Replication Forks - After proofreading, the error rate is 1...

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Replication Forks DNA synthesis occurs at numerous different locations on the same DNA molecule (hundreds in a human chromosome). These form bubbles of replication with a replication fork at the growing edge. The replication rate of eukaryotic DNA is 500 to 5000 base pairs per minute. A human cell typically requires a few hours to duplicate the 6 billion base pairs. Repair of damaged DNA Changes in the DNA code are called mutations. Repair enzymes repair most of the errors that occur in DNA. There are three different classes of repair mechanisms. 1. Proofreading corrects errors made during the DNA replication process. 2. Mismatch repair corrects base pair mismatching (A-T and G-C). 3. Excision repair removes and replaces small segments of damaged DNA. Errors corrected as a result of DNA synthesis Proofreading The overall error rate during DNA replication in E.coli is one base in one million (10 6 ). DNA polymerase proofreads the new strand of DNA as it is synthesized and it removes mismatched bases and replaces them with the correct bases.
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Unformatted text preview: After proofreading, the error rate is 1 in 1 billion (10 9 ) base pairs. Mismatch Repair After DNA is replicated, some enzymes function to locate mismatched base pairs, remove a short segment of nucleotides containing the error, and replace the segment with the correct nucleotides. The new segment is then sealed to the original strand by DNA ligase. Recall that this is the enzyme that seals the Okazaki fragments during DNA synthesis. When repair enzymes detect a pairing error, how do they know which DNA strand contains the error? The repair enzymes are capable of distinguishing between the original strand of DNA and the new strand that contains the error because the new strand is not methylated . Methylation involves adding methyl groups (CH 3 ) after DNA is synthesized. Shortly after DNA is synthesized, however, the new strand is not yet methylated. . Mismatch repair enzymes are able to detect which strand is not methylated....
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