Chemical and physical agents that can cause DNA damage include ionizing radiation, ultraviolet light, alkylating agents, hydrogen ions, and hydroxy radicals.
UV-induced DNA damage and repair
DNA damage
The purine and pyrimidine bases of nucleic acids absorb UV radiation strongly. UV light can be classified into three groups based on the wavelength: UV-A (315-400nm), UV-B (280-315nm) and UV-C (280 nm). UV light with longer wavelength (UV-A and UV-B) affects DNA significantly. Besides UV, other exogenous factors such ionizing radiation and endogenous factors such as free radicals are known to interfere with genome integrity. DNA damage leads to several events:
*Bases misincorporation
- Deamination, depurination, and depyrimidination of bases
- Oxidative damage
- Alkylation of bases
- Single or double-strand breaks
However, the major classes of mutagenic DNA lesions induced by UV radiation are cyclobutane–pyrimidine dimers (CPDs) and 6–4 photoproducts (6–4PPs). These dimers and photoproducts lead to replication arrests and double-strand breaks. Among several types of DNA damage, double-strand breaks are the most dangerous because they affect both DNA strands and can lead to the loss of genetic material.
DNA repair
Double-strand DNA breaks can be repaired by two main pathways: homologous recombination and non-homologous end-joining. In simple eukaryotes such as yeast, homologous recombination is the main pathway, whereas in higher eukaryotes such as mammals, non-homologous end-joining become the main pathway.
同源重组involves several protein such as Rad52 protein, MRE11-Rad50-NSB1 complex, and Rad51. Homologous recombination involves relationship between damaged DNA and undamaged DNA molecule with which it share sequence homologies. This undamaged DNA molecule is used as a template for DNA repair. Steps in homologous recombination are as follows:
核苷酸的限制the damage DNA by MRE11-Rad50-NSB1 complex.
Binding of 3'-single-stranded DNA by heptameric ring complex that is formed by Rad52 protein.
Interaction between Rad51 and Rad52 stimulating the DNA strand exchange activity of Rad51.
Rad51 protein catalyzes the strand exchange between damage DNA and template DNA, displacing one strand as D-loop.
Polymerase filling the gap of DNA double-strand breaks.
The resulting structure are resolved by Holiday junction.
Rad52
Rad52 protein inSacchromyces cerevisaeplays an important role in double-strand break repair by homologous recombination. Rad52 forms heptameric ring that catalyses DNA annealing and mediates Rad51-mediated strand invasion. Rad52 protein also protect 3' single-stranded DNA from exonucleolytic digestion in homologous recombination process.