recombination

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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 also 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 also 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 also 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 also 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 also 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 also Gene; Gene action; Genetic engineering; Genetics; Restriction enzyme.

The reunion, in the same or different arrangement, of formerly united elements that have been separated; in genetics, the formation of new gene combinations due to crossing over by homologous chromosomes. Recombination occurs between viruses such as influenza or bluetongue which have segmented genomes. Called also reassortment.

• r. frequency — the frequency of exchange between two genes on the same chromosome.

Process by which a conduction band electron gives up energy (in the form of heat or light) and falls into a valence band hole.

Look up recombination in Wiktionary, the free dictionary.

Recombination may refer to:

• Genetic recombination, the process by which genetic material is broken and joined to other genetic material
• Carrier generation and recombination, processes by which mobile electrons and electron holes are created and eliminated
• Crossover (genetic algorithm), also called recombination
• Recombination, a phase in the timeline of the Big Bang
• Recombination (chemistry), the opposite of Dissociation (chemistry)

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