Question

What is DNA Repair? What are the types of DNA repair?  Explain briefly.

Answer 1

DNA repair:

• The requirement that the DNA sequence of a gene is preserved despite the presence of numerous agents that can mutate DNA has caused organisms to develop methods which prevent mutation by repairing damage to DNA.
• The repair mechanisms are complex but essentially 3main types occur.
• They are Excision , Direct and Mismatch repair.

Excision repair:

• This is a complex system which is probably the most common form of DNA repair.
• Many different types of damage are repaired including pyrimidine dimers. Initially, one of a number of enzymes recognizes nucleotides that are damaged and marks them for repair.
• The mark can take the form of a nick in one of the strands of the double helix adjacent to the area of damage or a damaged base may be removed leaving a gap.
• In the next stage, a nuclease removes the marked nucleotide as well as a number of its neighbors. A DNA polymerase (DNA polymerase I in E. coli) then synthesizes new DNA to replace the missing bases and DNA ligase joins the new DNA to the existing molecule restoring the DNA to its original structure.

Direct repair:

• This is a much less common form of DNA repair which involves reversal of structural alterations that occur in nucleotides.
• An important example of direct repair is photoreactivation which repairs pyrimidine dimers produced by UV radiation.
• Enzymes called DNA photolyases are induced by visible light and repair pyrimidine dimers by breaking the links that form on dimerization.
• DNA photolyases occur in bacteria, microbial eukaryotes and plants.

Mismatch repair:

• This system corrects errors introduced during DNA replication by identifying mismatched nucleotides.
• A number of enzymes mark the mismatch or can repair it directly.
• It is important that the system can determine which of the mis matched bases is correct and it does this by distinguishing between the parental DNA strand which has the correct sequence and the daughter strand containing the mutated sequence.
• In E. coli the parental strand is easily recognized because it is tagged with methyl groups attached to adenine bases.

DNA Repair:

DNA repair is the collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome.