Question

Describe the mechanism in bacteria for repair of DNA containing mismatches, chemically modified bases, and double strand breaks.

Answer 1

DNA repair mechanisms:

1. Direct repair-

A. Photo reactivation:

• Ultra violet (UV) radiations (at about 254 nm) may cause DNA damage, resulting in formation of pyrimidine dimers (like T-T, cytosine dimers may also be formed).
• Enzyme called photolyase, can repair such damages by reversing the dimers to normal bases.
• Photolyase can utilize energy from near UV-spectrum or blue region (about 300 - 450 nm).
• This is utilized to break the covalent bonds between the dimers and thus repairing of DNA. The process of repair is called photoreactivation.
• Bacteria Deinococcusradioduranshave mechanism of protection from UV damage and is highly resistant to exposure to UV light.

B. Repair by alkyltransferase:

• Repair by alkyltransferase enzyme include reversal mechanism caused by mutation by alkylating agents.

2. Mismatch repair-

• The enzyme DNA polymerase cannot initiate polymerization reaction.
• It requires existing nucleotide sequence to add nucleotide at the 3’-OH.
• Primase synthesize RNA primer, on which DNA polymerase can add the nucleotides.
• DNA polymerase have proof reading activity.
• After replication DNA is compared with daughter DNA.
• If errors are missed during proof reading.
• The errors from parental to daughter strands are marked by hemi-methylation.
• The error is then removed. The 3' to 5' exonuclease activity of polymerase help in removal of mispaired base.
• This is called mismatch repair.

3. Excision repair-

• The errors are identified and nick is generated on few bases away from the error.
• Nick refers to a break created by nuclease activity that cleaves the phosphodiester linkages between nucleotides,.
• Nick translation refers to inducing a nick in
• There remains a gap of some bases resulting in overhangs.
• DNA repair proteins remove the two overhanging nucleotides (DNA glycosylases)
• DNA polymerases then repairs the nick.