Question

Name and detail the mechanism(s) through which this DNA damage can occur. What mechanism(s) have cells developed to deal with this damage when it is encountered? Detail how this led to the elucidation of DNA repair enzymes in your answer.

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.

· Compared to D. radiodurans, E. coli is very less resistance or exhibit sensitivity to UV radiation when exposed to UV radiation for certain time.

· Thus, when time of exposure of UV radiation was increased for E. coli, after 40 s growth decreased and almost no growth occurred after 80s.

· While for D. radiodurans no difference in growth resulted in such short period of UV exposure.

· Bacteria that can form spores are less susceptible to damage by UV radiation than non-spore forming bacteria.

· S. marcescens (Serratia marcescens) does not form spores when subjected to UV radiation, thus, is more susceptible.

· B. subtilis (Bacillus subtilis) can form spore and can resist the effect of UV radiation.

                                    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.

· 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.