DNA Mutation 8.21 Flashcards
three factors that contribute to the low rate of actual DNA replication errors
low error rate of DNA polymerase to begin with
3’ exonuclease activity that is actively proofreading
repairs systems
Major sources of DNA mutations
replication error
chemical instability of nitrogenous bases
environmental mutagens
ionizing radiation
Give a scenario in which nucleotides might be mis-incorporated during replication
Adenine tautomer base-pairing with cytosine
Give examples of how nitrogenous bases can change due to their instability
Deamination of cytosine to form uracil
hydrolysis of N-glycosidic bond - depurination
Explain how environmental mutagens can affect DNA
chemicals can act as deaminating agents (cysteine to uracil) (guanine to xanthine) (adenine to hypoxanthine)
alkylating agents (usually adds methyl group) - can turn guanine into O6-methylguanine which can hydrogen bind thymine
intercalating agents - insert or delete one or more base pairs, causing a frameshift mutation
how can ionizing radiation damage DNA
creation of pyrimidine dimers
what are the five types of DNA repair
mismatch repair, base-excision repair, nucleotide-excision repair, direct repair, recombinational repair
how does the double stranded nature of DNA preserve fidelity
the parent strand will typically be a reliable template to base repairs off of
Why is methylation important in DNA repair? How does it occur?
identifies the parent strand - Adenine in 5’-GATC sequences on the parent strand are methylated prior to replication
after replication, there is a short period where the daughter strand is unmethylated - afterwards, it is methylated and becomes undistinguishable from the parent strand
explain the mechanism for mismatch repair
MutS binds the mismatched base pair and recruits MutL - this complex draw in the DNA from both directions until it encounters a MutH protein (MutH proteins bind to methylated adenines on the parent strand)
MutH then cleaves the daughter strand at the point of methylation
Helicase, exonuclease, and polymerase then unwind, strip, and relay the DNA on the daughter strand from the point of cleavage to the mismatched base
describe what base-excision repair is and how base-excision repair occurs
Removes and repairs abnormal bases
DNA glycosylase can recognize abnormal bases (such as uracil) - it then cleaves the base at its glycosidic linkage
AP endonuclease cleaves the phosphate backbone on the defective strand so that DNA polymerase can come in to insert the correct base
DNA ligase then seals the nick
describe what nucleotide excision repair is and how it occurs
repairs large structural damage (such as pyrimadine dimers
excinuclease cleaves the phosphate backbone of the damaged strand at both ends of the defect
DNA helicase removes the damages portion
DNA polymerase fills in the gap
DNA ligase seals the nicks
explain what direct repair is and how it occurs for methylated guanine
enzymatic reversal of specific types of DNA damage
guanine methylation is a mutation that results in O6-methylguanine
MGMT - O6-methylguanine methyltransferase removes the methyl group from guanine
how does MGMT interfere in cancer treatment? how is this countered
certain treatments target oncogenes by methylating guanine - MGMT can actively hinder these efforts and cancer cells can even upregulate MGMT expression
some treatments include MGMT inhibition
what is recombinational repair?
repair of double-stranded break, cross-links, and lesions that cannot be accomplished using an undamaged complementary strand - information comes from a separate homologous chromosome (similar but not identical) - damage comes about from ionizing radiation and oxidative reactions
two main types of repair - homologous recombination repair or non-homologous end joining
