L10: Maintenance Of Genomic Integrity Flashcards
How does DNA damage occur due to
Error in DNA replication
Spontaneous depurination
Exposure to agents e.g uv light, tobacco, ionising radiation
If we do not repair the damaged dna what can occur
Mutation that leads to cancer development
What are the 5 major dna repair mechanism
1) direct reversal damage
2) base excision by ROS deamination
3) nucleotide excision repair
4) homologous recombination repair and non homologous end joining
5) dna mismatch repair
What are the 3 different types of dna damage that can occur
7-methyl guanine
O-6alkyl guanine
Thymine dimers
How does 7-methyl guanine occur
A methyl group adds onto the guanine
What does 7-methyl guanine cause a problem with
DNA replication
How does a 0-6 alkyl guanine occur
A drug called ethyl methane sulphonate causes guanine to mutate to 0-6 alkyl guanine
What is the normal nucleotide paring
A-t
C-g
When we get a 0-6 alkyl guanine what nucleotide instead of cytosine (C) does it pair with
Thymine
What happen in the dna with 0-6 alkyl guanine during dna replication
Formation of AT pair rather than CG
Why does thymine diners occur
Due to uv light exposure
What are thymine diners
Formation of covalent bond between adjacent thymine in the dna
What does thymine diners cause a problem with
DNA replication
Broadly speaking what does dna repair involve
- enzymatic reversal of the damage
Or - Removal and replacement of the damage
With enzymatic reversal how are uv induced thymine reversed
Via monomeric action by action of visible light and photolyase (enzyme)
In enzymatic reversal how are 0-6 alkyl guanine reversed
Alkyl transferase remove the alkyl group on the guanine
In enzyme reversal how are strands breaks in sugar phosphate backbone reversed
By ligation
What are the substrates of base excision repair
1) spontaneous hydrolyic depurination of dna
2) deamination of cytosine
3) formation of dna adducts
Describe what happens in the base excision repair
1) altered dna base is excised from the dna by glycoslyase
2) this leaves an abasic site
3) apurinic site (sugar and phosphate) is removed
4) a new nucleotide is added and ligated via DNA ligase.
What are the type of dna that nucleotide excision repair works on
Double stranded DNA (not single stranded dna)
What does nucleotide excision repair notice
DNA distortions
What can nucleotide excision repair remove
Thymine dimers
Describe how nucleotide excision repair occurs when there are thymine dimers
1) thymine dimers are recognised by XPC and XPE
2) we then have the recruitment of TF2H complex that contains XPB and XPD proteins
3) XPA also binds to TF2H complex which verifies the damage
4) XPB and XPD are helicases which cause the unwinding of the dsDNA
5) XPF and XPG are then recruited which are nucleases that cut the sides of the DNA that contain the thymine dimers
6) polymerase then detects the gap and replaces the bases that were removed
What is a daughter strand gap repair
When dna that has thymine dimers undergoes dna replication, gaps are left opposite the dimers as the dimers are not recognised.
DNA will gradually repair itself
After the repair of the gaps does thymine dimers remain
Yes
When are thymine dimers removed
Later by nucleotide excision repair
What is xeroderma pigmentosum (XP)
An autosomal receives disorders where patients show excessive sun sensitivity
What type of tumours does patients with XP show
Skin tumours
Where does the skin tumours appear only at
Areas where skin is exposed to light
What defect does XP patients have
Defect in DNA nucleotide exision repair
Why do patient with XP have a defect in the dna exision repair
Defeats in the XP genes that are required for NER
Name examples of XP genes that XP patients can show
XPA
XPC
What defect does the XP variant of xerderma pigmentosum show a defect in
Replication of dna following exposure to uv light i.e daughter strand gap repair
What happens in XP when the thymine dimers remix
Further mutation
What gene can be further mutated due to unexcised thymine dimers
PTCH1 gene
What does PTCH1 mutated gene cause
Basal cell naveous syndrome
What are the 2 ways of repairing dna double strand breaks
Homologous recombination repair
Non homologous end joining
What molecules does the homologous recombination repair involve
BRCA1
BRCA2
When there is a double stranded dna break which module binds to the end of the breaks to protect it form damage
53BP1
What is the role of BRCA1
Removes 53BP1
Why does BRCA1 remove 53bp1 from the breaks
To promote resection and recombination to occur
What molecule allows the recombination to occur
RAD51
How is RAD51 brought to the right position
By BRCA2
What other role does BRCA1 have
Cell cycle checkpoints
What happens to the a cell that contains BRCA2 mutations
Cannot perform homologous recombination repair
How do we treat BRCA2 deficient tumours
Use PARP inhibitor
How does PARP inhibitors work
1) inhibit PARP which usually repair single stranded breaks
2) if you inhibit the single stranded break then it will form a double stranded break during DNA replication
3) with PARP inhibitors defect is made worse and leads to cell death
When does dna mismatch usually occur
During dna replicaton
What are the types of mismatchs that occur in dna
1) Base to base mismatch
2) insertion, deletion loops due to polymerase slippage during replication
What does insertion and deletion loops give rise to
Micosatellites
What does micro satellites lead to
Micro satellite instability
What is the difference between a base and nucleotide
Nucleotide: a single unit of a polynucleotide chain made of sugar, phosphate group and nitrogenous base
Base: the type of nitrogenous base which is either a pyrimidine or purine (A,T,C,G)
