A modification of overlap extension PCR may also be used to construct directed mutations; this is termed megaprimer PCR .

TLS Online TPP Program

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#Unit 3. Fundamental Processes

In Escherichia coli MMR system, MutL, in turn, activates MutH, an enzyme that causes an incision or nick on one strand near the site of the mismatch.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

Nicking is followed by the action of a specific helicase (UvrD) and one of three exonucleases.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

Eukaryotic cells also repair mismatches and do so using homologs to MutS (called MSH proteins for “MutS homologs”) and MutL (called MLH and PMS

TLS Online TPP Program

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#Unit 3. Fundamental Processes

In BER, A DNA glycosylase recognizes a damaged base (in this case, a uracil) and cleaves between the base and deoxyribose in the backbone.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

n BER,  An AP endonuclease cleaves the phosphodiester backbone near the AP site

TLS Online TPP Program

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#Unit 3. Fundamental Processes

In BER,  DNA polymerase I initiates repair synthesis from the free 3’ hydroxyl at the nick, removing (with its 5’-3’ exonuclease activity) and replacing a portion of the damaged strand.