DNA repair

DNA Repair:-
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome
Agents that Damage DNA
• Certain wavelengths of radiation
o ionizing radiation such as gamma rays and X-rays
o ultraviolet rays, especially the UV-C rays (~260 nm) that are absorbed strongly by DNA but also the longer-wavelength UV-B that penetrates the ozone shield.
• Highly-reactive oxygen radicals produced during normal cellular respiration as well as by other biochemical pathways.
• Chemicals in the environment
o many hydrocarbons, including some found in cigarette smoke
o some plant and microbial products, e.g. the aflatoxins produced in moldy peanuts
• Chemicals used in chemotherapy, especially chemotherapy of cancers
Types of DNA Damage:-
1. All four of the bases in DNA (A, T, C, G) can be covalently modified at various positions.
o One of the most frequent is the loss of an amino group ("deamination") — resulting, for example, in a C being converted to a U.
2. Mismatches of the normal bases because of a failure of proofreading during DNA replication.
o Common example: incorporation of the pyrimidine U (normally found only in RNA) instead of T.
3. Breaks in the backbone.
o Can be limited to one of the two strands (a single-stranded break, SSB) or
o on both strands (a double-stranded break (DSB).
o Ionizing radiation is a frequent cause, but some chemicals produce breaks as well.