Correct option is A
The correct answer is (a). Here's why:
- In wild-type E. coli, the mismatch repair (MMR) system uses the methylation pattern at the GATC sequence to distinguish the parental strand (which is methylated) from the newly synthesized strand (which is not methylated in a normal scenario). The MMR system will then correct any errors that are found on the newly synthesized strand, recognizing it as the strand that needs to be repaired.
- In the Type A mutant (hypermethylation), GATC sequences are immediately methylated after DNA synthesis. This rapid methylation on the newly synthesized strand can make it harder for the MMR system to identify the newly synthesized strand from the parental strand. Since the newly synthesized strand is methylated similarly to the parental strand, the MMR system becomes less effective in identifying errors. As a result, Type A mutants will have a greater effect on the MMR system and may accumulate more spontaneous mutations because the mismatch repair process becomes less efficient.
- In the Type B mutant, where GATC sequences are never methylated, the MMR system can clearly distinguish the newly synthesized strand (unmethylated) from the parental strand (methylated). Therefore, Type B mutants will not accumulate as many spontaneous mutations, as the MMR system can still function normally in distinguishing and correcting errors on the newly synthesized strand.
Information Booster:
- Hypermethylation in Type A mutants means that the DNA is methylated at GATC sequences as soon as the daughter strand is synthesized, making it difficult for the MMR system to differentiate between the parental and daughter strands, leading to errors in repair and an accumulation of spontaneous mutations.
- In Type B mutants, the lack of methylation at the GATC sequences of the daughter strand means that the MMR system works effectively and corrects errors more efficiently.
Additional Information:
- Type B > Type A (Option 2): This is incorrect because the Type A mutant (with methylation on both strands) leads to a greater effect on the MMR system and higher spontaneous mutations, not Type B.
- Type A = Type B (Option 3): This is incorrect because Type A has a greater effect on the MMR system than Type B, as explained.
- Type B mutants will not accumulate spontaneous mutations (Option 4): This is incorrect because Type B mutants still accumulate mutations, but not as many as Type A mutants. The MMR system still works in Type B, though errors may still accumulate due to the absence of methylation.
