MCAT Biochemistry Question 90: Answer and Explanation

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Question: 90

15. During which phase of the cell cycle are DNA repair mechanisms least active?

A. G₁
B. S
C. G₂
D. M

Correct Answer: D

Explanation:

Mismatch repair mechanisms are active during S phase (proofreading) and G₂ phase (MSH2 and MLH1), eliminating choices (B) and (C). Nucleotide and base excision repair mechanisms are most active during the G₁ and G₂ phases, also eliminating choice (A). These mechanisms exist during interphase because they are aimed at preventing propagation of the error into daughter cells during M phase (mitosis).

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