Question

When two bases are mismatched, how does the cell know which base to repair?

A. UvrA and B system recognizes the mismatch and knows which base to cut out.

B. Dam methylation system marks the GATC sequence in the original strand and unmethylated daughter strand is repaired.

C. Cell is not able to distinguish new and old DNA strands, hence it does not repair a mismatch.

D. RecBC recognizes mismatch in the daughter strand and corrects it by replacing DNA fragment using the Okazaki fragment.

Answer 1

The incorrect base is identified in the mismatch repair process by the methylation pattern of the DNA strands. The methylated parental strand guides repair enzymes, such as MutS, MutL, and MutH, to correct the unmethylated daughter strand. Therefore, the correct answer is B. The Dam methylation system marks the GATC sequence in the original strand for repair.

Step-by-step explanation:

When two bases are mismatched during DNA replication, the mismatch repair system is employed to correct the error. One crucial component of this system is the pattern of methylation on the DNA strands. In Escherichia coli (E. coli), for instance, after DNA replication, the newly synthesized daughter strand remains unmethylated for some time, while the parental strand is already methylated. The methylated parental strand serves as a cue for repair enzymes to identify the original strand.

Proteins such as MutS, MutL, and MutH are involved in the mismatch repair process. MutH specifically cleaves the unmethylated strand, signifying it as the new DNA. An exonuclease then removes the segment of the strand that includes the erroneous base, and DNA polymerase III fills in the gap with the correct base, followed by DNA ligase sealing the repair.

Therefore, the correct answer to the question is B. The Dam methylation system marks the GATC sequence in the original strand so that the unmethylated daughter strand is the one to be repaired.