Question

Explain the following repair mechanisms:

-removal of thymine dimers

-base excision repair

-nucleotide excision repair.

Answer 1

Removal of thymine dimer:

Pyrimidine dimers formed by UV radiation are repaired by a light-dependent direct system called photoreactivation.In E. coli, the process requires the enzyme, photolyase. When stimulated by a light with a wavelength between 300-500nm, the enzyme binds to pyrimidine dimmers and converts them back to monomeric nucleotides.

Base excision repair:

This process is used to repair minor damages like alkylation, deaminationresulting from exposure to mutagenic agents. Enzyme DNA glycosylase initiates the repair process. DNA glycosylase cleaves the N-glycosidic bonds, liberating the altered base and generating an apurinic or an apyrimidinic site, called AP sites. The resulting AP site is then repaired by an AP endonuclease repair pathway.

Nucleotide excision repair:

It is similar to base excision repair pathway but is not preceded by the removal of a damaged base and act on more substantially damaged areas of DNA. This process involves the breaking of a phosphodiester bond on either side of the lesion, on the same strand, resulting in the excision of an oligonucleotide. This excision leaves a gap that is filled by repair synthesis, and a ligase seals the breaks. Uvr system removes a short oligonucleotide of 12 nucleotides length. The key enzyme is made up of three subunits: UvrA, UvrB, and UvrC, and is called ABC exinuclease. ABC exinuclease binds to DNA at the site of lesion. First, UvrA and UvrB attach to the DNA at the damaged site. UvrA recognizes the damage. Departure of UvrA allows UvrC to bind , forming UvrBC dimer which cleaves the DNA strand at the 8^th phosphodiester bond on the 5’ side of the lesion and at the 4^th or 5^th phosphodiester bond on the 3’ side. UvrD is a helicase (also called helicase II) that helps to unwind the DNA to allow the release of single strand between the cuts. The resulting gap is filled by DNA polymerase and sealed by ligase.