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The process whereby one or more gene alleles present in one chromosome may be exchanged with their alternative alleles on a homologous chromosome to produce a recombinant (crossover) chromosome which contains a combination of the alleles originally present on the two parental chromosomes. Genes which occur on the same chromosome are said to be linked, and together they are said to compose a linkage group. In eukaryotes, crossing-over may occur during both meiosis and mitosis, but the frequency of meiotic crossing-over is much higher. See Allele, Chromosome, Gene, Linkage (genetics)
Crossing-over is a reciprocal recombination event which involves breakage and exchange between two nonsister chromatids of the four homologous chromatids present at prophase I of meiosis; that is, crossing-over occurs after the replication of chromosomes which has occurred in premeiotic interphase. The result is that half of the meiotic products will be recombinants, and half will have the parental gene combinations. Using maize chromosomes which carried both cytological and genetical markers, H. Creighton and B. McClintock showed in 1931 that genetic crossing-over between linked genes was accompanied by exchange of microscopically visible chromosome markers. See Recombination (genetics)
In general, the closer two genes are on a chromosome, that is, the more closely linked they are, the less likely it is that crossing-over will occur between them. Thus, the frequency of crossing-over between different genes on a chromosome can be used to produce an estimate of their order and distances apart; this is known as a linkage map.
Since each chromatid is composed of a single deoxyribonucleic acid (DNA) duplex, the process of crossing-over involves the breakage and rejoining of DNA molecules. Although the precise molecular mechanisms have not been determined, it is generally agreed that the following events are necessary: (1) breaking (nicking) of one of the two strands of one or both nonsister DNA molecules; (2) heteroduplex (hybrid DNA) formation between single strands from the nonsister DNA molecules; (3) formation of a half chiasma, which is resolved by more single-strand breakages to result in either a reciprocal crossover, a noncrossover, or a nonreciprocal crossover (conversion event).
the interchanging of sections of paired chromosomes that occurs as a result of the rupture and union in a new order of each strand, or chromatid, and that leads to a redistribution, or recombination, of the linked genes.
Crossing-over is an extremely important mechanism of ensuring combinatory variability; consequently, it is one of the most important factors of evolution. As a rule, the process occurs during the prophase of the first division of the germ cells (meiosis), when the chromosomes are represented by four strands. At the site of the crossing-over the characteristic configuration of the crossed chromosomes, called the chiasma, has been discovered cytologically. The result of the crossing-over may be revealed by the new combination of linked genes (if the alleles of the homologous chromosomes participating in the process are heterozygous). This method, discovered by the American geneticist T. Morgan, made it possible to demonstrate the linear distribution of the genes in the chromosome and to develop a method of establishing their relative arrangement (genetic mapping). In 1933 the German scientist K. Stern cytologically demonstrated the process of crossing-over with the exchange of genes between the chromosomes.
The frequency of crossing-over depends roughly on the linear distance between the genes. In the event that a double or multiple exchange between two genes occurs all at once on a given sector, the frequency of recombination of these genes is decreased. The term “unequal crossing-over” refers to cases in which the sites of the ruptures in the chromosomes that are exchanging sectors are not strictly matched. One of the chromosomes receives extra genetic material, and its homologue receives too little.
In higher organisms, crossing-over has also been discovered in the somatic cells, where it leads to mosaic characters. The process of crossing-over may involve both strands of the DNA molecule or only one. It may affect a large section of the chromosome, containing several genes, or only part of a single gene (intragenic crossing-over). Both the rupture and the refusion of the chromosomes during crossing-over are accomplished with the participation of a number of enzymes. However, the molecular mechanism of crossing-over has not been elucidated conclusively.
REFERENCEKushev, V. V. Mekhanizmy geneticheskoi rekombinatsii. Leningrad, 1971.
V. N. SOIFER