Biochem 501: Part IV.4: Mutagenesis and Repair
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Causes of DNA mutation (repair mechanism)
- -Incorrect base incorporation (DNA Polymerase proofreading/mismatch repair)
- -Chemical changes (i.e.)
- -Base alkylation (O6-methylguanine instead of adenosine)
- -Deamination (loss of exocyclic amino group, cytosine-->uracil)
- -Depurination (N-glycosidic bond between base/pentose cleaved)
- -Pyrimidine dimer (bond forms between adjacent T or C nucleotides)
Mismatch repair relies on what?
- Distinguishing template from new strand.
- Repairs according to template
- Relies on which is methylated, new strand won't be methylated for a few minutes
Difference between DNA methylation and alklylation
- Methylation: Added to distinguish template strands, not harmful, added to area that IS NOT the nucleotide base pairing
- Alkylation: Can be addition of ethyl/methyl. Added to place that effects base pairing. HARMFUL
Direct repair of alkylation:
Demethylation by oxidative demethylation/methyltransferase
Deamnation can turn what into what
Base deamination allows:
- Only effect purines (A, G)
- Cuts base off of pentose
- Can cause mutations during replication
- -One side lacks a base, gets crimped and a missing base pair, phase shift
Fixing Deamination or Depurination
- Base excision repair:
- Cuts out bad part, DNA polymerase goes and trys again.
- Only effects pyrimidines (CT)
- Neighboring bases can bind each other
- Repair mechanism: Nucleotide excision
Repairing double strand breaks/SS breaks
- Homologous recombinations/non-homologous end-joining
Chromosomes are composed of
Sister chromosomes have
- -DS Break converted to DS gap with exonucleases
- -Strands with 3' ends are degraded less
- -Exposed 3' end pairs with intact homologue in sister chromatid
- -3' end extends along the template of intact homologue until both ends are complete and a double crossover holliday intermediate is created.
- -DNA is recreated on opposite side
- Each newly finsihed side is snipped off and each sister is left with a section of the other
A single holliday junction has how many potential cleavage planes?
2, leading to two possible resolved forms.
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