Ectopic recombination is an atypical form of recombination in which a crossing over takes place between two homologous DNA sequences located at non-allelic chromosomal positions. Such recombination often results in dramatic

chromosomal rearrangement In genetics, a chromosomal rearrangement is a mutation that is a type of chromosome abnormality involving a change in the structure of the native chromosome. Such changes may involve several different classes of events, like deletions, duplicati ...

, which is generally harmful to the organism.Montgomery, E., B. Charlesworth, and C. H. Langley. 1987. A test for the role of natural selection in the stabilization of transposable element copy number in a population of Drosophila melanogaster. Genet. Res. 49:31–41 Some research, however, has suggested that ectopic recombination can result in mutated

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

s that benefit the organism. Ectopic recombination can occur during both

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

and

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

, although it is more likely occur during meiosis. It occurs relatively frequently—in at least one yeast species (

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have be ...

) the frequency of ectopic recombination is roughly on par with that of allelic (or traditional) recombination.Licthen, M, R.H. Borts, and J.E. Haber. 1986. Meiotic gene conversion and crossing over between dispersed homologous sequences occurs frequently in saccharomyces cerevisiae. Genetics 115: 233-246 If the alleles at two loci are

heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

, then ectopic recombination is relatively likely to occur, whereas if the alleles are homozygous, they will almost certainly undergo allelic recombination. Ectopic recombination does not require loci involved to be close to one another; it can occur between loci that are widely separated on a single chromosome, and has even been known to occur across chromosomes. Neither does it require high levels of homology between sequences—the lower limit required for it to occur has been estimated at as low as 2.2 kb of homologous stretches of DNA nucleotides. In

tobacco Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...

plant

somatic cell In cellular biology, a somatic cell (), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Somatic cells compose the body of an organism ...

s, DNA double-strand break-induced recombination between ectopic homologous sequences appears to serve as a minor DNA repair pathway for double-strand breaks.Puchta H. Double-strand break-induced recombination between ectopic homologous sequences in somatic plant cells. Genetics. 1999 Jul;152(3):1173-81. doi: 10.1093/genetics/152.3.1173. PMID 10388832; PMCID: PMC1460648 The role of

transposable element A transposable element (TE), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome. The discovery of mobile genetic elements earned Barbara McClinto ...

s in ectopic recombination is an area of active inquiry. Transposable elements—repetitious sequences of DNA that can insert themselves into any part of the

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

—can encourage ectopic recombination at repeated homologous sequences of nucleotides. However, according to one proposed model, ectopic recombination might serve as an inhibitor of high transposable element copy numbers. The frequency of ectopic recombination of transposable elements has been linked to both higher copy numbers of transposable elements and the longer lengths of those elements.Petrov, D.A, Y.T. Aminetzach, J.C. Davis, D. Bensasson, and A.E. Hirsh. 2003. Size matters: non-LTR retrotransposable elements and ectopic recombination in drosophila. Mol Bio Evol 20: 880-892 Since ectopic recombination is generally deleterious, anything that increases its odds of occurring is selected against, including the aforementioned higher copy numbers and longer lengths. This model, however, can only be applied to single families of transposable elements in the genome, as the probability of ectopic recombination occurring in one TE family is independent of it occurring in another. It follows that transposable elements that are shorter, transpose themselves less often, and have mutation rates high enough to disrupt the homology between transposable element sequences sufficiently to prevent ectopic recombination from occurring are selected for.

References

{{reflist Cellular processes Modification of genetic information Molecular genetics