Recombination (redirected from Recombination frequency)

recombination, process of "shuffling" of genes by which new combinations can be generated. In recombination through sexual reproduction, the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents. In recombination by crossing over, alleles of genes are exchanged between homologous chromosomes during meiosis. This exchange results in the generation of new combinations of alleles on segments of chromosomes, counteracting the tendency of linked genes, i.e., genes on the same chromosome, to be always transmitted as a group. Various mechanisms for introducing new genetic material have been discovered in bacteria. These mechanisms have been used extensively to study gene structure and function. In transformation, a fragment of free deoxyribonucleic acid (DNA) is inserted in a recipient bacterium (see nucleic acid). The free DNA fragment comes from the chromosome of a bacterial cell that has been lysed, or dissolved. In transduction, genetic material is transferred from one bacterium to another by a carrier virus. When a virus enters a bacterium, its DNA can be inserted into the bacterial chromosome, reproducing along with the host chromosome in cell division. Subsequently, sometimes many bacterial generations later, the viral genetic material may detach from the bacterial chromosome, taking some of the bacterial chromosomal material along with it. The bacterium then lyses, the viral particle enters a new bacterium, and the viral particle, together with the bacterial genes it carries, is inserted into the chromosome of the new host. In conjugation, which occurs between bacteria of the same species and also between some bacteria of different species, either an entire chromosome or a part of one is transferred from a bacterium of a donor-mating strain to a bacterium of a recipient-mating strain. Donor strains, denoted male, contain a sex-factor particle composed of a nucleic acid; recipient, or female, strains lack the particle (see episome). Conjugation has been used to construct genetic maps, i.e., the ordering of genes along a chromosome. Evidence from transformation experiments was used to support the idea that DNA was the genetic material.

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recombination

In genetics, regrouping of the maternal and paternal genes during the formation of sex cells (gametes). Recombination occurs randomly in nature as a normal event of meiosis. It is enhanced by crossing-over (see linkage group). Recombination acts to ensure that no two daughter cells are identical, nor are any identical in genetic content to the parent cell. Laboratory study of recombination has contributed significantly to the understanding of genetic mechanisms, allowing scientists to map chromosomes, identify linkage groups, isolate the causes of certain genetic mistakes, and manipulate recombination itself by transplanting genes from one chromosome to another. See also genetic engineering, molecular biology.

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Recombination (genetics)

The formation of new genetic sequences by piecing together segments of previously existing ones. Recombination often follows deoxyribonucleic acid (DNA) transfer in bacteria and, in higher organisms, is a regular feature of sexual reproduction. See Deoxyribonucleic acid (DNA), Reproduction (animal), Reproduction (plant)

The fact that recombinants occur in sexual reproduction is due to reciprocal exchanges between chromosomes (crossing over) that take place in the first meiotic division. See Crossing-over (genetics)

Crossing-over between homologous chromosome pairs can also occur during the prophase of mitotic nuclear division. The frequency is very much lower than in meiosis, presumably because the mitotic cell does not form the synaptic apparatus for efficient pairing of homologs. See Mitosis

Recombination was once thought to occur only between genes, never within them. Indeed, the supposed indivisibility of the gene was regarded as one of its defining features, the other being that it was a single unit of function. However, examination of very large progenies shows that, in all organisms studied, nearly all functionally allelic mutations of independent origin can recombine with each other to give nonmutant products, generally at frequencies ranging from a few percent (the exceptionally high frequency found in Saccharomyces) down to 0.001% or less. Recombination within genes is most frequently nonreciprocal.

Bacteria have no sexual reproduction in the true sense, but many or most of them are capable of transferring fragments of DNA from cell to cell by one of three mechanisms. (1) Fragments of the bacterial genome can become joined to plasmid DNA and transferred by cell conjugation. (2) Genomic fragments can be carried from cell to cell in the infective coats of bacterial viruses (phages), a process called transduction. (3) Many bacteria have the capacity to assimilate fragments of DNA from solution and so may acquire genes from disrupted cells. Fragments of DNA acquired by any of these methods can be integrated into the DNA of the genome in place of homologous sequences previously present. Homologous integration in bacteria is similar in its nonreciprocal nature to recombination within genes of eukaryotic organisms. See Bacterial genetics, Bacteriophage, Transduction (bacteria)

Bacteriophages, plasmids, bacteria, and unicellular eukaryotes provide many examples of differentiation through controlled and site-specific recombination of DNA segments. In vertebrates, a controlled series of deletions leads to the generation of the great diversity of gene sequences encoding the antibodies and T-cell receptors necessary for immune defense against pathogens. All these processes depend upon interaction and recombination between specific DNA sequences, catalyzed by site-specific recombinase enzymes. The molecular mechanisms may have some similarities with those responsible for general meiotic recombination, except that the latter does not depend on any specific sequence, only on similarity (homology) of the sequence recombined.

Techniques have been devised for the artificial transfer of DNA fragments from any source into cells of many different species, thus conferring new properties upon them (transformation). In bacteria and the yeast S. cerevisiae, integration of such DNA into the genome requires substantial sequence similarity between incoming DNA and the recipient site. However, cells of other fungi, higher plants, and animals are able to integrate foreign DNA into their chromosomes with little or no sequence similarity. These organisms appear to have some system that recombines the free ends of DNA fragments into chromosomes regardless of their sequences. It may have something in common with the mechanism, equally obscure, whereby broken ends of chromosomes can heal by nonspecific mutual joining. See Transformation (bacteria)

The science of genetics has been revolutionized by the development of techniques using isolated cells for specific cleaving and rejoining of DNA segments and the introduction of the reconstructed molecules into living cells. This artificial recombination depends on the use of site-specific endonucleases (restriction enzymes) and DNA ligase. See Gene, Gene action, Genetic engineering, Genetics, Restriction enzyme