Genes can be damaged by a variety of factors, such as ultraviolet light, cigarette smoke, or certain types of viruses. Such damage, if left unrepaired, can cause mutations, which can lead to disease. The "first resort" for genetic repair is most often a mechanism that works on an "all or nothing" basis: when unable to precisely correct the damage, it stops in its tracks, leading to what can be an even more harmful effect the death of the cell.
Fortunately, nature has provided cells with two alternative, last-resort repair systems that can take command when the first rescue mechanism fails. One system is inaccurate it repairs genes while permitting the formation of a relatively small number of mutations. Though this poses a certain risk, it ensures the cell's continued existence. Equally important, it increases genetic diversity, allowing natural selection, the driving force of evolution, to come into play.
The other last-resort repair system was hypothesized by scientists in the 1960's, yet was never proved until the current study. This system, which relies on the help of "sister chromosomes," enables the cell to repair genetic damage without the risk of creating mutations. (During the process of cell division, each chromosome -- the structure in the nucleus that contains DNA -- gives rise to two identical "sister" chromosomes. These move on to the two separate cells created from the dividing cell.)
According to this theory, if one of the sister chromosomes
Contact: Jeffrey Sussman
American Committee for the Weizmann Institute of Science