40. In E. coli, mismatch during replication is corrected by the mismatch repair (MMR) pathway. For this, the
MMR proteins differentiate between the parent and daughter strand by:
1. Identifying the methylation on the daughter strand
2. Identifying the methylation on the parent strand
3. Identifying methylation on both strands
4. Identifying lack of methylation on both strands

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Mismatch Repair Pathway in E. coli: Identifying Parent and Daughter Strands

In Escherichia coli (E. coli), the process of DNA replication is highly efficient but occasionally errors occur, such as mismatched base pairs. To maintain the integrity of the genome, E. coli employs the mismatch repair (MMR) pathway, a mechanism that corrects these replication errors. An essential part of this repair process involves distinguishing between the parent (template) strand and the newly synthesized daughter strand. But how does the MMR system differentiate the two?

The Mismatch Repair (MMR) Pathway in E. coli

The MMR pathway in E. coli relies on the ability to differentiate between the old (parental) strand and the newly synthesized (daughter) strand. This distinction is crucial because only the daughter strand is the one with potential errors that need to be corrected.

• The key mechanism for distinguishing the strands is DNA methylation. In E. coli, the parent strand is methylated at specific sites (adenine residues in the sequence GATC). This methylation is not immediately present on the newly synthesized daughter strand after DNA replication.

How the MMR Pathway Works:

1. Methylation Identification:

   • The parent strand is methylated at certain adenine residues (GATC sequences), marking it as the correct strand.

   • The daughter strand, being newly synthesized, has not yet been methylated at these positions, making it the target for repair.

2. Mismatch Detection:

   • The MMR proteins recognize mismatches in the newly synthesized (daughter) strand. Once a mismatch is identified, the MMR system uses the methylation pattern to differentiate the strands.

   • The repair machinery then excises the mismatched section from the daughter strand and replaces it with the correct sequence, using the parent strand as the template.

The Correct Answer:

• Answer: 2. Identifying the methylation on the parent strand.

The MMR proteins identify the methylation on the parent strand, which allows them to correctly differentiate the parent strand from the newly synthesized daughter strand. This methylation on the parent strand serves as a marker for which strand is correct, ensuring that errors in the daughter strand are repaired.

Summary:

In E. coli, the mismatch repair (MMR) pathway is a crucial system for maintaining genomic integrity by correcting errors that occur during DNA replication. The MMR proteins distinguish between the parent and daughter strands based on DNA methylation, identifying the parent strand as the methylated one. This system ensures that replication errors, such as base mismatches, are corrected using the parent strand as the template for repair.