Question 1

Which experimental evidence most strongly supports the semiconservative model of DNA replication?

Accepted Answer

The observation of two distinct bands after two rounds of replication in Meselson-Stahl's density gradient experiment using $^{15}N$ and $^{14}N$ isotopes.

Question 2

During DNA replication, which enzyme is responsible for unwinding the double helix ahead of the replication fork?

Accepted Answer

DNA helicase

Question 3

What is the primary function of telomerase in eukaryotic cells?

Accepted Answer

To extend the ends of linear chromosomes by adding repetitive DNA sequences using an RNA template.

Question 4

Which of the following best describes the directionality of DNA synthesis by DNA polymerases?

Accepted Answer

DNA polymerases synthesize DNA in the $5' \rightarrow 3'$ direction by adding nucleotides to the 3' hydroxyl group.

Question 5

A mutation in the gene encoding single-stranded DNA binding proteins (SSB) would most likely result in which cellular defect?

Accepted Answer

Failure to keep DNA strands separated during replication, leading to premature reannealing and replication stalling.

Question 6

Which enzyme in E. coli is responsible for removing RNA primers from Okazaki fragments and replacing them with DNA?

Accepted Answer

DNA polymerase I, using its $5' \rightarrow 3'$ exonuclease activity.

Question 7

Why is the lagging strand synthesized discontinuously during DNA replication?

Accepted Answer

Because DNA polymerase can only synthesize in the $5' \rightarrow 3'$ direction, requiring multiple RNA primers and the formation of Okazaki fragments.

Question 8

What is the role of topoisomerase during DNA replication?

Accepted Answer

To relieve supercoiling tension ahead of the replication fork by making transient cuts in the DNA.

Question 9

In eukaryotic cells, what is the function of the origin recognition complex (ORC)?

Accepted Answer

It binds to origins of replication and recruits other proteins to initiate DNA unwinding and replication.

Question 10

A patient with Werner syndrome exhibits premature aging due to a defect in which cellular process?

Accepted Answer

Telomere capping and protection, leading to accelerated telomere shortening.

Question 11

Which of the following statements about DNA polymerase proofreading is correct?

Accepted Answer

Proofreading is performed by the $3' \rightarrow 5'$ exonuclease activity of DNA polymerase, which removes incorrectly paired nucleotides.

Question 12

Which of the following best explains why telomerase activity is restricted in most multicellular organisms?

Accepted Answer

To limit the number of cell divisions and prevent uncontrolled cell proliferation, which could lead to cancer.

Question 13

Which protein complex is responsible for coordinating DNA synthesis and unwinding at the replication fork?

Accepted Answer

The replisome

Question 14

In the context of DNA replication, what is the function of DNA ligase?

Accepted Answer

To join adjacent Okazaki fragments by forming covalent bonds between DNA segments.

Question 15

Which of the following is a consequence of the end-replication problem in eukaryotic cells?

Accepted Answer

Progressive shortening of chromosome ends with each round of DNA replication.

Question 16

Which of the following statements about Okazaki fragments is true?

Accepted Answer

They are short DNA fragments synthesized on the lagging strand during DNA replication.

Question 17

Theta ($	heta$) replication is characteristic of which type of DNA molecule?

Accepted Answer

Circular DNA molecules, such as those found in bacteria.

Question 18

Which of the following is a key difference between DNA replication in prokaryotes and eukaryotes?

Accepted Answer

Eukaryotes initiate replication at multiple origins, creating many replicons per chromosome.

Question 19

A researcher observes that a cell line can be passaged indefinitely and produces telomerase. What is the most likely explanation for this observation?

Accepted Answer

The cell line is immortalized, similar to HeLa cells, due to active telomerase maintaining telomere length.

DNA Replication, Repair, and Recombination (Chapter 17 Study Notes)

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