Question 1

Which experiment provided definitive evidence for the semiconservative model of DNA replication, and what was the key result supporting this model?

Accepted Answer

The Meselson-Stahl experiment; after one replication cycle in $$^{14}N $$medium, DNA showed an intermediate density band.

Question 2

During DNA replication in E. coli, which enzyme is primarily responsible for synthesizing the majority of the new DNA, and what is its directionality?

Accepted Answer

DNA polymerase III; synthesizes DNA in the $$5' \rightarrow 3'$$ direction.

Question 3

A patient with xeroderma pigmentosum is highly sensitive to sunlight. Which DNA repair pathway is defective in this condition, and what type of DNA damage does it fail to repair?

Accepted Answer

Nucleotide excision repair; fails to repair thymine dimers caused by UV radiation.

Question 4

In eukaryotic DNA replication, what is the function of the origin recognition complex (ORC), and how does it contribute to replication licensing?

Accepted Answer

ORC binds to the replication origin, recruiting helicase loaders and MCM proteins to form the pre-replication complex, ensuring replication occurs only once per cell cycle.

Question 5

Which of the following best describes the end-replication problem in eukaryotes, and how is it addressed?

Accepted Answer

DNA polymerase cannot replicate the 5' ends of linear chromosomes, leading to loss of coding regions; telomerase adds repeated sequences to telomeres to protect important DNA.

Question 6

A DNA sample is exposed to nitrous acid ($$HNO_2$$), which increases the rate of deamination. What is the most likely base change that will occur, and which repair pathway corrects this damage?

Accepted Answer

Cytosine is converted to uracil; base excision repair corrects the damage.

Question 7

During bacterial DNA replication, what is the role of DNA gyrase, and why is it essential?

Accepted Answer

DNA gyrase creates swivel points by making and sealing breaks to relieve supercoiling ahead of the replication fork, preventing strand breaks.

Question 8

Which protein complex is responsible for keeping DNA polymerase attached to the DNA during replication, and how does its function differ between the leading and lagging strands?

Accepted Answer

Sliding clamp; remains attached to the leading strand throughout replication but must be reloaded for each Okazaki fragment on the lagging strand.

Question 9

A cell exposed to UV radiation develops pyrimidine dimers. Which repair mechanism is used in humans to correct this damage, and what is the sequence of steps involved?

Accepted Answer

Nucleotide excision repair; excinuclease cuts the DNA on both sides of the lesion, helicase unwinds the segment, DNA polymerase fills the gap, and ligase seals the nick.

Question 10

Which of the following statements about Okazaki fragments is correct regarding their synthesis and processing in eukaryotic cells?

Accepted Answer

Okazaki fragments are synthesized discontinuously on the lagging strand, RNA primers are removed by RNAse H and FEN1, and fragments are joined by DNA ligase.

Question 11

A mutation causes loss of function in the Ku70 and Ku80 proteins. Which DNA repair pathway is most directly affected, and what is the consequence for double-strand break repair?

Accepted Answer

Nonhomologous end-joining (NHEJ); double-strand breaks cannot be efficiently joined, increasing error-prone repair.

Question 12

Which of the following best explains why telomerase activity is restricted to germ cells and certain proliferating cells in multicellular organisms?

Accepted Answer

Telomerase prevents loss of coding DNA during replication, but unrestricted activity could lead to uncontrolled cell division and cancer.

Question 13

A DNA polymerase incorporates a base analogue, 5-bromodeoxyuridine (BrdU), during replication. What is the potential consequence for the DNA sequence in subsequent generations?

Accepted Answer

BrdU can undergo tautomeric shifts, pairing with guanine and leading to AT to GC transitions in future replication cycles.

Question 14

Which mechanism allows for accurate repair of double-strand breaks using a homologous template, and what is the key protein involved in strand invasion?

Accepted Answer

Synthesis-dependent strand annealing (SDSA); Rad51 is the key protein.

Question 15

During homologous recombination, what is the significance of the Holliday junction, and how does its resolution affect genetic diversity?

Accepted Answer

The Holliday junction is a temporary structure that, when resolved via opposite-sense resolution, results in crossing over and increased genetic diversity.

Question 16

Which of the following best describes the movement of retrotransposons in the human genome?

Accepted Answer

Retrotransposons move via an RNA intermediate, using reverse transcriptase to synthesize DNA that is integrated at a new site.

Question 17

A cell line is immortalized and can be passaged indefinitely. Which molecular feature is most likely responsible for this property?

Accepted Answer

Production of telomerase, which maintains telomere length and prevents the Hayflick limit.

Question 18

Which of the following is a plausible outcome if a mutation disables the 3'→5' exonuclease activity of DNA polymerase III in E. coli?

Accepted Answer

DNA replication proceeds normally, but the error rate increases significantly due to loss of proofreading.

Question 19

Which of the following best explains why DNA uses thymine instead of uracil as one of its four bases?

Accepted Answer

Using thymine allows base excision repair to distinguish between normal thymine and uracil formed from cytosine deamination.

DNA Replication, Repair, and Recombination: Study Guide

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