Question 1

Which experiment provided the strongest evidence for the semi-conservative model of DNA replication, and what was the key finding?

Accepted Answer

The Meselson-Stahl experiment; it demonstrated that each daughter DNA molecule contains one parental and one newly synthesized strand.

Question 2

During prokaryotic DNA replication, which enzyme is responsible for relieving the torsional strain ahead of the replication fork, and what is the chemical reaction it catalyzes?

Accepted Answer

DNA gyrase (topoisomerase II); it introduces negative supercoils by breaking and rejoining double-stranded DNA.

Question 3

Which of the following best describes the difference between leading and lagging strand synthesis in eukaryotic DNA replication?

Accepted Answer

The leading strand is synthesized continuously by DNA polymerase ε, while the lagging strand is synthesized discontinuously as Okazaki fragments by DNA polymerase δ.

Question 4

A mutation changes a codon from UAU (tyrosine) to UAA (stop codon). What type of mutation is this, and what is its likely effect on the protein?

Accepted Answer

Nonsense mutation; it causes premature termination of translation, producing a truncated protein.

Question 5

Which DNA repair mechanism is most likely to correct a thymine dimer caused by UV radiation, and what enzyme is directly involved?

Accepted Answer

Direct repair (photoreactivation); photolyase

Question 6

In the context of eukaryotic DNA replication, what is the primary function of the PCNA protein?

Accepted Answer

It serves as a sliding clamp to increase the processivity of DNA polymerases ε and δ.

Question 7

A patient is diagnosed with a trinucleotide repeat disorder, such as Huntington's disease. Which molecular mechanism most likely caused the expansion of the nucleotide repeats?

Accepted Answer

Slippage of DNA polymerase during replication in repetitive sequences.

Question 8

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

Accepted Answer

Eukaryotic chromosomes are linear and much larger, so multiple origins ensure timely and complete replication.

Question 9

Which enzyme is responsible for removing RNA primers and filling in the resulting gaps with DNA in prokaryotic DNA replication?

Accepted Answer

DNA polymerase I

Question 10

A mutation changes a codon from GAG (glutamic acid) to GTG (valine), resulting in sickle cell anemia. What type of mutation is this, and what is its effect?

Accepted Answer

Missense mutation; substitutes one amino acid for another, altering protein function.

Question 11

Which of the following best describes the role of the β-clamp (sliding clamp) in prokaryotic DNA replication?

Accepted Answer

It holds DNA polymerase III onto the DNA template, increasing processivity.

Question 12

Which DNA repair pathway is most likely to introduce mutations during the repair process, and why?

Accepted Answer

Non-homologous end joining (NHEJ); because it can result in the loss or addition of nucleotides at the break site.

Question 13

Which of the following is a key difference between prokaryotic and eukaryotic DNA replication?

Accepted Answer

Eukaryotes have multiple origins of replication, while prokaryotes typically have a single origin.

Question 14

A DNA repair enzyme removes an oxidized guanine base, creating an abasic site. Which repair pathway is this, and what is the next enzymatic step?

Accepted Answer

Base excision repair; cleavage of the DNA backbone at the abasic site by AP endonuclease.

Question 15

Which of the following best explains why telomerase activity is essential in germ cells but not in most somatic cells?

Accepted Answer

Telomerase prevents the progressive shortening of chromosome ends during repeated rounds of DNA replication.

Question 16

Which protein complex is responsible for loading the MCM helicase onto DNA during eukaryotic DNA replication initiation?

Accepted Answer

ORC (origin recognition complex) with Cdc6 and Cdt1

Question 17

A frameshift mutation occurs in a gene. Which of the following is the most likely consequence?

Accepted Answer

The reading frame is altered, potentially resulting in a completely different and nonfunctional protein downstream of the mutation.

Question 18

Which of the following best describes the function of histone chaperones such as CAF-1 and ASF1 during DNA replication?

Accepted Answer

They assist in assembling nucleosomes on newly synthesized DNA.

Question 19

Which of the following is a real-life consequence of a loss-of-function mutation in a tumor suppressor gene?

Accepted Answer

Uncontrolled cell division leading to cancer.

DNA Replication and Repair: Mechanisms, Errors, and Cellular Strategies

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