Practice question
Question
Mismatch excision requires which helicase?
Explanation
Following MutH nicking of daughter strand at hemimethylated GATC, intervening DNA between nick and mismatch must be unwound and removed. Helicase II, named UvrD, is recruited through direct protein interaction with MutL. Using ATP hydrolysis, UvrD translocates 3' to 5' direction, unwinding duplex flanking mismatch region. Single-stranded DNA binding protein stabilizes exposed strands preventing reannealing. Subsequently, appropriate exonucleases degrade unwound strand: 3' to 5' exonucleases if nick is upstream, 5' to 3' RecJ if nick downstream. DNA polymerase III then fills gap accurately.
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