Question

once a mutation has occured in a particular gene, several types of events can occur to reverse the effects of the original mutation, give 3 of those types :

Answer 1

• Mutations lead to variations in the DNA sequence, which results in new allele and thus genetic variation.
• However, most genomic alterations do not change the protein sequences. Most of the mutations are detected and repaired before expression into proteins.

DNA repair mechanisms:

• 1. Direct repair-
     1. 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 Deinococcus radiodurans have 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.