Correct option is A
The mismatch repair (MMR) system in E. coli uses DNA methylation as a signal to distinguish the template strand from the newly synthesized strand. In the hypermethylation mutant (type A), DNA is methylated on the GATC sequences immediately after DNA replication. This helps the MMR system recognize the template strand for correction.
In contrast, in type B, where the GATC sequences are never methylated, the MMR system cannot differentiate between the parental and the daughter strand effectively, which can lead to a failure to repair mismatches.
Therefore, Type A will have a greater effect on the MMR system and will lead to the accumulation of spontaneous mutations, as the repair system works correctly and preferentially corrects the newly synthesized strand in normal cases.
Information Booster:
The mismatch repair (MMR) system in E. coli relies on the methylation of GATC sequences to identify the template strand and repair newly incorporated mismatches during DNA replication.
Type A mutants (hypermethylation mutants) maintain proper methylation patterns, which allows the MMR system to function normally and correct mismatches. Therefore, the system is efficient in correcting errors and preventing spontaneous mutations.
Type B mutants, however, lack methylation of the GATC sequences, which disrupts the MMR system's ability to distinguish the parental strand from the newly synthesized strand. This disruption prevents the proper correction of errors, leading to an accumulation of mutations over time.
The methylation signal is crucial for the accuracy of the MMR system, which plays a key role in preventing the accumulation of spontaneous mutations.
Additional Information:
(b) Type B > Type A: This is incorrect because, in type B mutants, the lack of methylation leads to an impaired MMR system, which will likely result in the accumulation of mutations rather than preventing them.
(c) Type A = Type B: This is incorrect because type A maintains normal methylation patterns, which helps the MMR system function properly and reduce mutations, whereas type B mutants do not have this methylation signal, leading to more spontaneous mutations.
(d) Type B mutants will not accumulate spontaneous mutations: This is incorrect because the absence of methylation in type B mutants causes a failure in the mismatch repair system, leading to the accumulation of spontaneous mutations.
