Lecture 7 - Checkpoints and cancer Flashcards
Describe the function of mismatch repair proofreading proteins
Detect structural distortions that occur when the wrong base is inserted during DNA synthesis. In mammals the detection is responsibility of 2 protein gene products - MSH2 and MLH1 - detect this and correct it.
Coupled to passing replication fork. Enzymes are able to recognise the breaks that form the boundaries of Okazaki fragments in the lagging strand and use it to recognise the newly synthesised daughter molecule as opposed to the template molecule. Consequence of this is that the intervening base pairs are removed and then resynthesised to fill the gap. For the leading strand in bacteria the methylation status of the newly synthesised DNA compared to the template strand is the differential point. These genes are often mutated in cancer cells.
What are the effects of mismatch repair defects on short sequence repeats?
Polymerases “stutter” when they encounter repeats - add or miss out a base. These regions especially reliant on functional mismatch repair problems.
TGF- β is a gene product known to contain a SSR and therefore prone to mismatches and sensitive to loss of mismatch repair. Responds to external signals on surface of the cell. Functions in G1 phase to respond to anti-growth signals - has to be activated for cell to receive restraining signals. If receptor is not functioning, cell cannot respond to anti-growth signals and is more likely to proliferate in an uncontrolled way.
What are the triggers of double strand breaks?
Reactive oxygen species/Ionizing radiation
DNA topisomerase inhibitors (chemotherapies)
‘Replication stress’
What is homology-based repair?
homology-based repair (HR) - uses the other chromatid as a template
melting of the helix on the normal chromatid in the appropriate region
invasion by the free end of the severed DNA strand
extension of the severed strand by a DNA polymerase
re-joining of the severed strand with DNA from the other side of the break
Functions primarily to repair lesions at the replication fork and in G2 and is error-free repair
What is non-homologous end-joining?
Enzymes detect and bind to the free end of the DNA and catalyze their ligation
No need for extensive sequence homology
Involves loss of sequence
Repairs DSBs at all stages of the cell cycle and is error-prone
