3 categories of mutations
point mutations: single point! (eg SNPs)
substitution or INDELS
deletion or larg-scale rearrangements of DNA or insertion of mobile genetic elements
many point mutations occur spontaneously through…
tautomerisation
- affects base pairing by having a change in a base which makes it pair with a base it normally wouldnt be able to
eg AC—> AT
chemical changes to bases
- deamination (converts cytosine to uracil)
- makes CG –> TA
2 ways to the cell reduces rate of spontaneous mutation
DNA proofreading
Mismatch repair system (MMR)
Proof reading
DNA pol I and pol II - excising incorrect bases and pol has another chance to put correct base
mismatch repair system
detect wrong base via MutS (directed by adenine methylation at GATC sequences) - this causes distortion on dna double helix
- this recruits MutL and MutH
Adenine methylation occurs after DNA synthesis therefore MutH endonucleus cuts newly synthesised (temporarily unmethylated) strand containing incorrect base
MutL-MutH activates excision with DNA helicase UvrD + exonucleases
DNA pol III repairs the single stranded gap, DNA ligases generates covalent bonds
