Lecture 6 Part 1

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25 Mar 2012
Lecture 6 (Part I) Feb. 9/12
DNA Replication (III)
DNA proofreading
1) 3’-5’ proofreading exonuclease
2) Strand-directed mismatch repair
a. MutS and MutL proteins
b. Base excision repair
c. Nucleotide excision repair
Proofreading exonuclease
Improper nucleotide pairing prevents further elongation
Cleaves improperly matched nucleotide
Occurs at end of synthesized strand
Found on the “e” (exonucleolytic) site of DNA polymerase
Polymerase: Polymerizing “P” site and “e” site occur almost simultaneously
Reason of 5’-3’ synthesis
Allows for efficient error correction/chain growth
DNA polymerase requires energy to catalyze synthesis
Only 5’-3’ can provide consistent energy through dNTP pyrophosphate cleavage
Only 5’-3’ has correct orientation to cleave pyrophosphate off dNTP
3’-5’ can only provide energy from the primer strand once at first dNTP addition
MutS and MutL proteins
Occurs after DNA polymerase, but before Ozaki fragment nicks have sealed
Scan for geometrical distortions caused by mismatched base pairs
Proteins stop scanning when they bump into a nick on the newly synthesized strand
Proteins remove synthesized strand section from its geometrical distortion to its nick
DNA polymerase re-fills the empty strand space
Can occur on both leading and lagging strands because of bi-directional growth
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