Fidelity of DNA Replication
-DNA must be replicated fully with minimal errors
-failure to maintain high fidelity of DNA replication causes defective genomes, possibly resulting in disease (cancer) and/or death
DNA Repair Mechanisms
-mediated by enzyme complexes, ensuring that the replication error rate is 1: 1,000,000,000 nucleotides
Proofreading of DNA Polymerase
-optimum conformation of the DNA Polymerase III active site and the incoming dNTP allows the addition of correct base pairs, but mistakes are still made (1: 10, 000)
-DNA Polymerase III can detect a mistake and uses 3’ to 5’ exonuclease activity to remove the most recently added nucleotides and resumes synthesis of the new strand (5’ to 3’)
-reduces mismatches to 1: 10, 000, 000
DNA Mismatch Repair (MMR)
-MMR covers for replication errors not corrected by proofreading
-Mut S and Mut L recognize and bind the DNA at the site of the mismatch
-Mut H endonuclease nicks the daughter strand several nucleotides away from the mismatch
-Exo 1 5’ to 3’ exonuclease removes nucleotides on the daughter strand around the mismatch
-DNA Polymerase III fills the gap and repairs the mismatch
-DNA Ligase seals the nick to complete the repair
DNA Damage
-mutagens are chemical agents that alter DNA bases
-DNA bases can be altered through:
-oxidation
-deamination
-alkylation
-UV radiation
-x-ray exposure
Chemical Modification of Bases
-aflatoxin B1 chemically modifies N-7 of guanine and prevents replication
-guanine is not recognized by DNA Polymerases
Base Excision Repair
-a mechanism to correct damage made to nitrogenous bases that are damaged by chemical modifications such as oxidation, alkylation, or deamination
Enzymes Involved in Base Excision Repair
-DNA glycosylases remove the modified base forming an AP site (a spot on the pentose-phosphate backbone without a base)
-AP Endonuclease removes the AP site
-DNA Polymerase I adds the correct nucleotide
-DNA Ligase seals the nick
Thymine Dimer
-UV radiation covalently links adjacent pyrimidines along the DNA strand
-this results in a bulge in the DNA double helix
Nucleotide Excision Repair
-a mechanism to correct damage made from exposure to UV radiation or chemical mutagens
-repair enzymes recognize distortions in the double helix, excise the distorted segment, and replace it
Enzymes Involved in Nucleotide Excision Repair
-UVr ABC exinuclease cuts the damaged strand at two sites
-DNA Polymerase I adds the correct nucleotides
-DNA Ligase seals the nick
