Cycles of Life:
EXPLORING BIOLOGY
Module 3: The Continuity of Life
Segment 1: Chromosomes: Information for Life
Objectives:
1. Describe the basic features of chromosome structure and function in different organisms.
2. Recognize that the typical cell contains two sets of chromosomes composed of matching pairs,
called homologues.
Video Synopsis:
Chromosomes were first revealed by Walther Fleming’s use of synthetic dye, hence the name. In
this segment, Drs. Christopher Wills and Gary Karpen tell about the significance of this discovery,
and how the structure of chromosomes is gradually being defined. Cytogenics specialist Arlene
Kumamoto shows how the chromosomes of different cells are identified, matched and counted using a
procedure known as “karyotyping.”
Questions:
1. What functions do chromosomes provide for the cell?
• Chromosomes house and organize DNA, the genetic material.
• Chromosomes insure that the DNA gets apportioned appropriately during cell division.
2. Why is the term chromosome, or colored body, an appropriate one?
• As Fleming established, the DNA readily picks up biological stains which are basic in nature.
(These stains cling to the negative charges on the DNA.)
3. How can the long DNA molecules fit into the relatively small chromosomes?
• The DNA undergoes several levels of extensive coiling along with being packaged in proteins.
4. What are the characteristics of homologous chromosomes?
• Homologous chromosomes are the same size.
• They have the same shape as determined by the position of their centromeres.
• They exhibit the same banding pattern when stained with a particular dye.
5. Do the number of chromosomes increase with more complex organisms?
• There appears to be no correlation between complexity and number of chromosomes.
• Where chromosomes are fewer in number, each one can have a greater DNA content per
chromosome.
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Follow-up Activities:
1. In the 1950s, it was thought that human cells had forty-eight chromosomes. What number is
considered to be accurate today? Research this topic in order to explain the discrepancy.
(Hint: Look for the name Painter in the literature.)
2. Using an appropriate text, describe the levels of coiling that are present in a condensed
eukaryotic chromosome.
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Cycles of Life:
EXPLORING BIOLOGY
Module 3: The Continuity of Life
Segment 2: Mitosis: Copycat Cells
Objectives:
1. Understand the activities of the cell cycle, and specify where mitosis fits into this cycle.
2. Summarize the events that occur during mitosis.
3. Identify several purposes which mitotic cell divisions are able to accomplish.
Video Synopsis:
The ancient art of wine making depends on a fundamental biological process: mitosis. In this
segment we see how a typical California winery refines and preserves the quality of specific grape
varieties over successive generations. Feature computer animation of the phases of mitosis, from the
replication of DNA to its subsequent distribution.
Questions:
1. What are the major stages of the cell cycle?
• Interphase
• Nuclear division - mitosis
• Cytoplasmic division - cytokinesis
2. What invisible activities take place during interphase?
• Growth, the accumulation of biomass.
• Synthesis, the exact duplication of the genetic material.
• Production of proteins used to build the spindle fiber for cell division.
3. What is the immediate accomplishment of mitosis?
• In mitosis, the duplicated chromatids are separated into two identical nuclei.
4. List and describe the major events in a mitotic division.
• Genetic material coils appearing as chromosomes containing two sister chromatids.
• Spindle fibers begin to form as nuclear membrane disappears.
• A spindle fiber attaches to each chromatid and chromosomes are nudged to center of the cell.
• Sister chromatids are pulled to opposite poles along spindle fibers.
• At poles, chromosomes begin to unwind again and nuclear membranes reform.
• After mitosis, cytokinesis divides the cytoplasm, forming two new cells.
5. What roles can mitotic cell division perform?
• Increase the size of an organism.
• Replace worn out cells or repair damaged tissue.
• Reproduce identical organisms, or clones.
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Follow-up Activities:
1. Understanding the factors that control the timing of the cell cycle may give us some information
that will be useful in the treatment of certain cancers. Research and report on what is currently
known about the timing of the cell cycle.
2. Construct a model of the events occurring during mitosis. In the past, students have used such
things as pipe cleaners, wires, pop-it beads or simply paper cutouts to represent chromosomes.
Perhaps you have a better idea!
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Cycles of Life:
EXPLORING BIOLOGY
Module 3: The Continuity of Life
Segment 3: Meiosis: Shuffling the Genetic Deck
Objectives:
1. Contrast meiosis and mitosis with regard to purpose, location, and resulting number of
chromosomes per cell produced.
2. Summarize the major events occurring during meiosis.
3. Contrast asexual and sexual reproduction, and suggest why sexual reproduction seems to be the
preferred mode of the most successful organisms.
4. Outline several ways by which meiosis and gamete activity contribute to the genetic variation of
a population.
Video Synopsis:
Meiosis is integral to the process of sexual reproduction. In this segment, Drs. Christopher Wills and
Arlene Kumamoto describe how zoologists were able to identify two previously undetected species of
Dik-Dik antelope by the difference in the number of chromosomes present in the cells of each. This
case provides a springboard for discussion of the mechanisms of meiosis and the advantages of sexual
reproduction.
Questions:
1. What are gametes and what role do they play?
• Gametes are haploid cells which mature into eggs and sperm.
• Gametes are the products of meiotic cell division.
2. What are the main events of meiosis?
• Like mitosis, meiosis begins with the coiling of chromosomes, the disappearance of the
nuclear membrane, and the organizing of the spindle fibers.
• Next, homologous chromosomes match up and exchange corresponding pieces, a process
called “crossing over”.
• The paired homologues then line up randomly on the midline of the cell.
• The members of each homologous pair separate and go to opposite poles and two cells are
formed.
• These cells begin a second division without further replication of genetic material.
• Each chromosome, containing its two sister chromatids, lines up independently in the center
of the cell.
• The sister chromatids separate from one another and are drawn to opposite poles, where
nuclear division is completed and followed by cytokinesis.
• The resulting four cells are haploid, i.e., each contains one complete set of chromosomes,
and are ready to mature into gametes.
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3. How do meiosis and mitosis differ regarding purpose, location, and numbers of chromosomes?
• Mitosis is for growth, repair and sometimes asexual reproduction, while meiosis produces the
haploid gametes needed for sexual reproduction.
• Mitosis takes in all somatic cells; meiosis is restricted to the gonads, ovaries and testes.
• Mitosis produces diploid (2N) daughter cells; meiosis, on the other hand, produces haploid
(1N) gametes.
4. Why do asexually reproducing organisms often become extinct sooner than those reproducing
sexually?
• Asexually reproducing organisms lack variation.
• Therefore, they are less able to respond to environmental changes.
5. How does the process of sexual reproduction contribute to genetic variation?
• “Crossing over” exchanges pieces of homologous chromosomes with a potential for
rearrangement of genes.
• The random positioning of homologous chromosomes on the midline of the cell (at
metaphase), shuffles maternal and paternal chromosomes.
• Fertilization brings together genetic information from two different sources, sperm from the
father and an egg from the mother.
Follow-up Activities:
1. As was suggested for mitosis, construct a physical model for meiosis. Be sure that your
model shows “crossing over” and the random lining up of homologous chromosomes at
the cell’s center.
2. Human karyotypes are prepared much like the antelope karyotypes seen in the video. Discover
and report on the laboratory steps required in this procedure.
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Cycles of Life:
EXPLORING BIOLOGY
Module 3: The Continuity of Life
Segment 4: DNA: Blueprint of Life
Objectives:
1. Cite the experiment of Hershey and Chase which confirmed that DNA is indeed the molecule
that codes for inherited traits.
2. Describe the parts of a nucleotide and explain how nucleotides are linked together to make DNA,
following the model of Watson and Crick.
3. Explain how DNA is replicated including the materials needed for this process.
Video Synopsis:
This segment catalogues the discovery of DNA, featuring interviews with pioneering geneticists
Alfred Hershey and James Watson. Dr. Leroy Hood helps us explore the structure of DNA,
including the nitrogen-containing bases that form the “steps” of this molecular staircase. We see
how a limited number of nucleotides can produce a seemingly limitless genetic variability simply by
the way they are sequenced in the chromosome.
Questions:
1. What did the Hershey and Chase experiment confirm?
• DNA as the genetic material.
• By using a virus specific for infecting bacterial cells (a bacteriophage), they were able to
radioactively label the proteins and/or DNA of the virus.
• Only when the viral DNA was labeled did radioactivity enter the bacterial cell.
• It must be this DNA , therefore, which provides the genetic information to make more
viruses.
2. What is a nucleotide?
• Nucleotides are the building blocks of nucleic acids.
• Each nucleotide contains a pentose sugar, a phosphate group, and one of four nitrogenous
bases.
• In DNA, the sugar is deoxyribose and the bases are adenine, guanine, cytosine and thymine.
3. According to the Watson and Crick model, how are nucleotides arranged to form the DNA
molecule?
• The DNA molecule is made up of two strands of nucleotides with each strand held together by
bonds between adjacent sugars and phosphates.
• The strands are joined together by bonds between the bases of each strand.
• The bonding is very regular with adenine always binding to thymine, and guanine to cytosine.
4. What part of the DNA molecule actually contains the genetic information?
• The information is maintained in the base sequence along one of the DNA strands.
• The genetic language, therefore, has four letters in it.
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5. Describe the events in DNA replication.
• An unwinding enzyme separates the two DNA strands.
• A polymerase enzyme adds the appropriate nucleotides to form a new strand.
• The order in which the energized nucleotides are added is determined by one of the original
strands that remains intact.
• Each new molecule then contains an original strand and a complimentary new strand, a
method of replication known as semiconservative.
Follow-up Activities:
1. Hershey and Chase’s experiment confirming DNA as the genetic material was necessary because
another hypothesis, namely that protein was the genetic material, remained entrenched in the
minds of many scientists even as late as the early 1950s. Research the topic to determine what
earlier experiments showed DNA to be the genetic material.
2. In developing their model of DNA structure, Watson and Crick “stood on the shoulders” of a
number of other scientists. See if you can find out the scientists involved and the contributions
they had made.
3. Using styrofoam balls or other materials, build a technically accurate model of a DNA molecule.
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Cycles of Life:
EXPLORING BIOLOGY
Module 3: The Continuity of Life
Segment 5: Genetic Engineering
Objectives:
1. Identify the goal of gene therapy.
2. Demonstrate an understanding of the basics of recombinant DNA technology including
restriction enzymes, vectors, gene splicing and gene cloning.
Video Synopsis:
Andrew Gobea inherited a disease called Sever Combined Immune Deficiency, or SCID. The disease
interferes with the production of white blood cells and hinders the overall effectiveness of the
immune system. This results when a fragment of the DNA is missing. Dr. Donald Kohn uses
realtively new techniques of gene therapy to try to identify the defective DNA and reintroduce
normal genetic content.
Questions:
1. How does gene therapy work?
• Seeks to insert a normal gene into an individual possessing two defective genes.
• It is hoped that the cells that receive a normal gene will reproduce themselves so that the
normal gene product can continue to be produced by the genetically challenged individual.
2. Why are restriction enzymes aptly described as molecular scissors?
• Restriction enzymes cut the DNA at very specific sites when they recognize a characteristic
base sequence.
• The most useful enzymes cut the DNA asymmetrically, creating sticky ends.
3. In genetic engineering, what are vectors?
• Vectors are vehicles which carry engineered DNA into a host cell.
• Vectors that have been used include plasmids, or small rings of extrachromosomal bacterial
DNA, and viruses.
4. How are genes spliced into vectors?
• Treating both the source DNA and vector DNA with the same restriction enzymes creates
pieces of DNA with complimentary, or matching, sticky ends.
• When the source and vector fragments are mixed together, they then can join by their
complimentary sticky end to form a recombinant molecule.
• Finally, a ligase or joining enzyme, restores the covalent backbone of the double stranded
recombinant DNA.
5. What is accomplished when vectors are cloned?
• Cloning produces multiple copies of vectors or cells which all contain the same DNA.
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Follow-up Activities:
1. Identify a genetically engineered product that is currently in use. Research the development of
this product and how it benefits society. Also identify potential risks to society which might
accrue from its use.
2. Prepare an argument for or against genetic engineering use and development. If possible,
debate someone with an opposing view.
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Cycles of Life:
EXPLORING BIOLOGY
Module 3: The Continuity of Life
Segment 6: Proteins: Building Block of Life
Objectives:
1. Elaborate on why protein molecules are so critical to life.
2. Distinguish DNA from a second type of nucleic acid known as RNA, and identify the three
common classes of RNA.
3. Describe the process of transcription whereby some of the information in DNA gets transferred
to RNA.
4. Discuss the process of translation including the involvement of mRNA, tRNA, and the
ribosomes.
Video Synopsis:
Dr. James Lake introduces the how and why of RNA transcription, followed by a demonstration of
the process at the molecular level. In animation, we see how enzymes unwind the DNA molecule,
making room for nucleotides that will eventually become mRNA, tRNA and RNA. We also see how
the components for protein synthesis converge on the ribosome and assemble the amino acids into
three- dimensional polypeptide chains. Dr. Lake suggests that certain primitive bacteria function
like humans at the level of protein synthesis.
Questions:
1. What central role do proteins play in the chemistry of life?
• The genetic information in DNA is expressed in the proteins it codes for.
• The proteins produced include those which serve as structural components and those that
function as enzymes controlling all of metabolism.
2. How do the nucleic acids DNA and RNA differ?
• In DNA the sugar is deoxyribose, while in RNA it is ribose.
• In RNA the nitrogenous base thymine is replaced by uracil.
• Although not mentioned in the video, RNA molecules are single stranded compared to the
doubled stranded DNAs.
3. What are the three common classes of RNA?
• mRNA - bearer of the code for a protein.
• tRNA - “taxis” which carry specific amino acids to ribosome.
• rRNA - components of the ribosomes where proteins are made.
4. What is accomplished by transcription?
• In transcription, a portion of the DNA unwinds, allowing one of its strands to act as a
template or model, for the construction of an RNA strand which is complimentary to the
DNA template strand.
• With transcription, the genetic information in the DNA is now encoded in the RNA.
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5. Outline the major events in the process of translation.
• mRNA attaches to a small ribosomal subunit.
• The initiating tRNA, carrying its specific amino acid, binds to the starting sequence of the
mRNA.
• A large ribosomal subunit joins to form an intact ribosome with two binding sites.
• The next coding sequence is recognized by the proper tRNA, carrying its specific amino acid.
• When brought into contact, the neighboring amino acids are enzymatically joined together.
• Movement of the mRNA along the ribosome exposes new coding sequences and the process
continues until a stop coding sequence signals the end of the protein.
Follow-up Activities:
1. A table showing the genetic dictionary is included in most text books. Research the literature to
find out how workers such as Khorona, Nirenberg, Ochoa and Holly were able to decipher the
code. Establish the meaning of the terms redundancy and degeneracy as they apply to the code.
2. Along with several of your classmates, have a contest to see who is the first one to determine the
entire primary structure ( sequence of amino acids) of a protein given a sequence of mRNA
nucleotides. Each participant needs to have a copy of the genetic dictionary as well as an RNA
sequence composed of 60 bases. The RNA sequence should be randomly generated using the four
RNA bases: U,A,C and G by a nonpartisan classmate. (This classmate must scan the sequence he
or she generates to be sure it does not contain any nonsense codons.)
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