Damage, repair and recombination of DNA sequences Flashcards
What 3 things can damage DNA?
1) Oxidation (H removed or O added)
2) Hydrolysis (Water splitting bonds)
3) Uncontrolled random methylation to a base
How does oxidation damage DNA?
Opens up bonds (double bonds)
Affects the ability of the bases to bond with its pair
How does hydrolysis damage DNA?
Can release amino groups on the bases (DEAMINATION)
OR
Can separate a purine base from the deoxyribose sugar (DEPURINATION)
How does uncontrolled methylation damage DNA?
Methylation of the base at N atom - affects the ability of the bases to interact with their counterparts
What is oxidation?
H removed
OR
O added
What is the result of DEPURINATION?
Liberation of a purine (guanine or adenine) from its sugar attachment:
- Un-purinated sugar has no coding potential
- Gap in the DNA sequence (read by DNA polymerase as no DNA information
- Deletion of a bas pair in that position
What is the result of DEAMINATION?
Removal of a amino group from the base:
- Changes the identity of the base (cytosine to uracil)
- Causes wrong base pairing (adenine instead of guanine)
In deamination, what is the base change?
Cytosine to Uracil
What is BASE Excision Repair and what is the process of it?
Repair of point mutations (deamination and depurination):
1) Uracil DNA glycosylase recognises uracil in the DNA as foreign - cleaves uracil from the sugar
2) AP endonuclease cuts the inside of the DNA and removes the SUGAR
3) Followed by cleavage of the phosphate in the DNA backbone by phosphodiesterase (removes the PHOSPHATE)
4) Single stranded break and loss of nucleotide is formed - this is a signal for DNA polymerase (as this is a primer-template junction)
5) DNA polymerase repairs the gap
6) DNA ligase seals the nick
What causes the formation of pyrimidine dimers?
UV radiation
What are pyrimidine dimers?
What does this cause?
Formation of covalent bonds between the carbon atoms of two adjacent pyrimidines (cytosine or thymine)
- Distorts the structures of the pyrimidines - they can no longer pair properly with purine counterparts
- No longer can form correct H bonds
- Arrests DNA replication or cause mis-reading of the DNA sequence by DNA polymerase
What is the NUCLEOTIDE Excision Repair?
What is the process of repairing a pyrimidine dimer?
Repairs many types of DNA damage (including pyrimidine dimers)
1) Covalently linked dimers distort the helical structure of DNA - are readily detected
2) Recruits excision nuclease - Cleaves single-stranded DNA segment containing pyrimidine dimer defect
3) DNA helicase liberates the DNA fragment by disrupting the H bonding
4) Causes a gap in the DNA - primer template junction recognised by DNA polymerase
5) DNA ligase seals the nick
What disorder does DEFECTIVE Nucleotide Excision Repair machinery cause?
How?
Xeroderma pigmentosum (XP) - Extreme sensitivity to sunlight-induced skin cancer
Mutations in 7 different genes can cause this (including XPA, XPC, XPD, XPF, XPG)
These genes all encode enzymes which participate in NER pathway for the correction of UV-induces DNA lesion
SO, cause a defect in the repair of UV induced dimers
What is up-regulated in response to UV light?
What happens if exposed to TOO MUCH UV light?
Enzymes involved in the Nucleotide Excision Repair Pathway
(Genes: XPA, XPC, XPD, XPF, XPG)
If too much light - repair system becomes overloaded and can lead to mutations that cannot be repaired
What are the homologues of XP genes in E.coli?
Uvr protiens (UvrA, UvrB, UvrC, UvrD)