Damage Tolerance Function can allow DNA replication to continue despite presence of DNA damage (e.g. thymidine dimer) Process DNA polymerase stalls at dimer sliding clamp releases regular DNA polymerase and binds the one of two translesion polymerases error free recognizes that the dimer is normally a thymidine and the polymerase adds an adenosine opposite and continues replication error prone polymerase adds any base opposite the lesion and continues replication Mismatch Repair Process repairs G/T or A/C pairing sometimes misincorporated due to tautomerization of the nucleotide involves MutS, MutH, MutL enzymes strand specific recognizes which is the new strand because it is unmethylated and the old strand is methylated Deficiency hereditary nonpolyposis colorectal cancer aka Lynch syndrome cause hereditary absence of one copy of enzyme hMLH1 or hMSH2 second copy lost due to somatic mutation known as the two-hit model common to many DNA repair deficiencies presentation microsatellite instability di-, tri-, tetranucleotide repeats that can be amplified constant in number in normal cells diagnostic in Lynch syndrome ↑↑ risk of colorectal cancer NOT preceded by benign polyps Base Excision Repair Function specific endonucleases (glycosylases) remove bases that have been modified by several common mechanisms of damage e.g. deaminated cytosines (C → U) removed by uracil glycosylase can take place anytime during the cell cycle but occurs primarily in G1 Process glycosylase specific for the damaged nucleotide removed damaged base by breaking glycosidic bond damaged base removed sugar remains but base removed creates an apurinic/apyrimidinic (AP) site gap filled by DNA polymerase I this protein has 5' to 3' exonuclease activity ligation of strand nick by DNA ligase III Nucleotide Excision Repair Function removes thymidine dimers caused by UV-B light removes damaged bases caused by chemicals Process maintenance repair XPC recognises DNA lesion and recruits XPA XPB-G binds DNA and removes a chunk spanning the damaged segment DNA polymerase fills the gap DNA ligase seals the nick transcription-coupled repair RNA polymerase stalls at DNA lesion CSB and XPG recognize stalled RNA polymerase CSA joins complex and removes damaged site and allows transcription to continue Deficiency xeroderma pigmentosum (XP) cause lack any enzyme XPA - XPG presentation cannot repair UV damage sunlight sensitivity ↑↑↑ prevalence of skin cancer corneal ulcers diagnosis measurement of repair mechanisms in white blood cells treatment avoidance of sunlight Cockayne syndrome cause lack of CSA or CSB AR presentation growth failure photosensitivity nervous system abnormalities can affect any organ system Homologous Recombination Function repair double-strand breaks requires a sister chromatid to use as a template therefore must occur after S phase of cell cycle Process double-strand break recognized by MRN complex BRCA and BLM enzymes involved in end processing Holliday junctions are formed cross-shaped structures that mediate strand rejoining junctions are resolved may result in loss of heterozygosity due to the use of the opposite strand as a template Deficiency Bloom syndrome cause lack of BLM helicase enzyme presentation short stature rash from sun exposure café-au-lait spot leukemias, lymphomas, carcinomas BRCA-1 involved in breast, prostate, and ovarian cancer BRCA-2 involved in breast cancer Non-Homologous End Joining Function repair double-strand breaks these breaks may be caused by ionizing radiation or oxidative free radicals mechanism of cancer radiation therapy occurs when a sister chromatid is not available to use as a template (prior to S phase of cell cycle) Process break recognized by MRN complex additional enzymes (Artemis, XLF, Pol μ) cut ends so they can bind DNA ligase IV joins ends together Deficiency severe combined immunodeficiency disease (SCID) one of many causes