direct reversal of DNA damage
Cells can eliminate three types of DNA damage by chemically reversing it. These mechanisms do not require a template, since the types of damage they counteract can only occur in one of the four bases. Such direct reversal mechanisms are specific to the type of damage incurred and do not involve breakage of the phosphodiester backbone.
1. Thymidine dimers
The formation of thymine dimers (a common type of cyclobutyl dimer) upon irradiation with UV light results in an abnormal covalent bond between adjacent thymidine bases.
The photoreactivation process directly reverses this damage by the action of the enzyme photolyase, whose activation is obligately dependent on energy absorbed from blue/UV light (300–500nm wavelength) to promote catalysis.
2. Methylation of guanine bases
Another type of damage, methylation of guanine bases, is directly reversed by the protein methyl guanine methyl transferase (MGMT), the bacterial equivalent of which is called as ogt.
This is an expensive process because each MGMT molecule can only be used once; that is, the reaction is stoichiometric rather than catalytic.
A generalized response to methylating agents in bacteria is known as the adaptive response and confers a level of resistance to alkylating agents upon sustained exposure by upregulation of alkylation repair enzymes.
3. Methylation of cytosine and adenine bases
The third type of DNA damage reversed by cells is certain methylation of the bases cytosine and adenine.
See also
DNA damage
DNA repair