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Chapter 19: Problem 32

During mismatch repair, why is it necessary to distinguish between the template strand and the newly made daughter strand? How is this accomplished?

Short Answer

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Distinguishing between the template and the daughter strand during mismatch repair is crucial in maintaining the accuracy of DNA replication as it allows the system to correct errors in the newly synthesized strand based on the correct sequence in the template strand. This is accomplished by recognizing the methylation pattern of the template strand which remains fully methylated after DNA replication, while the newly synthesized daughter strand is not yet methylated.

Step by step solution

01

Understand the Necessity for Distinguishing Strands

Mismatch repair is a highly important process for maintaining the accuracy of DNA replication. For the system to function properly, it has to identify which strand is the original template and which one is newly synthesized. This is necessary because the system corrects errors by replacing erroneous bases in the newly synthesized strand, leaving the original template strand intact. If the mismatch repair system could not distinguish the two strands, errors could be introduced rather than corrected.
02

Define the Mechanism of Distinguishing Strands

The mechanism for distinguishing the original template from the daughter strand is based on methylation. In E.coli, after DNA replication, the template strand is fully methylated while the newly synthesized daughter strand is non-methylated. So, the repair system recognizes the methylation pattern of the parental strand and introduces corrections on the newly made, not-yet-methylated daughter strand.
03

Describe the Process of Methyl-directed Mismatch Repair

Once the mismatched bases are identified between the originally methylated template and the not-yet-methylated daughter strand, the repair system excises the segment of the daughter strand containing the wrong base. Then, DNA polymerase fills in the gap by synthesizing new DNA using the original template strand as a guide. DNA ligase finally seals the nick.

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Most popular questions from this chapter

Is each of the following mutations a silent, missense, nonsense, or frameshift mutation? The original DNA strand is \(5^{\prime}\)-ATGGGACTAGATACC-3'. (Note: Only the coding strand is shown; the first codon is methionine.) A. 5'-ATGGGTCTAGATACC \(-3\) ' B. 5'-ATGCGACTAGATACC \(-3^{\prime}\) C. 5'-ATGGGACTAGTTACC \(-3^{\prime}\) D. 5'-ATGGGACTAAGATACC \(-3\) '

In E coli, a methyltransferase encyme encoded by the dam gene recognizes the sequence \(5^{\prime}-\mathrm{GATC}-3\) and attaches a methyl group to the nitrogen at position 6 of adenine. E. coli strains that have the dam gene deleted are known to have a higher spontaneous mutation rate than normal strains. Explain why.

Achondroplasia is a rare form of dwarfism. It is caused by an autosomal dominant mutation within a single gene. Among \(1,422,000\) live births, the number of babies born with actoondroplasia was 31. Among those 31 babies, 18 of them had one parent with achondroplasia. The remaining babies had two unaffected parents. What is the mutation frequency for this disorder among these \(1,422,000\) babies? What is the mutation rate for achondroplasia?

A point mutation occurs in the middle of the coding sequence for a gene. Which types of mutations-silent, missense, nonsense, and frameshift-would be most likely to disrupt protein function and which would be least likely?

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