AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

Excision repair

EXCISION REPAIR ( DARK REPAIR )

• Involves sequence of enzyme catalysed steps in which the thymine dimers are removed from the DNA molecule and a new segment of DNA is synthesized.
• Excision repair occurs both in darkness and in the presence of blue light.
• First, an endonuclease recognizes the thymine dimer and cleaves the phosphodiester bond near the site of damage.
• Then an exonuclease, probably the 5’ 3 exonuclease activity of DNA polymerase-I removes a segment of the strand adjacent to the endonuclease cut, including the dimer.
• DNA polymerase-I then fills the gap using the complementary strand as the template.
• DNA ligase then catalyses the formation of phosphodiester linkage between adjacent nucleotides.

Repair by Uracil DNA glycosylase

• Uracil is produced in the DNA by deamination of cytosine (by nitrous acid).
• The enzyme uracil-DNA glycosylase removes uracil from DNA creating an AP site (apyrimidinic / apurinic site).
• Such AP sites undergo excision repair mechanism described earlier.