DNA Repair

There are a variety of repair mechanisms for DNA that help to reduce replication error. Such repair mechanisms include DNA mismatch repair, double-stranded DNA break repair, and nucleotide excision repair. There are genes that control these processes.

These repair mechanisms are not just important in the germ line. Somatic cells with genetic instability can give rise to neoplasms.

One example of an abnormality in DNA repair is the disease known as xeroderma pigmentosa (XP). Persons with this condition are highly susceptible to the effects of exposure to ultraviolet light. when UV light strikes the skin, the absorbed UV energy causes formation of pyrimidine dimers in the cellular DNA. Ordinarily, there are are a succession of enzymes (helicase to unwind the DNA, endonuclease to cut the DNA at the dimer, exonuclease to remove the dimer, polymerase to fill in new bases, and ligase to connect the corrected DNA strand). In XP, the inherited defect is in nucleotide excision repair (helicase or endonuclease genes). The propensity for skin cells to undergo mutation leads to a high risk for skin cancer.