Essentials of Genetics

William S. Klug, Michael R. Cummings, Charlotte A. Spencer

Chapter 10

DNA Replication - all with Video Answers

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Chapter Questions

Problem 1

In this chapter, we focused on how DNA is replicated and synthesized. In particular, we elucidated the general mechanism of replication and described how DNA is synthesized when it is copied. Based on your study of these topics, answer the following fundamental questions:
(a) What is the experimental basis for concluding that DNA replicates semiconservatively in both bacteria and eukaryotes?
(b) How was it demonstrated that DNA synthesis occurs under the direction of DNA polymerase III and not polymerase I?
(c) How do we know that in vivo DNA synthesis occurs in the $5^{\prime}$ to $3^{\prime}$ direction?
(d) How do we know that DNA synthesis is discontinuous on one of the two template strands?
(e) What observations reveal that a "telomere problem" exists during eukaryotic DNA replication, and how did we learn of the solution to this problem?

Jennifer Stoner

Jennifer Stoner

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Problem 2

Review the Chapter Concepts list on p. $182 .$ These are concerned with the replication and synthesis of DNA. Write a short essay that distinguishes between the terms replication and synthesis, as applied to DNA. Which of the two is most closely allied with the field of biochemistry?

Jennifer Stoner

Jennifer Stoner

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Problem 3

Compare conservative, semiconservative, and dispersive modes of DNA replication.

Rachel Griffin

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Problem 4

Describe the role of $^{15} \mathrm{N}$ in the Meselson-Stahl experiment.

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Problem 5

Predict the results of the experiment by Taylor, Woods, and Hughes if replication were (a) conservative and (b) dispersive.

Rachel Griffin

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Problem 6

Reconsider Problem 30 in Chapter $9 .$ In the model you proposed, could the molecule be replicated semiconservatively? Why? Would other modes of replication work?

Shiksha Dutta

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Problem 7

What are the requirements for in vitro synthesis of DNA under the direction of DNA polymerase I?

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Problem 8

How did Kornberg assess the fidelity of DNA polymerase I in copying a DNA template?

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Problem 9

Which characteristics of DNA polymerase I raised doubts that its in vivo function is the synthesis of DNA leading to complete replication?

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Problem 10

Kornberg showed that nucleotides are added to the 3 '-end of each growing DNA strand. In what way does an exposed $3^{\prime}$ - OH group participate in strand elongation?

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Problem 11

What was the significance of the polA1 mutation?

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Problem 12

Summarize and compare the properties of DNA polymerase I, II, and III.

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Problem 13

List and describe the function of the ten subunits constituting DNA polymerase III. Distinguish between the holoenzyme and the core enzyme.

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Problem 14

Distinguish between (a) unidirectional and bidirectional synthesis, and (b) continuous and discontinuous synthesis of DNA.

Rachel Griffin

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Problem 15

List the proteins that unwind DNA during in vivo DNA synthesis. How do they function?

Rachel Griffin

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Problem 16

Define and indicate the significance of (a) Okazaki fragments, (b) DNA ligase, and (c) primer RNA during DNA replication.

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Problem 17

Outline the current model for DNA synthesis.

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Problem 18

Why is DNA synthesis expected to be more complex in eukaryotes than in bacteria? How is DNA synthesis similar in the two types of organisms?

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Problem 19

If the analysis of DNA from two different microorganisms demonstrated very similar base compositions, are the DNA sequences of the two organisms also nearly identical?

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Problem 20

Several temperature-sensitive mutant strains of $E .$ coli display the following characteristics. Predict what enzyme or function is being affected by each mutation.
(a) Newly synthesized DNA contains many mismatched base pairs.
(b) Okazaki fragments accumulate, and DNA synthesis is never completed.
(c) No initiation occurs.
(d) Synthesis is very slow.
(e) Supercoiled strands remain after replication, which is never completed.

Rachel Griffin

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Problem 21

Many of the gene products involved in DNA synthesis were initially defined by studying mutant $E .$ coli strains that could not synthesize DNA.
(a) The dnaE gene encodes the $\alpha$ subunit of DNA polymerase III. What effect is expected from a mutation in this gene? How could the mutant strain be maintained?
(b) The $d n a Q$ gene encodes the $\varepsilon$ subunit of DNA polymerase. What effect is expected from a mutation in this gene?

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Problem 22

Assume a hypothetical organism in which DNA replication is conservative. Design an experiment similar to that of Taylor, Woods, and Hughes that will unequivocally establish this fact. Using the format established in Figure $10.5,$ draw sister chromatids and illustrate the expected results depicting this mode of replication.

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Problem 23

Describe the "end-replication problem" in eukaryotes. How is it resolved?

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Problem 24

In $1994,$ telomerase activity was discovered in human cancer cell lines. Although telomerase is not active in human somatic tissue,
these cells do contain the genes for telomerase proteins and telomerase RNA. since inappropriate activation of telomerase may contribute to cancer, why do you think the genes coding for this enzyme have been maintained in the human genome throughout evolution? Are there any types of human body cells where telomerase activation would be advantageous or even
necessary? Explain.

CB

Christopher Bazell

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