome are due to the individual having an extra chromsome 21. 178 MHR • Unit 2 Genetic Processes Trisomies and Monosomies The condition in which one chromosome is lost due to non-disjunction is called monosomy. In this case, the gamete is missing one chromosome of a homologous pair. For example, Turner syndrome involves a missing X chromosome. Individuals with this disorder have female sexual characteristics that are underdeveloped. The condition in which there is a gain of an extra chromosome due to nondisjunction is called trisomy. The most common trisomies are found in chromosomes 21, 18, and 13, and in abnormalities in the number of sex chromosomes. Some of these are listed in Table 4.2. There are no therapies that directly treat or cure these disorders. Medical treatment of affected people focusses on managing the health problems that are associated with the disorder. Trisomies of the other human chromosomes and most monosomies are lethal and have been found in miscarried fetuses. monosomy the loss of a chromosome as a result of non-disjunction trisomy the gain of an extra chromosome as a result of non-disjunction Table 4.2 Chromosomal Abnormalities in Humans Conditions Number of Live Births Syndrome Characteristics SuggestedInvestigation Autosome Trisomy 21 1 in 800 Down Intellectual disabilities, abnormal pattern of palm creases, almond-shaped eyes, flattened face, short stature Trisomy 18 1 in 18 000 Edward Intellectual and physical disabilities, facial abnormalities, extreme muscle tone, early death Trisomy 13 1 in 15 000 Patau Intellectual and physical disabilities, wide variety of defects in organs, large triangular nose, early death XXY 1 in 1000 males Klinefelter Sexual immaturity (inability to produce sperm), breast swelling XYY 1 in 1000 males Jacobs Typically no unusual symptoms; some individuals may be taller than average XXX 1 in 1500 females Triple X Tall and thin, menstrual irregularity Turner Short stature, webbed neck, sexually underdeveloped ThoughtLab Investigation 4-B, Chromosomal Abnormalities Sex Chromosome XO 1 in 5000 (1 X chromosome, only) females Prenatal Genetic Testing Prenatal genetic testing refers to tests performed on a fetus (a developing baby still in the womb) that are based on testing for genetic-based abnormalities. Until recently, prenatal genetic testing was only offered to pregnant women in high-risk situations. Such situations included women over 35 years who are at a higher risk for non-disjunction disorders, women with a family history of a genetic disorder, and women with other significant risk factors. Now, a family doctor may refer women of all ages for prenatal testing. Through this referral process, the cost of all approved procedures is covered by the Ontario Health Insurance Plan (OHIP). Deciding whether to have prenatal genetic testing performed and what to do with the information once received can be difficult personal decisions. Such decisions are often complicated by many ethical dilemmas. Some of the ethical issues related to prenatal genetic testing include pregnancy termination and potential discrimination against persons with disabilities. Chapter 4 Cell Division and Reproduction • MHR 179 Prenatal Testing Procedures Typically, prenatal testing initially involves the expectant mother having blood tests and an ultrasound. These tests can provide information about potential physical and chromosomal abnormalities, and indicate whether there is high risk for Down syndrome. Fetal proteins in the expectant mother’s bloodstream are analyzed, and an image of the fetus and measurement of fluid at the back of the fetus’s neck are obtained. Maternal blood tests and ultrasound are considered non-invasive tests, since they do not require direct sampling of fetal cells. Depending on the results from the non-invasive tests and on factors such as the health and family history of the expectant mother, invasive prenatal testing may be performed. Invasive tests involve collecting a DNA sample of the fetus. Figure 4.22 summarizes key points about amniocentesis and chorionic villus sampling, which are common invasive tests. Invasive Tests Chorionic Villus Sampling (CVS) Amniocentesis ultrasound scanner ultrasound scanner catheter amniotic fluid chorion placenta A sample of amniotic fluid (fluid surrounding the fetus), which contains fetal cells, is taken after the 14th week of pregnancy. A sample of cells from the chorion (part of the placenta) is taken after the 9th week of pregnancy. Figure 4.22 In amniocentesis and chorionic villus sampling, chromosome abnormalities, genetic disorders, and certain malformations of the spine and brain are monitored. Activity 4.4 Prenatal Genetic Testing: Considering the Options Genetic tests provide people with important information. Deciding what to do with that information can involve many ethical considerations. What are the benefits of and concerns about prenatal genetic testing? Materials • computer with Internet access • Are there any regulations that restrict who has access to the results of this test? If so, what are they? Are there any regulations about what decisions can be made once the results are known? If so, what are they? • What is the cost of having this test done? • What counselling services are available for the parents? Procedure 2. Summarize the information you have collected and present your findings to the class. 1. Research one prenatal genetic test that is currently available. Report on the following points, and on any other information that you find interesting. Questions • What is the procedure for obtaining the sample to be analyzed, and what are the health risks of having it done? What information does the test provide? • Why does a doctor and/or expecting parents decide to have this test done? • What are the expecting parents’ rights regarding the decision to have or not have this test done? 180 MHR • Unit 2 Genetic Processes 1. How can knowing the test results be a positive thing? How can knowing be a negative thing? 2. What did you find most interesting about the genetic test you chose? Explain why. 3. What did you find most controversial about the genetic test you chose? Explain your answer. Section 4.2 RE V IE W Section Summary • Meiosis involves two nuclear divisions, resulting in haploid gametes from diploid parent cells. It leads to genetic variation in gametes through independent assortment and crossing over of chromosomes. • Errors during meiosis include changes in chromosome structure and chromosome number that result from mistakes during crossing over and non-disjunction of chromosomes. • Prenatal genetic testing can be used to detect errors in the number and structure of chromosomes in the fetus. Testing involves using non-invasive methods, such as ultrasound, and invasive methods, such as amniocentesis and chorionic villus sampling. Review Questions 1. 2. K/U Use the terms meiosis, mitosis, and fertilization to identify the following processes. a. produces haploid cells from diploid cells b. produces diploid cells from haploid cells c. produces genetically identical cells 7. T/I Describe the alignment of chromosomes during prophase I, and its significance. 8. What are similarities between mitosis and meiosis II? What is the significant difference between them? T/I The somatic cells in a horse have 64 chromosomes. a. What is the diploid number for a horse? b. What is the haploid number for a horse? c. How many chromosomes are present in a normal gamete? d. How many chromosomes are present in a cell at prophase I? 9. C Use a graphic organizer to compare and contrast spermatogenesis and oogenesis. T/I 10. T/I A diploid organism has five pairs of chromosomes in each somatic cell. Assuming that no crossing over occurs, how many genetically distinct gametes can this organism produce? 11. C Distinguish between the terms independent assortment and crossing over. Use a sketch to illustrate how both lead to increased variation in the cells produced during meiosis. 3. K/U What process produces diploid cells from haploid cells? 4. K/U What two outcomes does meiosis achieve that mitosis does not? 12. C Sketch the four types of errors in chromosome structure that can occur. 5. T/I Use the diagram below to answer the following questions. 13. K/U How does non-disjunction lead to abnormities in chromosome number? Describe two types of genetic disorders that can result from non-disjunction. 14. Determining a karyotype is an important clinical method for diagnosing genetic disorders. Explain why it is useful for diagnosing monosomies and trisomies. 15. T/I Non-invasive methods of prenatal genetic testing are used before invasive methods. Why do you think that is? 16. C Prenatal genetic testing has many significant benefits. However, many ethical dilemmas are also associated with it. Write a paragraph explaining why you are either for or against prenatal genetic testing. If you found it difficult to support only one side of the issue, explain why. a. What stage of meiosis is shown in the diagram? b. Use a labelled sketch to describe the next step for the chromosomes in the diagram. c. Why are parts of the chromosomes shown with different colours in the diagram? d. What is the diploid number for this cell? 6. A K/U In what part(s) of the human body does meiosis take place? Chapter 4 Cell Division and Reproduction • MHR 181 SECTION 4.3 Key Terms selective breeding artificial insemination embryo transfer in vitro fertilization cloning gene cloning Reproductive Strategies and Technologies For thousands of years, humans have used reproductive technologies for the development of livestock and plant crops with desired traits. Today’s technologies now enable us to genetically manipulate organisms in ways that were once considered science fiction. Recall the Opener for this chapter—Trakr’s clones. The genetic make-up of those dogs was not left to chance. But what if this were to extend beyond selecting a beloved pet by manipulating its genes? What if humans could select their own offspring in the same way? As you will learn in this section, developments in genetics research have led to cures, treatments, and inventions that can inspire hope as well as stir controversy. recombinant DNA therapeutic cloning Reproductive Strategies in Agriculture reproductive cloning The origins of genetics lie in the earliest practices of agriculture. Traditional agriculture involves the controlled breeding of plants and animals with specific combinations of useful or desirable traits. This practice is called selective breeding. Traditional agriculture is often imprecise, because it combines many genes (and, therefore, many traits) at a time. Nevertheless, in the hands of skillful and patient breeders, selective breeding has produced many plants and animals, such as the Appaloosas shown in Figure 4.23. Today, however, many reproductive technologies are used to help with this selective process. Artificial insemination is the artificial transfer of semen into a female’s reproductive tract. Typically, the semen is processed and stored prior to introduction. The most significant benefit of artificial insemination over more traditional methods is that it makes semen from high-quality males more widely available, through breeders and on-line sources. This allows farmers and pet owners to choose desirable traits for the male parent. Another method of providing genetic variation is embryo transfer. This process involves fertilizing an egg artificially and then transferring it into a recipient female. Embryos can be shipped very easily, which eliminates the need to physically ship an animal from one place to another. Studies indicate that animals born and raised in their native environment do better than those that are imported. Embryo transfer, like artificial insemination, can be coupled with modern genetic techniques to ensure the quality of the embryos that are being implanted. stem cell selective breeding the process of breeding plants and animals for desirable traits artificial insemination the process by which sperm are collected and concentrated before being introduced into the female’s reproductive system embryo transfer the process by which an egg that has been fertilized artificially is transferred into a recipient female’s uterus. Figure 4.23 Horses were reintroduced to North America in the 1500s. The Nez Perce, a Native American nation in the Pacific Northwest, are famous for their selective breeding of horses. The Appaloosa, bred for its leopard-spotted coat and striped hooves, is now one of the most popular breeds of horses. 182 MHR • Unit 2 Genetic Processes Reproductive Technologies for Humans There are a number of reproductive technologies that are now available for couples who are not able to conceive a child. These techniques are often referred to as Assisted Reproductive Technologies (ART). Artificial insemination has also been used in humans. Typically, the sperm are collected and concentrated before being introduced into the woman’s vagina. The donor sperm can be from the woman’s male partner, or from an unknown source, such as a sample from a sperm bank. In vitro fertilization, or IVF, offers a reproductive solution for women who have blocked Fallopian tubes. Immature eggs are retrieved from the woman. The eggs are combined with sperm in laboratory glassware, as shown in Figure 4.24. After fertilization, the developing embryo is placed in the uterus. Because fertilization takes place in laboratory glassware, babies conceived by this method are often referred to as “test-tube babies.” A variation of this method involves injecting the sperm into the egg when there is low or no penetration of the egg by the sperm. In Britain on July 25, 1978, a girl named Louise Joy Brown was the world’s first test-tube baby. This provided immeasurable hope to many couples who could not conceive a child on their own. Over 1.5 million babies conceived through IVF have been born since 1978. in vitro fertilization the technique used to fertilize egg cells outside the female’s body Magnification: 625× Figure 4.24 After the sperm and egg are put together in laboratory glassware, they are incubated together for about 18 hours to allow fertilization. Preimplantation Genetic Diagnosis Parents who have a history of genetic disorders in their family may choose to use a process that allows for the diagnosis of genetic disorders soon after fertilization. IVF is used in these cases. Since the genetic testing is done before the embryo is implanted in the uterus, this process is called preimplantation genetic diagnosis (PGD). Once IVF is performed, zygotes are allowed to divide over two days, and then one cell from each of the developing embryos is analyzed for the presence of a genetic disorder. After the genetic analysis, healthy embryos are implanted in the female’s uterus. Parents of sick children have used PGD to “engineer” a genetic match in another sibling. As a genetic match, the newborn sibling is able to donate umbilical cord blood, which contains stem cells that can be used to treat a number of diseases. You will learn more about stem cells later in this section. Further developments to IVF-based procedures continue to improve the success rates for human reproduction. They have allowed many individuals to have children who they would not have been able to have otherwise. However, with these advancements have come ethical and social debates. Many wonder if people will find ways to abuse the technology and if there is a limit to how far we should go with them. Chapter 4 Cell Division and Reproduction • MHR 183 Cloning: Reproduction of Exact Copies cloning a process that produces identical copies of genes, cells, or organisms gene cloning the use of DNA manipulation techniques to produce multiple copies of a single gene or segment of DNA recombinant DNA a molecule of DNA that includes genetic material from different sources In general, cloning is defined as a process that produces identical copies of genes, cells, or organisms. The word cloning can mean very different things, according to what is being copied—a gene, a cell, or an organism. Therefore, it is important to understand what the different types of cloning are and how they are used. Gene Cloning Gene cloning involves manipulating DNA to produce multiple copies of a gene or another segment of DNA in foreign cells. The cloned DNA can be used for further study, or for mass production of the protein that the gene codes for. Proteins produced in this way have numerous commercial and medical applications. For example, insulin, a hormone that enables the body to use sugar, is absent in people diagnosed with type I diabetes. Before gene cloning, people with diabetes used purified insulin from animal sources. This procedure was labour-intensive and made insulin expensive to produce. Since the early 1980s, human insulin has been produced in bacteria through cloning of the insulin gene. The general experimental approach to gene cloning is described below. Refer also to Figure 4.25, which summarizes the steps in cloning a gene in bacteria. 1. Isolate the segment of DNA to clone, and choose a vector for cloning. Vectors act as carriers of the DNA to be cloned so that the DNA can be copied in a foreign cell. One commonly used vector for cloning in bacteria is called a plasmid. Plasmids are small, circular pieces of DNA that remain distinct from the bacterial chromosome. 2. Insert the chromosomal DNA into the vector. This relies on the use of reagents that can cut DNA and help different pieces to join together. The resulting DNA molecule, which includes genetic material from different sources, is called recombinant DNA. 3. Treat foreign cells, such as bacterial cells, so that they take in the recombinant DNA. The process of taking up the recombinant DNA is called transformation. Once the recombinant DNA plasmid is taken into the cell, many copies of the cloned gene or DNA fragment will be made by the host cell. chromosomal DNA cell of interest gene of interest recombinant DNA vector DNA bacterial cell gene cloning: many copies of a gene of interest vector DNA Figure 4.25 A gene or piece of DNA can be cloned. Many copies of it or the protein product that the gene codes for can be produced and isolated. Compare How does the recombinant DNA molecule differ from the vector DNA? 184 MHR • Unit 2 Genetic Processes Learning Check 19. What advantages does artificial insemination have over traditional methods of selective breeding? 22. What is one benefit of developing genetically engineered medical products such as insulin? 20. How do artificial insemination and embryo transfer increase the genetic variation in animals? 23. Why would a company use embryo transfer? 21. What is the function of a vector in gene cloning? 24. Some human proteins that are cloned in and isolated from bacteria are not as active as when they are purified from the natural tissue source. Why do you think this can happen? Therapeutic Cloning and Reproductive Cloning Therapeutic cloning involves producing genetically identical cells that are used to treat various diseases. This includes using the cloned cells to grow new tissues and organs. Reproductive cloning also involves production of cell clones, but with the aim of producing a genetically identical organism. Unlike gene cloning, therapeutic cloning and reproductive cloning are surrounded by controversy because there are ethical questions about how they are used. Both reproductive and therapeutic cloning use a process called somatic cell nuclear transfer (SCNT) to generate the cloned cells, shown in Figure 4.26. In this technique, an egg cell’s nucleus is removed and replaced with the nucleus of a somatic cell of a donor. somatic body cell with desired genes nucleus fused with denucleated egg cell cell clones Therapeutic Reproductive cloning cloning egg cell therapeutic cloning the process of replacing an egg cell’s nucleus with the nucleus from a somatic donor cell to produce a cell line of genetically identical cells reproductive cloning the process of producing genetically identical organisms Figure 4.26 Therapeutic and reproductive cloning involve inserting the nucleus from a somatic cell of the donor into an egg cell that has had its nucleus removed. Explain Why is the nucleus removed from the egg before SCNT is performed? nucleus removed surrogate female tissue culture Reproductive Cloning in Animals This chapter was introduced using an example of the application of reproductive cloning—Trakr’s clones. However, reproductive cloning in animals is not very successful. The birth rate ranges from only 0.5 to 6 percent. In addition to low birth rates, cloned offspring tend to have a high mortality rate, as well as high incidences of disease and premature ageing. Nevertheless, research into cloning animals continues because of the potential applications. For example, one aim of animal cloning is to use it to repopulate an endangered species. The first cloning of an endangered animal took place in 2001. The cloned animal was an Asian gaur, which is a rare, ox-like mammal native to India and Southeast Asia. The animal was cloned from a dead gaur’s skin cells, which were fused with a domestic cow’s egg cell. The egg was then transplanted into a surrogate mother, also a domestic cow. The cow successfully gave birth, but the cloned offspring died about two days later. Chapter 4 Cell Division and Reproduction • MHR 185 Therapeutic Cloning and Stem Cells stem cell an undifferentiated cell that can develop and become specialized into different cell types of the body One ethical issue concerning therapeutic cloning is that the cells produced by SCNT are stem cells. Stem cells are undifferentiated (unspecialized) cells that, under the right conditions, can develop into any one of the more than 200 types of somatic cells. If human cells produced through SCNT are implanted into a surrogate, they could develop into an embryo and produce a human clone. Another controversy is due to the initial use of embryos as a source of stem cells. Over the years, scientists have used three different sources for stem cells: • embryonic stem cells, which are obtained from embryos • adult stem cells, which are somatic cells that have retained the ability to differentiate into some other cell types • induced pluripotent stem cells, which are specialized adult stem cells that have been induced to return to a stem-cell-like state Now that scientists can use pluripotent stem cells, their reliance on embryonic tissue will be reduced or eliminated. Potential applications of stem cell research are shown in Figure 4.27. The key to stem cell research is learning how to “program” stem cells to become certain cell types. Successes in stem cell research include improving heart function and formation of blood vessels by injecting stem cells into the circulatory systems of animals. In addition, starting with stem cells, scientists have “grown” blood vessels, heart valves, skin, and a urinary bladder in the lab. Thus, stem cell research holds great promise for regenerative medicine—the creation of tissues and organs to replace those damaged or lost due to age, disease, trauma, or genetic defects. Since the stem cells are generated from a patient’s own somatic cells, they are a genetic match to the patient. This means that tissues that are formed from these stem cells are unlikely to be rejected by the immune system. This solves the problem of tissue rejection, and it provides a source of organs to supplement those that are already in short supply. stem cells bone marrow cells for treating types of cancer nerve cells for treating neurological diseases cardiac cells for treating heart disease pancreatic cells for treating diabetes Figure 4.27 Stem cells can be stimulated to differentiate into specific tissue types under the right conditions. Potential applications for stem cells include treating diseases and in regenerative medicine. 186 MHR • Unit 2 Genetic Processes Transgenic Organisms Researchers have developed techniques for inserting foreign DNA into plants and animals to produce transgenic organisms—organisms whose genetic material includes DNA from a different species. Transgenic organisms are a type of genetically modified organism, or GMO. In general, a GMO is an organism that has had the sequence of its genome altered for a specific purpose. Applications of Transgenic Plants Transgenic crop plants account for over half the corn and canola grown in North America. Many have been modified to increase their resistance to herbicides, insect pests, or viruses. A great promise of plant genetic engineering is the production of plants with increased nutritional value. In many developing countries where rice is the main staple food, symptoms of iron and vitamin A deficiencies affect hundreds of thousands of people. In 2000, Swiss researchers developed a genetically modified strain of rice known as golden rice, shown in Figure 4.28. This rice has been genetically engineered to increase its iron and vitamin A content. Golden rice is now part of the food aid delivered to many developing countries. Transgenic plants can also be used for medical purposes. A Canadian company has inserted the human insulin gene into a safflower plant. The transgenic safflower plant produces insulin as it grows. Producing insulin this way is much less expensive, so it can make diabetes treatment more affordable worldwide. Beans Aspergillus fungus Wild rice Daffodil Ferritin gene is transferred into rice from beans. Phytase gene is transferred into rice from a fungus. Metallothionein gene is transferred into rice from wild rice. Enzymes for β-carotene synthesis are transferred into rice from daffodils. Fe Pt Ferritin protein increases iron content of rice. Activity rice chromosome Phytate, which inhibits iron reabsorption, is destroyed by the phytase enzyme. 4.5 A1 S Metallothionein protein supplies extra sulfur to increase iron uptake. A2 A3 A4 Figure 4.28 This transgenic product, golden rice, contains four different foreign genes. Three of these genes come from other plants, and one comes from a fungus. Infer How could the development of such a disease-resistant plant be economically advantageous? β-carotene, a precursor to vitamin A, is synthesized. Assessing the Use of Transgenic Plants 2. Prepare a summary of the information you have gathered. Different agencies oversee the development and use of transgenic products. These agencies consider criteria such as the potential social, economic, and environmental costs and benefits. Questions Materials 1. What advantages has the development of the product provided to Canadian citizens? • computer with Internet access Procedure 2. Have there been any negative consequences associated with the use of the product? 1. Research a transgenic plant product or crop that has been approved for use in Canada. Describe the review process it has gone through for approval in Canada. Chapter 4 Cell Division and Reproduction • MHR 187 Applications of Transgenic Animals Animals such as mice, fruit flies, and roundworms are widely used in research laboratories around the world to study diseases and develop ways to treat them. Transgenic milk-producing animals, such as goats, are being used to produce medical protein products that include human growth hormone and anti-clotting factors. Figure 4.29 shows the main steps in creating a herd of goats that are genetically modified to secrete specific proteins in their milk. Similar steps have been used by a Canadian research company to insert a spider gene into goats. The transgenic goats secrete spider silk in their milk. The aim is to eventually spin the silk into lightweight, strong fibres that can be used for such things as clothing and nets. Another area of research involves developing transgenic animals that can serve as organ donors for humans. Usually, transplanting organs from donor animals, such as pigs, into humans has very limited success because of tissue rejection. Some genetic engineering research teams are conducting work to develop transgenic pigs that are more compatible with human tissues. Research such as this also raises difficult issues, however. Some people are concerned about the risk of transferring diseases from pigs to humans. Other people ask whether it is ethical to create new kinds of animals purely for the purpose of harvesting their organs. Figure 4.29 Genetic engineering can create transgenic animals that secrete human proteins or other substances in their milk. human gene Explain In your opinion, is it ethical to use animals in this way? Why or why not? egg donor egg micro-injection of human gene development within a host goat milk containing a medical product transgenic goat Regulating the Use of Transgenic Organisms Despite a thorough review process, many organizations and citizen groups oppose the use of transgenic organisms. Some of the risks cited are listed below. • Environmental threats: The use of herbicide-resistant plants could encourage the use of stronger herbicides, which may get into the water or soil system. There is also evidence that genes can cross to other species, which may create “superweeds” and “superbugs.” • Health effects: Not enough is known about the long-term effects of consuming transgenic products such as food and medicine. • Social and economic issues: Although there are benefits to human health and reducing world hunger, the amount of money spent on genetics research may be greater than the overall benefit. In addition, some people wonder if private enterprise is having too much influence over the global food market. Still others question the ethics of using other species solely for human benefit. 188 MHR • Unit 2 Genetic Processes STSE BIOLOGY Connections Stem Cells: Paralysis Cured? A race car driver is paralyzed in a crash. A teen is paralyzed after diving into shallow water. Until recently, these individuals would have little hope of regaining the full use of their bodies, but new research on adult stem cells shows promise for reversing paralysis. bone marrow stem cells CNS stem cells fat cells cardiac muscle cells epithelial cells blood cells HOW CAN STEM CELLS BE USED? Scientists are trying to find ways to grow adult stem cells in cell cultures and manipulate them to generate specific cell types. For example, stem cells might be used to repair cardiac tissue after a heart attack, to restore vision in diseased or injured eyes, to treat diseases such as diabetes, or to repair spinal cells to reverse paralysis. Late actor and paralysis victim Christopher Reeve was a strong supporter of stem cell research because he believed there is much potential to improve the condition of life for others who suffer from paralysis. STEM CELLS AND PARALYSIS In Portugal, Dr. Carlos Lima and his team of researchers found that tissue taken from the nasal cavity is a rich source of adult stem cells. These stem cells become nerve cells when transplanted into the site of a spinal cord injury. The new nerve cells replace the cells that were damaged. More than 40 patients with paralysis due to accidents have undergone the Portuguese procedure. All patients have regained some sensation in paralyzed body areas. Most have regained some motor control. With intensive physical therapy, about 10 percent of the patients now can walk with the aid of supportive devices, such as walkers and braces. This is promising news to the many individuals facing illnesses or injuries that have robbed them of the full use of their bodies. STEM CELLS AND THE FUTURE Scientists are eager to do the research necessary to make adult stem cell treatments a regular part of health care. Paralysis might not have to be permanent—stem cells could provide the cure. nerve cells skeletal muscle cells Stem cells from bone marrow or the central nervous system (CNS) can be manipulated to generate many cell types that can then be transplanted to treat illness or repair damage. Connect to Society Create a pamphlet that explains the benefits to society of adult stem cell research. Conduct research in order to include information about the research methods, treatment, examples, cell physiology, and a brief history of adult stem cell research. Be sure to illustrate your pamphlet. Chapter 4 Cell Division and Reproduction • MHR 189 Section 4.3 RE V IE W Section Summary • Artificial insemination and embryo transfer are two agricultural practices that allow for the selective breeding of high-quality animals around the world. • Genes can be cloned by inserting them into a vector and transforming host cells. Gene cloning is used to economically produce proteins for treating disease. In the future, cloning may have wider-reaching applications. • Therapeutic cloning involves using cloned cells to treat disease, and includes generating new tissues and organs from those cells. Reproductive cloning involves using cell clones to develop a cloned individual. • Recombinant DNA technology is used to create transgenic organisms. There are both risks and benefits with the use of transgenic products. Review Questions 1. How did farmers of the 18th century improve their herds? Explain the process. 10. What is the benefit of producing insulin from transgenic plants rather than transgenic bacteria? 2. A When choosing a pet or herd animal, there are certain traits that are desirable. Think about adding a pet to your family or animal to your farm. Make a list of desirable traits for that animal. How could you use current breeding technology to produce such an animal? 11. K/U Although animals have been successfully cloned, reproductive cloning is not considered to be very successful. Explain this statement. 12. A A breeder wishes to produce more prize-winning dogs from his 2-year-old male schnauzer. What are his options? Which method would produce the most exact copy? 13. K/U Explain the role that stem cells play in regenerative medicine. 14. K/U How can transgenic organisms help to achieve social, economic, or environmental goals? Give one example of a transgenic organism designed to meet one of these goals. 15. A A company has developed a transgenic carrot that secretes pesticide, making it resistant to damaging insects and worms. a. What are some of the risks and benefits that you think the Canadian government should consider when deciding whether to approve this plant for agricultural use? b. If approved, what advantages will this transgenic carrot offer to farmers? What are some of the potential drawbacks to farmers? c. Do you believe that foods produced with genetically modified ingredients should be labelled so that consumers can make informed choices? List your arguments. 16. A Many individuals have life-threatening allergies to particular foods, such as nuts. Researchers are developing transgenic peanuts that will not produce allergic reactions. Do you think this is a good use of research money? Explain. 3. K/U C Artificial insemination enables the agricultural industry to improve the genetic quality of farm animals. Should this be allowed in humans—perhaps using DNA of brilliant scientists or highly skilled artists? Write a paragraph that supports or criticizes this practice. 4. T/I Compare embryo transfer in animal populations with IVF in humans. 5. K/U What is a vector, and why is it important in cloning? 6. List the three types of cloning and differentiate between them. K/U 7. C Use a flowchart with diagrams to summarize gene cloning. 8. T/I Describe the process that produced the bacterial cells below. Why do some of the cells have more DNA than others? 9. T/I Describe the two cells used in somatic cell nuclear transfer. Describe the genetic make-up of the daughter cells produced from the parent cell. 190 MHR • Unit 2 Genetic Processes T/I Inquiry INVESTIGATION 4-A Skill Check Initiating and Planning ✓ Performing and Recording ✓ Analyzing and Interpreting ✓ Communicating Safety Precautions • Be sure your hands are dry when you plug in or disconnect the cord of the microscope. The Phases of Meiosis Meiosis takes place in the reproductive cells of all sexually reproducing plants and animals. The process of meiosis results in production of daughter cells that • have half as many chromosomes as the parent cells • are genetically distinct from each other, and from the parent cells In this investigation, you will identify and draw cells in various stages of meiosis from prepared slides of plant cells. • The glass or plastic slides and cover slips used to mount specimens are fragile and can break easily. Handle them carefully. Materials • compound microscope • prepared slides of cells undergoing meiosis (for example, lily anther, frog testis) Magnification: 500× This lily cell is in prophase II of meiosis. Pre-Lab Questions 1. Describe how you first view a prepared slide under a microscope. 2. When viewing an object under high power, describe how you adjust the focus. 3. Describe the features of a biological diagram. 4. List the phases of meiosis and an identifying feature of each phase. 5. What is the proper safety procedure to follow when working with glass microscope slides so that you avoid breaking one? What should you do if you accidentally break a glass microscope slide? Go to Using a Microscope in Appendix A for information about the proper care and use of a microscope. Go to Biological Drawing in Appendix A for help with making biological drawings. Question How can the features of a cell undergoing meiosis be used to identify the distinct phases? Chapter 4 Cell Division and Reproduction • MHR 191 Procedure 1. Set up a microscope. 2. Using the high-power setting of the microscope, find cells that are in different stages of meiosis in one field of view. 3. Draw diagrams of the cells that you can see in various stages of meiosis. Label the following on your diagrams where possible. • stage of meiosis • chromosome • chromatid • centromere • spindle fibres • equator • tetrad • crossing over Conclude and Communicate 6. Were you able to identify the phase of meiosis for each cell? Why or why not? Extend Further 7. INQUIRY How do you think meiosis in the cells of other types of organisms compares to meiosis in the cells that you observed? How could you support your hypothesis without physically observing these cells under the microscope? 8. RESEARCH How long does each phase of meiosis last? Use your research to compare the length of time for cells of different species in meiosis with what you have discovered in this investigation. 4. You may not find cells in all phases of meiosis on your slide. Be prepared to share your slides and observations with other members of the class. 5. While examining your slide, take note of the relative number of cells in each phase of meiosis. Which phase(s) are found most frequently? 6. If there are slides of more than one type of organism available, repeat step 2, and make notes to record any differences in meiosis between different species. 7. Clean up as directed by your teacher. Analyze and Interpret 1. Were you able to find cells in all phases of meiosis? If not, why do you think you could not find any cells in that phase? 2. Which phase of cells was found the most frequently? What can you infer, therefore, about the length of that phase? 3. During which phase(s) are chromosomes most easily identified? Explain why. 4. Which structure(s) were you not able to identify? Explain why. 5. Could you identify the chromosome number for this species? If not, why not? 192 MHR • Unit 2 Genetic Processes Magnification: 250× This cell, also from the lily, is in the telophase II stage of meiosis. ThoughtLab INVESTIGATION 4-B Skill Check Initiating and Planning ✓ Performing and Recording ✓ Analyzing and Interpreting ✓ Communicating Materials • paper karyotypes • paper • scissors • glue stick Chromosomal Abnormalities For expectant parents, a karyotype can provide information about chromosomal abnormalities, including changes in chromosome number or structure, that could affect their futures. In this investigation, you will act as a genetics researcher to assemble and analyze a karyotype to determine if there is a chromosomal abnormality. Pre-Lab Questions 1. What type of sample is required for karyotyping? 2. What types of disorders can be identified by karyotyping? What types cannot? 3. How is a karyotype analyzed? Question How are some genetic disorders identified through karyotyping? Organize the Data 1. If you are using a paper karyotype, cut out the chromosomes from the sheet provided by your teacher. 2. Complete the karyotype by matching the homologous pairs of chromosomes. The largest pair is #1. Continue to pair chromosomes until all of the chromosomes are paired (if possible). 3. Use the glue stick to fix your chromosomes onto a sheet of paper. Analyze and Interpret 1. Is the karyotype of a male or a female? How do you know? This karyotype is of a woman without a genetic disorder. 2. Does the individual have the correct number of chromosomes? Explain your answer. 3. Are any chromosome abnormalities in the karyotype? Explain. Conclude and Communicate 4. How useful was the karyotype in determining whether there was a chromosomal abnormality? What were the limitations? Extend Further 5. INQUIRY How could the analysis for a genetic abnormality be improved to provide more specific information? 6. RESEARCH Find out about a genetic disorder that is associated with a karyotype that you or a classmate analyzed. Include in your description the chromosome(s) affected, physical effects, treatment, and the status of the research into this genetic disorder. Chapter 4 Cell Division and Reproduction • MHR 193 Chapter 4 Section 4.1 SUMMARY Cell Division and Genetic Material Somatic cells reproduce through the processes of mitosis and cytokinesis. Daughter cells that are formed contain the same genetic information. KEY TERMS allele autosome centromere centrosome chromosome gene genetics Section 4.2 genome homologous chromosome karyotype sex chromosome sister chromatid somatic cell spindle fibre • The cell cycle is divided into three phases: interphase, mitosis, and cytokinesis. Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. • Genes are sections of DNA that contain genetic information for the inheritance of specific traits. Different forms of the same gene are called alleles. • Chromosomes in human somatic cells are organized into 23 pairs. One pair is the sex chromosomes, which determine the sex of the individual. The other 22 pairs are the autosomes. A karyotype is used to analyze chromosomes in a cell. Sexual Reproduction Gametes, the reproductive cells, are produced through the process of meiosis. This allows for genetic variation in sexual reproduction. KEY TERMS asexual reproduction crossing over diploid fertilization gamete haploid meiosis monosomy Section 4.3 KEY CONCEPTS • Somatic (body) cells divide to allow for the growth of the organism, to repair tissues and organs that have been damaged, and to replace dead or dying cells. non-disjunction oogenesis sexual reproduction spermatogenesis synapsis trisomy zygote KEY CONCEPTS • Meiosis involves two nuclear divisions, resulting in haploid gametes from diploid parent cells. It leads to genetic variation in gametes through independent assortment and crossing over of chromosomes. • Errors during meiosis include changes in chromosome structure and chromosome number that result from mistakes during crossing over and non-disjunction of chromosomes. • Prenatal genetic testing can be used to detect errors in the number and structure of chromosomes in the fetus. Testing involves using non-invasive methods, such as ultrasound, and invasive methods, such as amniocentesis and chorionic villus sampling. Reproductive Strategies and Technologies Modern technology has allowed scientists to manipulate the genetic make-up of life forms. This has led to the creation of techniques that benefit humanity in terms of reproduction and other medical advancements. KEY TERMS artificial insemination cloning embryo transfer gene cloning in vitro fertilization recombinant DNA reproductive cloning selective breeding stem cell therapeutic cloning KEY CONCEPTS • Artificial insemination and embryo transfer are two agricultural practices that allow for the selective breeding of high-quality animals around the world. • Genes can be cloned by inserting them into a vector and transforming host cells. Gene cloning is used to economically produce proteins for treating disease. In the future, cloning may have wider-reaching applications. • Therapeutic cloning involves using cloned cells to treat disease, and includes generating new tissues and organs from those cells. Reproductive cloning involves using cell clones to develop a cloned individual. • Recombinant DNA technology is used to create transgenic organisms. There are both risks and benefits with the use of transgenic products. 194 MHR • Unit 2 Genetic Processes Chapter 4 REVIEW Knowledge and Understanding Select the letter of the best answer below. 1. What are the components of a nucleotide? a. a centromere and two sister chromatids b. a sugar and a phosphate group c. a base and a phosphate group d. a sugar, a phosphate group, and a base e. a vector, a plasmid, and a cloned gene 2. Which of the following characteristics about homologous chromosomes in somatic cells is false? a. They are of similar size. b. Genes are in the same locations on each chromosome. c. They contain the same alleles. d. They contain the same genes. e. They have similar banding patterns when dyed. 3. At the end of meiosis II, how many haploid cells have been formed from the original parent cell? a. 0 b. 1 c. 2 d. 3 e. 4 4. The somatic cells of a dog contain 78 chromosomes. Which of the following statements is false? a. The diploid number for a dog is 78. b. The haploid number for a dog is 39. c. Sperm produced by a dog contain 39 chromosomes. d. A cell in metaphase of mitosis contains 78 chromosomes. e. A cell in metaphase II of meiosis contains 78 chromosomes. 5. What information can be inferred from a karyotype? a. whether amniocentesis is a possible prenatal test to perform b. whether a person has inherited a specific gene c. what the sex of a person is d. whether there has been a specific change in DNA sequence e. None of the above. 6. Which of the following correctly identifies the processes that are indicated by the letters A, B, C, and D in the diagram below? A haploid gametes (n = 23) sperm cell B C egg cell diploid zygote (2n = 46) multicellular diploid adults (2n = 46) D a. b. c. d. e. A: meiosis; B: meiosis; C: fertilization; D: mitosis A: meiosis; B: meiosis; C: mitosis; D: fertilization A: mitosis; B: meiosis; C: fertilization; D: mitosis A: mitosis; B: mitosis; C: fertilization; D: meiosis A: mitosis; B: mitosis; C: meiosis; D: fertilization 7. Which of the following correctly describes the changes in chromosome structure? a. inversion: part of one chromosome becomes attached to another chromosome b. deletion: a piece of a chromosome is inverted c. duplication: a complete chromosome is copied d. translocation: part of a chromosome is copied e. translocation: part of one chromosome becomes attached to another chromosome 8. Which of the following describes the role of a plasmid in gene cloning? a. It contains only the gene to be cloned. b. It is the vector, which acts as a carrier of the gene to be cloned. c. It is used to disrupt the chromosomes to extract the gene to be cloned. d. It is used to break open the bacterial host cells. e. It is a cell that is fertilized before implantation in a uterus. Chapter 4 Cell Division and Reproduction • MHR 195 Chapter 4 REVIEW Answer the questions below. 9. What are three functions of cell division in multicellular organisms? 10. Scientists often describe the structure of DNA by comparing it to a ladder. Explain how a DNA molecule is similar to a ladder. Then explain the limitations of this comparison. 11. Describe the similarities and differences in the karyotypes for males and for females. 12. One of the following diagrams represents metaphase I of meiosis. Which diagram is it? How do you know? What phase of which type of cell division is represented in the other diagram? A Thinking and Investigation 20. Chromosomes appear as unwound chromatin during interphase, but are coiled during mitosis. Explain why this is advantageous. 21. How does the complementary nature of the bases in DNA enable accurate replication? Why is accuracy important? 22. What plant or animal tissue would be best to use for studying each of the following processes? Explain your answer. a. mitosis b. meiosis 23. “The sex chromosomes in a human are a homologous pair.” Do you agree or disagree with this statement? Explain your position. 24. Explain what you think is happening in the following image. Why is this significant? B Magnification: 7800× 13. What are the two key outcomes of meiosis? 14. Why is only one egg produced during oogenesis, while four sperm are produced during the parallel process of spermatogenesis? 15. Distinguish between non-disjunction in anaphase I and non-disjunction in anaphase II. Describe a genetic disorder that results from non-disjunction. 16. Describe four errors that can occur to the structure of a chromosome during meiosis. 17. Explain the significance of the discovery that somatic cells can be induced to become undifferentiated (stem-cell-like)? 18. When an animal is cloned, three different adults may be involved in the process. What are their roles? Which animal is cloned? 19. List three applications of transgenic organisms. 196 MHR • Unit 2 Genetic Processes 25. What would happen if a chromosome synapsed with a non-homologous chromosome during meiosis, rather than with its homologue? 26. Assuming that no crossing over occurs, how many genetically distinct gametes can each of the following produce? a. A diploid cell with four pairs of homologous chromosomes. b. A somatic cell with a total of 16 chromosomes. c. An organism whose haploid number is three. 27. What is one major difference between selective breeding and genetic engineering? 28. Many chromosomal abnormalities go undetected until adulthood, where they appear as a decrease in fertility. Using your knowledge of meiosis, explain why cells with chromosome abnormalities may not successfully complete meiosis. Communication Application 29. Imagine that you have been asked to explain cell division to a Grade 5 class. You know that many people learn best when they can use their bodies as well as their minds, so you decide to have the class put on a 15-minute “mitosis play.” Create a list of characters and the dialogue for this play. Include a props list and production directions. 39. Some genetics researchers are developing artificial chromosomes as tools to help better understand how natural chromosomes function. What features would these chromosomes need to have for them to behave appropriately during cell division? 30. Use a graphic organizer to summarize important similarities and differences between mitosis and meiosis. 31. Sketch a homologous chromosome pair, and label the distinguishing features. 32. Illustrate how mistakes during meiosis can lead to changes in the number or structure of chromosomes. 33. Design an information pamphlet that describes and illustrates the different methods for prenatal testing. 34. Create a graphic organizer to differentiate between four methods of selecting for specific traits in animals. 35. The applications of stem cell research are not well understood by the general public. Use a graphic organizer to illustrate some of the potential benefits and the risks associated with the use of stem cells. 36. 37. Variability and diversity of living organisms result from the distribution of genetic materials during the process of meiosis. Meiosis provides much of the variation associated with sexual reproduction. Using diagrams, illustrate how and during which phases of meiosis genetic variation is introduced. Genetic and genomic research can have social and environmental implications. Imagine that you are a journalist writing an article for the magazine Ethics in a Changing World. You have been asked to research and explain the social and environmental implications of current research related to genetics. Suggested topics include stem cell research for regenerative medicine, transgenic crops to reduce hunger, genetic screening for specific diseases, and reproductive cloning. 38. Summarize your learning in this chapter using a graphic organizer. To help you, the Chapter 4 Summary lists the Key Terms and Key Concepts. Refer to Using Graphic Organizers in Appendix A to help you decide which graphic organizer to use. 40. Genetic testing is a controversial subject. Research one example of a genetic disorder that is associated with a chromosomal abnormality that was discussed in this chapter. a. Describe the chromosomal abnormality. b. Outline the possible options patients might have to choose from if they test positive. c. Write a supported opinion paragraph in favour of or against prenatal genetic testing for this disorder. 41. Think about the following case: Parents have a child who is ill with leukemia. They are considering conceiving a child through in vitro fertilization and having preimplantation genetic diagnosis performed to ensure the newly conceived child is a genetic match to their ill child. This would allow their new child to act as a stem cell donor for the ill sibling. a. Choose one of the following roles: the mother, the father, the sick child, a physician advising the parents. b. Write a paragraph from the perspective of this individual arguing for or against this procedure being done. 42. How can genetic engineering help us grow oranges in Ontario? 43. Research a drug or other form of medical treatment that was developed using recombinant DNA technology. Describe what it is, it use, and any risks or controversies there are associated with its use. 44. In the Launch Activity for this chapter, you were asked to provide your opinion on various cloning-related techniques. Look back at your list of answers. a. Have any of your opinions changed? If so, which ones, and why did they change? b. What new information did you learn? Did any of this new information surprise you? c. Did learning more about different sides of the issues make forming an opinion more or less difficult? Explain why. 45. How can the application of stem cell research be used to alleviate wait times for organ transplantation? Chapter 4 Cell Division and Reproduction • MHR 197 Chapter 4 SELF-ASSESSMENT Select the letter of the best answer below. T/I The graph below shows the relative amount of DNA found in the nucleus of a somatic cell over time. Which of the following statements correctly relates the numbers on the graph to the stage of the cell cycle described? Relative Amount of DNA Per Cell 1. 5. 3 2 2 1 a. b. c. d. e. 4 1 5 Time a. Molecules are synthesized for cell growth during stage 5. b. The cell has divided and each daughter cell has a complete set of chromosomes at stage 4. c. Mitosis occurs during stage 2. d. The cytoplasm is split during stage 1. e. DNA is in the form of chromatin during stage 3. 2. 3. 4. T/I The diploid number for the cell below is 2n = 8. In which stage is the cell pictured? A A new insecticide functions as a mutagen (that is, it causes errors in DNA). It does not appear to affect adult populations of insects, but offspring have a variety of defects that are lethal. Which of the following does the insecticide likely affect? a. mitosis in the gamete-producing cells b. mitosis in the somatic cells c. meiosis in the gamete-producing cells d. meiosis in the somatic cells e. both mitosis and meiosis in the somatic cells A The cancer drug vinblastine interferes with synthesis of microtubules. In mitosis, this would interfere with which of the following? a. spindle formation b. DNA replication c. carbohydrate synthesis d. disappearance of the nuclear membrane e. cytokinesis K/U How many chromosomes does a cell have during metaphase I of meiosis if it has 12 chromosomes during interphase? a. 3 b. 6 c. 12 d. 24 e. 36 198 MHR • Unit 2 Genetic Processes metaphase of mitosis metaphase I of meiosis anaphase I of meiosis anaphase II of meiosis telophase of mitosis 6. K/U Which of the following statements about homologous chromosomes is true? a. Homologous chromosomes are different lengths. b. Homologous chromosomes have different centromere positions. c. Homologous chromosomes pair up during meiosis I. d. Homologous chromosomes have the same alleles at the same location on the chromosome. e. Homologous chromosomes do not have common genes for traits. 7. K/U Which of the following statements about trisomy is false? a. Many trisomies are found in miscarried fetuses. b. The incidence of trisomy increases with maternal age. c. Trisomy 21 is also known as Down syndrome. d. Trisomy can be identified using karyotyping. e. Trisomy is one result of non-disjunction during mitosis. 8. K/U Which of the following reproductive strategies produces offspring that are genetically identical to the parent? a. selective breeding b. artificial insemination c. embryo transfer d. in vitro fertilization e. reproductive cloning 9. K/U Which of the following is a current use for gene cloning? a. mass produce drugs such as insulin b. grow tissues and organs for replacement c. repopulate extinct species d. cure diseases that result from errors in chromosome structure e. improve human reproduction 10. K/U Which of the following two processes are used in the creation of transgenic plants and animals? a. genetic engineering and selective breeding b. genetic engineering and IVF c. cloning and selective breeding d. cloning and IVF e. stem cells and cloning 19. T/I The image below shows chromosomes from a human cell. Use the image to answer the questions that follow. Use sentences and diagrams as appropriate to answer the questions below. 11. K/U Sketch a small section of DNA. Use the sketch to describe how DNA’s complementary nature enables accurate replication. 12. A Horticulturists grow thousands of genetically identical plants by using cuttings. Cuttings do not involve sexual reproduction. Describe one benefit and one risk of using cuttings to produce a certain type of plant. 13. T/I The process of meiosis is often called a reduction division. Explain the use of this term. 14. Use a diagram to show how the appearance of DNA during the G1, S, and G2 phases differs from the appearance of DNA during metaphase. a. Name and describe the technique used to obtain the data in this image. b. Identify the genetic abnormality. c. Describe how this type of genetic abnormality arises. C C A cell in prophase I of meiosis has a diploid number of 2n = 6. Draw the cell with the correct arrangement and number of chromosomes. On the diagram, label the following: chromatid, centromere, centriole, spindle fibre. 16. K/U Describe four differences between meiosis and mitosis. 20. K/U Down syndrome is a genetic disorder. a. What is the chromosomal abnormality that is associated with this disorder? b. Describe the biological process that results in this disorder. 21. The decision to have prenatal genetic testing done can involve many issues. Discuss reasons for and against this type of testing. 22. C Use a flowchart to illustrate how IVF and preimplantation genetic diagnosis (PGD) can be used to produce a healthy child. 15. 17. 18. Black hair and brown eyes often appear together. Assume the alleles for these features are located on one chromosome. Similarly, blond hair and blue eyes often appear together. Assume that the alleles for these two features are on a homologous chromosome. Using a sketch, show how crossing over can cause the features to no longer be inherited together. C A A horse has 64 chromosomes and a donkey has 62. Using your knowledge of meiosis, explain why a cross between these animals produces a sterile mule. 23. A C Use labelled diagrams to illustrate gene cloning. 24. Discuss the benefits and risks associated with the development of transgenic organisms. What limitations, if any, do you think should be placed on developing these types of organisms? 25. K/U Describe the use of therapeutic cloning in regenerative medicine. What are some of the ethical concerns associated with therapeutic cloning? 12 13 T/I Self-Check If you missed question ... 1 Review section(s) ... 4.1 2 3 4 5 6 7 8 9 10 11 14 15 16 17 18 19 20 21 22 23 24 25 4.1, 4.1, 4.1, 4.1, 4.1 4.2 4.1 4.2 4.3 4.3 4.3 4.1 4.1 4.2 4.1 4.2 4.2 4.2 4.2 4.2 4.3 4.3 4.3 4.3 4.2 4.2 4.2 4.2 Chapter 4 Cell Division and Reproduction • MHR 199