MCB 150 Lecture Notes - Lecture 23: Cytosine, Thymine, Guanine
15 Mar 2017
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Document Summary
Dna sequences can be changed by many factors. Among these, uncorrected mistakes in replication account for the most mutations. This is when billions of new bonds are formed. Frequency of mutations that end up in e coli is about . 04 mutations/cell division. Average in all organisms is 1 mistake per billion. But in vitro e coli dna pol make about 400 mistakes. This is about 10000 times more mistakes in vivo. If proofreading does not catch the mistake. Mismatch repair system in e coli scans recently synthesizing dna looking for mismatches and hemimethylated dna. New dna gets methylated at adenine residues within the sequence shortly after replication. If mismatch repair enzymes are missing, error freq is. One type of colon cancer can be traced to a mutation. Proofreading and mismatch repair each decrease error frequency by. The mistake is always in the new strand. Mismatch repair enzymes fix problem in unmethylated (new) strand.
