Chromosomal Crossover
From EvoWiki
| See Chromosomal Crossover in Wikipedia, the free encyclopedia. |
Chromosomal crossover (or simply "Crossing over") is the breaking of one homologous maternal and paternal chromosome, the exchange of corresponding sections of DNA, and the rejoining of the chromosomes. This process can result in an exchange of alleles between chromosomes. Crossing over is a form of recombination which occurs only in sexually reproducing eukaryotes. Some prokaryotes also undergo recombination, though in a different process.
Contents |
Consequences for evolution
Crossing over occurs in Prophase I of meiosis, during the formation of gametes (sex cells). In this way the shuffling of genes, or "independent assortment", by crossing over is heritable and increases the variation which feeds evolution. Crossing over does not lead to entirely independent assortment of genes, however, as chromosomes typically only cross-over about four times in meiosis. Two genes on the same chromosome that are close together are said to be linked and less likely to recombine.
Molecular process
The molecular processes of recombination were first discovered in the 1960s by Robin Holliday. Crossing over is controlled by recombinase enzymes. Recombinase enzymes line up the homologous chromosomes and split the double helices of the two DNA molecules and the two chromosomes are joined by sticking the homologous DNA strans together, creating a holliday junction, holliday intermediate or cross strand exchange. This tetrahedral intermediate is "pulled" by other enzymes to create a short "sticky" length on each chromosome. Thus the product of recombination is half of the maternal chromosome and half of the paternal chromosome joined by a short hybrid of sticky ends.
The maternal and paternal chromosomes are not identical however—otherwise recombination would make no difference anyway—so sticky ends can not be formed just anywhere on the chromosome. Sticky ends must form somewhere where there are identical sequences in all members of a population, such as Alu repeats, a form of noncoding DNA, often referred to as junk DNA.
See also
References
- Lodish, H., D. Baltimore, A. Berk, S.L. Zipursky, P. Matsudaira, J. Darnell, 1995. Molecular Cell Biology 3rd ed. New York: Scientific American Books, ISBN 0716723808
- Russel, P.J., 2003. iGenetics. San Francisco: Benjamin Cummins, ISBN 080534697X
- Wikipedia: Genetic recombination
