Lecture 3 Flashcards
What affect will loss of DNA repair mechanisms have?
Accelerated mutation rate
Therefore seen in many cancers
What are DNA lesions?
DNA damage which can become mutation when the cell divides if they are not fixed by DNA repair mechanisms
Why do some DNA lesions become mutations?
- Repair is not 100% effective and becomes less precise with age
- Repair pathways can themselves be compromised by mutations to their genes
- Overwhelmed due to number of damaged sites (Massive exposure to carcinogen)
How does could chemotherapy cause recurrence of cancer?
Although chemo is used to kill cancer cells it is itself a carcinogen and thereofre can lead to further mutations which will give rise to new tumours later on
How will the cell normally react to mutations? Why might this not happen?
If severe will cause apoptosis and cell death. Therefore if programmed cell death also becomes defective then a tumour will be born
What are the three main sources of error in DNA?
- Incorporation of errors during DNA synthesis
- Nucleotide Decay
- DNA Damage
What is the chance of DNA polymerase incorpoating an incorrect base? How does the cell decrease this error rate?
1 in 105
Proofreading subunit of subunit will detect the misincorporated nucleotide and cause a reverse of the polymerase to fix the erro; this will decrease incorporation error to 1 in 107
Mismatch repair reduces the error further to 1 in109
How has the importance of the proofreading function been demonstrated?
Mice with knocked out proofreading subunits shown by kaplan-meier plot to have 50% reduction in survival rate to wild type
Oultine mismatch repair
Proteins hMSH2 and hMLH1 detects imperfections in base pairing. Detect which is the template strand and which is replicated strand, using nicks present during replication. (Nicks only on lagging strand it is unknown how direction is determined on leading strand but mismatch repair still successful). Nick tells proteins in which direction to fix to error and a loop is formed between nick and incorrect base, the sequence within the loop is removed and the gap is filled by polymerase.
Where in the genome is mismatch repair known to be most important?
Give an example gene
Regions of short sequence repeats, which are frequently prone to incorrect base incorporations, due to polymerase finding repeats difficult.
TGFß gene has short sequence repeats and commonly mistakes in replication cause premature STOP codon incorporation and a truncated protein product. This will make a cell immune to TGFß and hence cell cycle progression will be unregulated
How has loss of mismatch repair been shown as an early event in cancer development?
Staining of endometrical tissue
Loss of hMLH1 seen in tumour tissue and in surrounding ‘normal’ tissue. Presence in ‘normal tissue’ suggests loss of hMLH1 pre-dates other tumourigenic changes.
What are the three types of nucleotide decay?
Why are they unstoppable?
- Depurination
- Depyrimidination
- Spontaneous Deamination
Natural process that is spontaneous within the cell
Name three types of DNA damage
- Oxidative Damage
- Chemical Damage
- Radiation
What effect does UV light have on DNA?
Formation of pyrimidine dimers - affects ability of double helix to fold and wind appropriately
- Cyclobutane - two rings are connected by two bonds
- 6,4 pyrimidine dimer - rings connected by one bound
How does a cell deal with pyrimidine dimers?
Normally the dimer is repaired however this may not work.
Therefore specialised DNA polymerases, called error prone polymerases, exist that can replicate past the lesion but they can incorporate the wrong base, AKA point muatation. This ‘fixes’ the damage as a mutation in the next generation.
