DNA single-strand breaks
Tuesday 7 March 2006
When only one of the two strands of a chromosome has a defect, the other strand can be used as a template to guide the correction of the damaged strand.
In order to repair damage to one of the two helical domains of DNA, there are numerous mechanisms by which DNA repair can take place.
Direct reversal of damage by various mechanisms that specialize in reversing specific types of damage. Examples include methyl guanine methyl transferase (MGMT) which specifically removes methyl groups from guanine, and photolyase in bacteria, which breaks the chemical bond created by UV light between adjacent thymidine bases. No template strand is required for this form of repair.
Excision repair mechanisms that remove the damaged nucleotide replacing it with an undamaged nucleotide complementary to the nucleotide in the undamaged DNA strand. These include:
- Base excision repair (BER), which repairs damage due to a single nucleotide caused by oxidation, alkylation, hydrolysis, or deamination;
- Nucleotide excision repair (NER), which repairs damage affecting 2−30 nucleotide-length strands. These include bulky, helix distorting damage, such as thymine dimerization caused by UV light as well as single-strand breaks.
- A specialized form of NER known as Transcription-Coupled Repair (TCR) deploys high-priority NER repair enzymes to genes that are being actively transcribed;
- Mismatch repair (MMR), which corrects errors of DNA replication and recombination that result in mispaired nucleotides following DNA replication.
DNA single-strand damage repair