A molecular biologist has discovered a new strain of bacteria that utilizes a sequence other than GATC to guide its mismatch repair activities. Which of the following sequences cannot be used by this new bacterium for mismatch repair?
The correct answer and explanation is :
The correct answer is GATC.
In molecular biology, mismatch repair (MMR) is a system for correcting errors that occur during DNA replication. It is crucial for maintaining the integrity of the genome. In most bacteria, the mismatch repair system involves recognizing and repairing mismatches in the DNA strand. The process is guided by the identification of the “parent” (original) strand and the newly synthesized strand.
In the case of Escherichia coli and many other bacteria, the sequence GATC plays a central role in guiding mismatch repair. The E. coli mismatch repair system is based on the recognition of a methylated GATC site. The parent strand is identified by the methylation of the adenine residue in the GATC sequence. The newly synthesized strand is typically unmethylated, allowing the mismatch repair machinery to recognize the error on the newly synthesized strand and repair it.
In this system, the MutS protein recognizes the mismatch, while MutL and other proteins direct the repair machinery to the error site. The sequence GATC provides a critical signal that helps to distinguish between the two strands, enabling the repair machinery to identify which strand is correct and which one needs to be corrected.
If the newly discovered strain of bacteria uses a sequence other than GATC to guide mismatch repair, it means that the mismatch repair system in that bacterium is likely based on different sequence motifs and/or mechanisms. However, since the question is asking which sequence cannot be used by this new bacterium for mismatch repair, GATC is the correct answer because it is the canonical sequence used by many bacteria for the methylation-dependent mismatch repair system, which is unlikely to be used if a different system exists in the newly discovered strain.
Thus, GATC cannot be used for mismatch repair in this new strain because the bacterium uses a different guiding sequence.