DNA Mutation and Repair Flashcards
Oxidized Base
Reactice O2 species produced as a by-product of cellular respiration and inflammatory response. Base excision repair.
Missing Base
Removal of purines by acid and heat, removal or uracil by DNA glycolysis. Base excision repair.
Altered Base
Ionizing radiation, alkylating agents. Nucleotide excision repair.
Linked Pyrimidines
UV irradiation. Nucleotide excision repair. Translesion DNA synthesis.
Cross-linked Strands
Bifunctional alkylating agents. Nucleotide excision repair. Translesion DNA synthesis.
Single or Double Stranded Breaks
Ionizing Radiation. Chemotherapy. Homologous recombination. Nonhomologous end joining.
Mutation
A heritable change in the sequence of DNA at the cellular level, ie a change in the genotype. The damage itself is not the mutation. The damage ultimately results in a mutation.
Mismatch Repair
Corrects incorrect bases incorporated during DNA replication that are not removed by DNA polymerase. In bacteria, they can distinguish via methylation status. Mammalian cells are less clear. If detection happens quickly when only the parent strand is methylated, the misincorperated base will be selectively removed from the newly synthesized, nonmethylated daughter strand. MutS (scans), MutL (binds and links S and H), and MutH (Endonuclease activity) recognize the mismatch in bacteria. DNA polymerase fills the gap.
Direct-Acting Carcinogens
Covalently modify DNA bases.
Indirect-Acting Carcinogens
Water insoluble and inert. Cannot modify DNA directly. Converted by liver cytochrome P-450 into highly reactive electrophiles that can modify DNA.
Nucleotide Excision Repair
Repairs environmentally induced DNA damage. Includes chemical modifications and shortwave UV component of sunlight. Distortion in DNA. Inherited include Xeroderma pigmentosum and trichothiodystrophy. A dimer senses the distortion (XPC and hHR23B). The dimer attracts TFIIH (9 subunits) and it unwinds the DNA helix by virtue of the subunits XPB and XPD that are helicases. XPA and RPA stabilize the unwound helix and recruit repair factors. XPG makes an incision in the 3’ end to the damaged region. ERCC1 + XRF join the complex and make a second incision at 5’. Gap filling DNA sythesis and ligation.
Xeroderma Pigmentosum
UV irradiation and chemical mutagens. Skin and eye photosensitivity, keratosis susceptibility to skin carcinoma and melanoma. Involved XPA, XPB, XPC, XPD, XPE, XPF, XPG.
Trichothiodystrophy
More severe than xeroderma pigmentosa. Same pathway, but subset mutations of XPB and XPD impair the activity of proteins in both pathways. Brittle hair, mental retardation, neuroskeletal anomalies.
Homologous Recombination Repair
Ionizing radiation and chemotherapy cause double-stranded DNA breaks, as do endogenous cellular processes such as replication fork collapse. Most lethal for eukaryotic cells. Two competing processes– homologous recombination repair (error-free) and the inaccurate non-homologous end joining. Homologous recombination involves homologous DNA used as a template to repair a double stranded break. Intact sister chromatid required so it only acts during S and G2 phases of the cell cycle. Break is sensed by a protein complex (NBS1, RAD50, ATM, and MRE11). A 5’ to 3’ exonuclease exposes free 3’ overhangs in a process called resection. A free 3’ end coated with recombination proteins (RAD 51 and RAD 52) invades homologous duplex DNA from the sister chromatid. DNA polymerase extends free 3’ end. Helicases bind the crossover points (Holliday junctions) and propagate branch migration. Holliday junctions are resolved, allowing the repaired duplexes to separate.
Nonhomologous End Joining (NHEJ)
Repair system for double stranded breaks. Error-prone. Ligating free double-stranded ends together, with no specificity. Translocations and inversions. Leukemia and Lymphoma. Each of the double stranded breaks binds a complex and forms a synapse. DNA is unwound and processed. Short single stranded in one end base-pairs with a homologous region in the other end. The unpaired 5’ regions are removed and free ends ligated by a DNA ligase complex.
