Friday, March 13, 2009
It is widely believed that recombination, or crossing over, only occurs at meiosis in diploid eukaryotes. Most textbooks reinforce this belief by associating crossovers with chiasmata, which are only seen at meiosis.
In spite of the textbook claims, most people are well aware of the fact that recombination takes place in somatic cells. After all, it's the basis of most recombinant DNA technology and underlies many of the mechanisms that cause cancer. Furthermore, some developmental processes, such as immunoglobulin gene rearrangements require recombination in somatic cells.
Mitotic recombination has been known to occur since the 1930s when it was used for fate mapping in Drosophila so it's somewhat surprising that crossing over is so intimately connected with meiosis in the textbooks. The frequency of mitotic crossing over may be lower than that seen during meiosis, although the differences may not be great in most species.
In yeast, the frequency of recombination during meiosis can be 10,000 times greater than the frequency of crossing over in somatic cells but that's partly because meiotic recombination is very high in yeast cells. Perhaps they have been selected in vitro for high rates of recombination.
Why does recombination occur in mitotic cells? Probably for the same reason it occurs during meiosis—it's a form of DNA repair.
There's a short review of mitotic recombination in the lastest issue of PLoS Genetics [Mitotic Recombination: Why? When? How? Where?]. Let's try and put an end to the false idea that recombination and crossing over only takes place during meiosis.