1. MECHANISMS OF DNA REPAIR

2. Mechanisms for the repair of damaged DNA are probably universal
Mechanisms for the repair of damaged DNA are probably universal. Escherichia coli possess 3 different mechanisms for the repair of DNA.

3. Postreplication recombination
DNA REPAIR MECHANISMS
Postreplication recombination
Photoreactivation
Excision repair

4. PHOTOREACTIVATION
Enzyme splits thymine dimers directly without removal of any nucleotide.
It binds to thymine dimers in DNA in dark.
It can’t catalyse cleavage of bonds joining thymine molecules without energy derived from visible light.
It is also active on cytosine dimers and cytosine-thymine dimers.
When UVR are mutagens, treatment id carried out in dark.

6. EXCISION REPAIR
Involves a sequence of enzyme catalysed steps in which the thymine dimers are removed from DNA molecule and new segment of DNA is synthesised.
Occurs efficiently in dark as well as in blue light.
Endonuclease recognises thymine dimers or distortion in double helix caused by them and cleaves the phosphodiester backbone of DNA strand containing at a site near the damage.

7. 5’ 3’ exonuclease activity of DNA polymerase I removes segment of the strand adjacent to endonuclease cut, including the dimer.
DNA polymerase I fills in the gap using the complementary strand as template.
DNA ligase catalyses covalent closure between adjacent nucleotides.
DNA polymerase I appears to be involved because mutants lacking this enzyme are defective in excision repair.
uvrA, uvrB and uvrC are required.

9. POSTREPLICATION RECOMBINATION
When DNA molecules containing thymine dimers are replicated, gaps are formed in the nascent complementary strands opposite the dimers because DNA polymerase can not use the distorted strands as template.
After a lag due to thymine dimer blockage, replication is reinitiated at secondary initiation sites beyond the dimer.
This results in progeny double helices with thymine dimers in one strand and gaps in complimentary strand.

10. If these 2 sister chromosomes recombine such that end up in one chromosome and the intact undamaged segments end up in the other chromosome, the latter will be functional and produce viable cells.
It includes both replication and recombination and takes place in dark.

12. URACIL DNA GLYCOSYLASE METHOD
Uracil DNA glycosylase removes uracil from DNA.
Called glycosylase rather than nuclease because it removes uracil glycosidic bond.
Produces apyrimidinic sites in DNA.
These sites undergo excision repair mechanism.