Question 1

Which of the following best describes the semiconservative model of DNA replication?

Accepted Answer

Each daughter DNA molecule consists of one parental strand and one newly synthesized strand.

Question 2

In the Meselson-Stahl experiment, what result after the first replication in $$^{14}N$$ medium ruled out the conservative model of DNA replication?

Accepted Answer

A single band of intermediate density DNA was observed.

Question 3

Which enzyme is responsible for unwinding the DNA double helix at the replication fork?

Accepted Answer

Helicase

Question 4

What is the function of single-strand binding proteins during DNA replication?

Accepted Answer

They stabilize unwound parental DNA strands and prevent them from re-pairing.

Question 5

Why is DNA synthesis always in the 5' to 3' direction?

Accepted Answer

DNA polymerases can only add nucleotides to the free 3' OH group of the growing strand.

Question 6

What is the role of primase in DNA replication?

Accepted Answer

It synthesizes a short RNA primer to provide a starting point for DNA polymerase.

Question 7

Which enzyme replaces RNA nucleotides of the primer with DNA nucleotides on the lagging strand?

Accepted Answer

DNA polymerase I

Question 8

What is the function of DNA ligase during DNA replication?

Accepted Answer

It joins the sugar-phosphate backbones of Okazaki fragments to form a continuous DNA strand.

Question 9

Which of the following best explains why eukaryotic chromosomes have multiple origins of replication?

Accepted Answer

Their DNA molecules are much longer, so multiple origins speed up the replication process.

Question 10

A mutation in the gene encoding a nucleotide excision repair enzyme can lead to which human disorder, characterized by extreme sensitivity to sunlight?

Accepted Answer

Xeroderma pigmentosum

Question 11

What is the main evolutionary significance of rare, uncorrected mutations in DNA?

Accepted Answer

They are the source of genetic variation on which natural selection acts.

Question 12

Which of the following is a correct description of an Okazaki fragment?

Accepted Answer

A short segment of DNA synthesized on the lagging strand during replication.

Question 13

Which enzyme relieves the strain caused by the unwinding of the DNA double helix ahead of the replication fork?

Accepted Answer

Topoisomerase

Question 14

During DNA replication, what provides the energy for the polymerization reaction?

Accepted Answer

Hydrolysis of pyrophosphate ($$2P_i) $$released from the incoming nucleotide triphosphate.

Question 15

Why do eukaryotic chromosomes have telomeres at their ends?

Accepted Answer

To protect genes from being eroded during successive rounds of DNA replication.

Question 16

If a cell lacked functional DNA ligase, what would be the most direct consequence during DNA replication?

Accepted Answer

Okazaki fragments on the lagging strand would not be joined into a continuous DNA strand.

Question 17

Which of the following best explains why DNA polymerase has a lower error rate in the completed DNA than during initial nucleotide pairing?

Accepted Answer

DNA polymerase proofreads each nucleotide and removes incorrectly paired nucleotides.

Question 18

Which of the following is NOT a function of DNA polymerase III in E. coli?

Accepted Answer

Replacing RNA primers with DNA nucleotides.

Question 19

A patient with a defect in mismatch repair enzymes is at increased risk for which of the following?

Accepted Answer

Cancer due to accumulation of replication errors.

Question 20

Which of the following statements about the directionality of DNA replication is correct?

Accepted Answer

New DNA strands are always synthesized in the 5' to 3' direction.

DNA Replication and Repair: Mechanisms and Molecular Players

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