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