Gene conversion

From Wikipedia, the free encyclopedia

Gene conversion is a phenomenon which occurs during meiotic division. It is a process by which DNA sequence information is transferred from one DNA helix (which remains unchanged) to another DNA helix, whose sequence is altered. It occurs occasionally during general recombination.

Meiosis is the cell division step that produces the gametes: briefly, the mother cell contains DNA from each of the organism's two parents; these two DNA strands duplicate into 4 strands (i.e. 2 copies of 2 copies of each gene), which then shuffle in a process called recombination. The strands then segregate, over two cell division steps, into four haploid daughter cells - with half the amount of genetic material each as the original mother cell.

If you have a gene at which an organism inherited different copies from each of its parents, it is called heterozygous: generically represented as genotype: Aa (i.e. one copy of variant (allele) 'A', and one copy of allele 'a'). When a heterozygote creates gametes, the alleles should split, and end up in a 1:1 ratio in the resulting cells. However, gene conversion refers to an error during meiosis, in which a ratio other than the expected 1A:1a is observed - for example 3A:1a, 1A:3a, 5A:3a or 3A:5a.

This conversion of one allele to the other is due to inappropriate base mismatch repair during recombination: if one of the four strands during meiosis pairs up with one of the four strands of a different chromosome, as can occur if there is sequence homology, mismatch repair can alter the sequence of one of the chromosomes, to match identically that on the other.

Nonreciprocal exchange of genetic information as a result of heteroduplex formation between non-sister chromatids. Mismatches in the region of the heteroduplex are corrected so that one strand acts as the acceptor, which is made to complement the second strand, the donor (ie, base changes occur on one strand only, the acceptor strand).

Also is one of the ways a gene may be mutated to create a cancerous gene.

images: http://www.web-books.com/moBio/Free/Ch8D4.htm and http://www.web-books.com/moBio/Free/Ch8D2.htm

[edit] Importance of Gene Conversion in the Cohesiveness of the Gene Pool

Gene conversion is the only force acting to homogenize the DNA sequences composing the gene pool of a species. Every gene conversion event takes as its substrate two DNA sequences that are homologous but not identical, because of sequence mismatches and yields two identical DNA sequences. Gene conversion forms the cohesive force that links DNA sequences within different organisms of a species. Over time, gene conversion events yield a homogenous set of DNA sequences, both for allelic forms of a gene and for multi gene families. Interspersed repeats act to break up this linkage and catalyze the formation of novel gene during evolution.