Q.1 In mismatch correction repair, the parental DNA strand is distinguished from the daughter strand by
(A) acetylation (B) phosphorylation (C) methylation (D) glycosylation

In mismatch repair, the parental DNA strand is distinguished from the newly synthesized daughter strand through methylation, making (C) the correct answer. This process ensures replication errors are corrected on the error-prone daughter strand. Understanding this mechanism is crucial for biotechnology and molecular biology studies.

Correct Answer

The correct option is (C) methylation. In prokaryotes like E. coli, the parental strand contains methylated adenine residues in GATC sequences, added by Dam methylase post-replication. The unmethylated daughter strand is targeted for excision and repair by proteins like MutS, MutL, and MutH, preventing mutations.

Option Explanations

Mismatch repair distinguishes strands to fix replication errors accurately, primarily via post-replication hemimethylation.

• (A) Acetylation: Incorrect; this modifies histones or proteins for gene expression, not DNA strand discrimination in repair.

• (B) Phosphorylation: Incorrect; this alters proteins for signaling or enzyme activity, unrelated to parental-daughter strand identification.

• (C) Methylation: Correct; transient hemimethylation marks the parental strand as methylated, directing repair to the daughter strand.

• (D) Glycosylation: Incorrect; this adds sugars to proteins or lipids, not involved in DNA mismatch repair strand selection.

Mechanism Overview

DNA mismatch repair (MMR) detects base mismatches post-replication. MutS recognizes the mismatch, MutL links it to MutH, which nicks the unmethylated daughter strand near GATC sites. Helicase and exonucleases remove the erroneous segment, followed by resynthesis using the parental strand as template and ligation. In eukaryotes, nicks or PCNA orientation replace methylation.