Question 1

You have discovered an "Exo^-" mutant form of DNA polymerase in which the 3′-to-5′ exonuclease function has been destroyed but the ability to join nucleotides together is unchanged.Which of the following properties do you expect the mutant polymerase to have?

Accepted Answer

The loss of the 3′-to-5′ exonuclease function means that the mutant polymerase will not be able to proofread and remove mismatched nucleotides. This will lead to an increased likelihood of mismatched base pairs being generated during DNA synthesis. The other choices are not necessarily affected by the loss of the exonuclease function.

Question 2

Which diagram accurately represents the directionality of DNA strands at one side of a replication fork? (a)

(b)

(c)

(d)

Figure 6-19

Accepted Answer

D

Question 3

The process of DNA replication requires that each of the parental DNA strands be used as a __________ to produce a duplicate of the opposing strand.

Accepted Answer

The parental DNA strands provide the template for the formation of the complementary DNA strand during replication. The template strand acts as a guide for the addition of nucleotides to the newly synthesized strand. Therefore, choice C, template, is the correct answer. Choices A, catalyst, and D, copy, do not accurately describe the role of the parental DNA strands in replication. Choice B, competitor, is irrelevant to the process of DNA replication.

Question 4

Which of the following statements correctly explains what it means for DNA replication to be bidirectional?

Accepted Answer

Bidirectional replication means that the replication forks formed at the origin move in opposite directions, with one fork moving in one direction and the other fork moving in the opposite direction. This allows for the replication of both strands of DNA simultaneously.

Question 5

DNA polymerase catalyzes the joining of a nucleotide to a growing DNA strand.What prevents this enzyme from catalyzing the reverse reaction?

Accepted Answer

Hydrolysis of pyrophosphate (PP_i) to inorganic phosphate (P_i) + P_i is an energetically favorable reaction, releasing enough energy to drive the reaction in the forward direction. The other options do not directly relate to preventing the reverse reaction.

Question 6

Which of the following statements about sequence proofreading during DNA replication is FALSE?

Accepted Answer

The exonuclease activity involved in proofreading during DNA replication cleaves DNA in the 3′-to-5′ direction, not the 5′-to-3′ direction. This allows the polymerase to remove incorrectly paired bases immediately after they are incorporated, ensuring high fidelity in DNA replication.

Question 7

You prepare bacterial cell extracts by lysing the cells and removing insoluble debris via centrifugation.These extracts provide the proteins required for DNA replication.Your DNA template is a small, double-stranded circular piece of DNA (a plasmid) with a single origin of replication and a single replication termination site.The termination site is on the opposite side of the plasmid from the origin.  Figure 6-11
What part of the DNA replication process would be most directly affected if a strain of bacteria lacking the exonuclease activity of DNA polymerase were used to make the cell extracts?

Accepted Answer

Lagging-strand completion would be most directly affected if a strain of bacteria lacking the exonuclease activity of DNA polymerase were used to make the cell extracts. Exonuclease activity is involved in proofreading the newly synthesized DNA strand during lagging-strand completion, so the lack of this activity would result in a higher error rate during this step. The initiation of DNA synthesis and leading-strand elongation would not be affected by the lack of exonuclease activity, and while Okazaki fragment synthesis also involves proofreading, it occurs during the synthesis of the fragments and not during their completion on the lagging strand.

Question 8

You prepare bacterial cell extracts by lysing the cells and removing insoluble debris via centrifugation.These extracts provide the proteins required for DNA replication.Your DNA template is a small, double-stranded circular piece of DNA (a plasmid) with a single origin of replication and a single replication termination site.The termination site is on the opposite side of the plasmid from the origin.  Figure 6-11
What part of the DNA replication process would be most directly affected if a strain of bacteria lacking helicase were used to make the cell extracts?

Accepted Answer

Because helicase unwinds the two DNA template strands, replication of both strands depends upon the activity of helicase at the time of initiation.

Question 9

DNA replication is considered semiconservative because

Accepted Answer

Semiconservative DNA replication means that each daughter DNA molecule consists of one old parental strand and one newly synthesized strand. This was first demonstrated by the Meselson-Stahl experiment in which bacteria were grown in a medium containing a heavy isotope of nitrogen (15N) that was incorporated into their DNA. The bacteria were then transferred to a medium containing a lighter isotope of nitrogen (14N) and allowed to replicate their DNA. After one round of replication, the DNA was observed to band at an intermediate density, indicating that each new DNA molecule had one heavy strand and one light strand. This supported the semiconservative model of replication.

Question 10

Initiator proteins bind to replication origins and disrupt hydrogen bonds between the two DNA strands being copied.Which of the factors below does not contribute to the relative ease of strand separation by initiator proteins?

Accepted Answer

The ease of strand separation by initiator proteins is primarily due to the fact that replication origins are rich in A-T base pairs, which only require two hydrogen bonds to be broken as opposed to the three needed for G-C base pairs. Initiator proteins also only separate a few base pairs at a time, which reduces the energy needed for strand separation. Additionally, once opened, other proteins of the DNA replication machinery bind to the origin to stabilize the unwound DNA strands. The reaction occurring at room temperature is not a factor in the ease of strand separation by initiator proteins.

Question 11

How does the total number of replication origins in bacterial cells compare to the number of origins in human cells?

Accepted Answer

Bacterial cells typically have a single origin of replication, while human cells have around 10,000 origins of replication to ensure the entire genome is replicated efficiently.

Question 12

You prepare bacterial cell extracts by lysing the cells and removing insoluble debris via centrifugation.These extracts provide the proteins required for DNA replication.Your DNA template is a small, double-stranded circular piece of DNA (a plasmid) with a single origin of replication and a single replication termination site.The termination site is on the opposite side of the plasmid from the origin.  Figure 6-11
What part of the DNA replication process would be most directly affected if a strain of bacteria lacking DNA ligase were used to make the cell extracts?

Accepted Answer

DNA ligase is responsible for completing the ligation, or joining, of Okazaki fragments on the lagging strand during DNA replication. Therefore, if a strain of bacteria lacking DNA ligase were used, lagging-strand completion would be most directly affected.

Question 13

You prepare bacterial cell extracts by lysing the cells and removing insoluble debris via centrifugation.These extracts provide the proteins required for DNA replication.Your DNA template is a small, double-stranded circular piece of DNA (a plasmid) with a single origin of replication and a single replication termination site.The termination site is on the opposite side of the plasmid from the origin.  Figure 6-11
Which of the following statements is TRUE with respect to this in vitro replication system?

Accepted Answer

The answer of You prepare bacterial cell extracts by lysing...

Question 14

The classic experiments conducted by Meselson and Stahl used isotopes of which element to distinguish newly synthesized DNA from parental DNA?

Accepted Answer

Meselson and Stahl used isotopes of nitrogen to distinguish newly synthesized DNA from parental DNA. Specifically, they used the heavy isotope nitrogen-15, which is less abundant than the more common isotope nitrogen-14. By growing bacteria in a medium containing only nitrogen-15, they were able to ensure that all of the bacteria's DNA would contain nitrogen-15. They then transferred the bacteria to a medium containing only nitrogen-14, and allowed them to replicate their DNA. By analyzing the density of the resulting DNA using centrifugation, they were able to distinguish the newly synthesized DNA (which contained a mix of nitrogen-14 and nitrogen-15) from the parental DNA (which contained only nitrogen-15).

Question 15

Which of the following statements about the newly synthesized strand of a human chromosome is TRUE?

Accepted Answer

In humans, DNA replication occurs at multiple origins along the chromosome due to the large size of the genome. The replication process involves both continuous (leading strand synthesis) and discontinuous (lagging strand synthesis, which forms Okazaki fragments) DNA synthesis.

Question 16

If the genome of the bacterium E.coli requires about 20 minutes to replicate itself, how can the genome of the fruit fly Drosophila be replicated in only 3 minutes?

Accepted Answer

Drosophila DNA contains more origins of replication than E.coli DNA, allowing multiple sections of the genome to be replicated simultaneously, which significantly speeds up the overall process of DNA replication.

Question 17

DNA polymerases are processive, which means that they remain tightly associated with the template strand while moving rapidly and adding nucleotides to the growing daughter strand.Which piece of the replication machinery accounts for this characteristic?

Accepted Answer

The sliding clamp is responsible for holding the DNA polymerase in place and allowing it to move rapidly along the template strand without dissociating. This contributes to the processivity of DNA replication. Helicase unwinds the double-stranded DNA to create a template for replication, single-strand binding proteins stabilize the unwound template strands, and primase synthesizes RNA primers to initiate DNA synthesis. While these components are important for DNA replication, they do not directly contribute to the processivity of DNA polymerase.

Question 18

You prepare bacterial cell extracts by lysing the cells and removing insoluble debris via centrifugation.These extracts provide the proteins required for DNA replication.Your DNA template is a small, double-stranded circular piece of DNA (a plasmid) with a single origin of replication and a single replication termination site.The termination site is on the opposite side of the plasmid from the origin.  Figure 6-11
What part of the DNA replication process would be most directly affected if a strain of bacteria lacking single-strand binding protein were used to make the cell extracts?

Accepted Answer

The answer of You prepare bacterial cell extracts by lysing...

Question 19

The chromatin structure in eukaryotic cells is much more complicated than that observed in prokaryotic cells.This is thought to be the reason why DNA replication occurs much faster in prokaryotes.How much faster is it?

Accepted Answer

DNA replication in prokaryotes is approximately 10 times faster than in eukaryotes due to the simpler chromatin structure and fewer regulatory mechanisms.

Question 20

How many replication forks are formed when an origin of replication is opened?

Accepted Answer

When an origin of replication is opened, two replication forks are formed, one moving in each direction from the origin. This allows for bidirectional replication of the DNA.

Quiz 6: DNA Replication and Repair

You have discovered an "Exo-" mutant form of DNA polymerase in which the 3′-to-5′ exonuclease function has been destroyed but the ability to join nucleotides together is unchanged.Which of the following properties do you expect the mutant polymerase to have?

Which diagram accurately represents the directionality of DNA strands at one side of a replication fork? (a) (b) (c) (d) Figure 6-19