Lecture 6 - DNA repair Flashcards
How can DNA damage be prevented?
Proofreading by DNA polymerase
Hydrolysis of damaged nucleotides
How can DNA damage be reversed?
Alkyltransferases remove methyl groups via enzymatic hydrolysis. These are an important way to reduce mutations by reverting the DNA damage.
Also process called photoreactivation – reversal of pyrimidine dimers
How are photodimers reversed?
Photolyases are activated by low energy light. These photolyases remove crosslinked bonds leading to normal base after the dimer reaction. This is called Photoreactivation and reduces dramatically the presence of these photodimers after uv damage.
What is base excision repair?
Base is taken out of DNA, but backbone remains intact. DNA glycosylase is used to remove base. Different glycosylases specialise for different bases
AP endonuclease used to cut AP site
AP lyase removes deoxyribose phosphate
DNA polymerase fills gap with correct nucleotide
DNA ligase joins the ends
What is nucleotide excision repair?
similar but BER but on a bigger scale. Has a very different mechanisms, a big DNA repair complex (including things like helicases. An oligonucleotide is removed, hence Nucleotide vs bases.
Recognition of base damage by repair complex Incision of DNA strand on both sides Removal of oligonucleotide Filling of gap by DNA polymerase Joining of ends by DNA ligase
What is mismatch repair?
Bacteria use methylation to discriminate between old and new strands. Old DNA has me on both strands. After replication, a single strand is methylated so bacteria knows which strand is the old strand.
Newly replicated DNA is hemimethylated.
Recognition of mismatch by MutS.
MutL and MutH bind, pulling DNA through the protein complex.
MutH cuts DNA before GATC.
What is non-homologous end-joining?
mends ds breaks Binding of ends by protein complex Trimming of ends Joining of ends Results in the loss of some bases!
What is homologous recombination?
Broken DNA is resynthesized using intact DNA strand as template
This works after DNA replication has happened (not in G1 phase)
