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Methyl-directed Mismatch Repair
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How many errors in DNA replication are too many? DNA polymerase makes an error in DNA synthesis approximately once in every thousand bases added to a DNA strand. An error rate of one in a thousand may sound very low. If a student only got one question in a thousand wrong in a course we would consider him or her an excellent student. In DNA replication every single error can have a potentially disastrous effect on the function of the gene being copied. Each gene is composed of hundreds or thousands of bases and organisms have thousands of genes. An error rate of one in a thousand is going to result in a completely nonfunctional cell in a handful of generations. After DNA replication other factors check for errors and correct them. These factors have similar error rates to DNA polymerase meaning that errors in DNA replication only occur about once in a million times.

View the animation below, then complete the quiz to test your knowledge of the concept.





1Arrange the following proteins in the proper sequence in which they participate in methyl-directed mismatch repair.
1. Mut S
2. Mut L
3. Mut H
A)1, 2, 3
B)2, 1, 3
C)2, 3, 1
D)3, 2, 1
E)3, 1, 2


2In methyl-directed mismatch repair, the strand that is repaired is
A)the methylated strand.
B)the non-methylated strand.
C)the hemi-methylated strand.
D)either the methylated or non-methylated, depending on the type of mismatch.
E)either the methylated or non-methylated, depending on the size of mismatch.


3Methylation of DNA typically occurs at sites with the sequence
A)ATAT.
B)CTGA.
C)AGGA.
D)GTTG.
E)GATC.


4DNA pol III and DNA ligase are required to fill in the repair gap.
A)True
B)False


5The MutH protein is directly responsible for recognizing mismatched DNA base pairs.
A)True
B)False





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