The following points highlight the five important types of DNA repair system. The types are: 1. Direct Repair 2. Excision Repair 3. Mismatch Repair 4. Double-Strand Break Repair 5. SOS Repair.
Type # 1. Direct Repair:
It involves the reversal of DNA damage.
Pyrimidine dinners (induced by UV rays) can be monomerised again by DNA photolyases in presence of visible light. Cleavage of the cyclobutane ring of pyrimidine dimers by DNA photolyases restores the original DNA structure (Fig. 13.15A). Photolyases have chromophores which absorb blue light to provide energy for the reaction.
Photo-reactivation is specific for pyrimidine dimers and an example of direct reversal and is error-free.
Mutagenic effect of alkylating agents is protected through direct reversal afforded by alkyltransferase enzyme. This inducible protein specifically removes an alkyl group from the O6 position of guanine and transfers it to protein itself, causing inactivation of the protein (Fig. 13.15B).
Type # 2. Excision Repair:
In nucleotide excision repair, an endonuclease makes nicks on either side of the lesion, which is then removed to leave a gap. This gap is filled by a DNA polymerase, and DNA ligase makes the final phosphodiester bond (Fig. 13.16A). In base excision repair, the lesion is removed by a specific DNA glycosylase.
The resulting AP site is cleaved and expanded to a gap by an AP endonuclease plus exonuclease. Thereafter, the process is like nucleotide excision repair (Fig. 13.16B).
Type # 3. Mismatch Repair:
Replication errors which escape proofreading have a mismatch in the daughter strand. Hemi-methylation of the DNA after replication allows the daughter strand to be distinguished from the parental strand. The mismatched base is removed from the daughter strand by an excision repair mechanism (Fig. 13.17).
Type # 4. Double-Strand Break Repair:
Double- strand breaks are repaired simply by bringing the ends back together called non-homologous end joining. This is accomplished by DNA ligase under the direction of multi-component protein complex (Fig. 13.18). Alternative repair mechanism relies on nucleotide sequences of homologous piece of DNA, such as sister chromatid or homologous chromosome, called homology- directed recombination.
Type # 5. SOS Repair:
SOS response is initiated by interaction of Rec A protein with Lex A repressor. Damage activates Rec A protein which brings about proteolytic degradation of Lex A protein. Thus all operons, to which Lex A is bound, are induced (Fig. 13.19). This may include a number of genes with SOS box coding for repairing enzymes. This facilitates increased capacity to repair DNA damage.