March%2017Posting - Error repair DNA polymerase...

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Unformatted text preview: Error repair DNA polymerase proof-reading function ( misses only 1 error in 1000 potential errors! 99.9% accurate ) Other repair systems Mutations: originate as errors in DNA replication DNA replication happens at a high rate and errors do occur. Overall, the error rate is about one mistake every 10 9 bp that are replicated. Rate is about the same in all organisms. This is a very low rate. Since there are about 1000 bp of coding region per gene, the rate of mutation per gene is 1000 times greater (1 error per replication of 10 6 genes) and Any one of the nucleotides in the gene could be mutated. E. coli DNA Polymerase III Holoenzyme has many subunits Function Structural Gene Polymerase polC (dnaE) 3'-exonuclease dnaQ Assembly on DNA dnaX, holE Sliding clamp, processivity dnaN The complex dnaX(Z), holA, holB, holC, holD Processivity = tendency to continue the process once started DNA polymerase III has a proofreading function The rate of error in selecting dNTPs by PolC is ~1/10,000. The overall error rate is low (1/10 9 ) because of the 3exonuclease function of DNAQ. Proof reading by 3'-exonuclease, i.e. the dnaQ gene product New DNA Template 5 A G G C 3 3 T C C G T C A G T 5 Base pairing error Mismatch Template New DNA DNA polymerase reverses direction, removes incorrect nucleotide and adds correct one. New DNA Template T C 5 A G G T 3 3 T C C G T C A G T 5 5 A G G T 3 3 T C C G T C A G T 5 Timing? Cells also have enzymes to specifically search and repair mismatches: Nucleotide Excision Repair Cells contain enzymes (around 10-20 different types) that scan DNA for mismatched bases between replication rounds. These are used for all types of mismatches. One set of these enzymes can cleave the DNA backbone to remove mismatches. The gaps are filled in by DNA polymerase I and ligase (just like removing primers during lagging strand synthesis). C G T T T T T G G A T C G C A A A A G C C T A G 5 3 5 3 5 3 5 3 5 3 5 3 C G T T T T T G G A T C G C A A A A A C C T A G Excision of nucleotides near mismatch by repair enzymes DNA pol I fills the gap DNA ligase seals up Nucleotide excision repair mechanism C G T T T T T G G A T C G C A A G 5 3 5 3 C G T T T T T G G A T C G C A A G Cells have a photoreactivating repair system for pyrimidine dimers. The enzyme DNA Photolyase specifically binds pyrimidine dimers. It absorbs light and uses the absorbed energy to break the bonds between the adjacent pyrimidines. Thymine dimers can be Photoreactivated Another repair mechanism for TT dimers: the sloppier copier DNA polymerase III and its associated proteins cannot replicate past a thymine dimer (or several other types of damage)....
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at The University of British Columbia.

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March%2017Posting - Error repair DNA polymerase...

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