The junction is then resolved either horizontally, which produces no recombination, or vertically, which results in an exchange of DNA. The next step, called branch migration, takes place as the junction travels down the DNA. Each end then crosses over and invades the other chromosome, forming a structure called a Holliday junction (Figure 2). In the single-stranded model, following the alignment of homologous chromosomes, a break is introduced into one DNA strand on each chromosome, leaving two free ends. The basic steps of recombination can occur in two pathways, according to whether the initial break is single or double stranded. This process occurs with a high degree of accuracy at high frequency in both eukaryotic and prokaryotic cells. It involves the alignment of two homologous DNA strands (the requirement for homology suggests that this occurs through complementary base-pairing, but this has not been definitively shown), precise breakage of each strand, exchange between the strands, and sealing of the resulting recombined molecules. As in eukaryotes, recombination also plays important roles in DNA repair and replication in prokaryotic organisms.Īlthough common, genetic recombination is a highly complex process. Although bacteria do not undergo meiosis, they do engage in a type of sexual reproduction called conjugation, during which genetic material is transferred from one bacterium to another and may be recombined in the recipient cell. Recombination also occurs in prokaryotic cells, and it has been especially well characterized in E. Evidence for this finding came from the fact that alleles first introduced into the cross on a knobbed chromosome later appeared in offspring without the knob similarly, alleles initially introduced on a knobless chromosome subsequently appeared in progeny with the knob (Figure 1). McClintock and Creighton then followed these alleles through meiosis, showing that alleles for specific phenotypic traits were physically exchanged between chromosomes. Using a strain of maize in which one member of a chromosome pair exhibited the knob but its homologue did not, the scientists were able to show that some alleles were physically linked to the knobbed chromosome, while other alleles were tied to the normal chromosome. Specifically, in 1931, Barbara McClintock and Harriet Creighton obtained evidence for recombination by physically tracking an unusual knob structure within certain maize chromosomes through multiple genetic crosses. The role of recombination during the inheritance of chromosomes was first demonstrated through experiments with maize. In these cases, a sister chromatid serves as the donor of missing material via recombination followed by DNA synthesis. Recombination is also used in DNA repair (particularly in the repair of double-stranded breaks), as well as during DNA replication to assist in filling gaps and preventing stalling of the replication fork. In this instance, the outcome of recombination is to ensure that each gamete includes both maternally and paternally derived genetic information, such that the resulting offspring will inherit genes from all four of its grandparents, thereby acquiring a maximum amount of genetic diversity. One important instance of recombination in diploid eukaryotic organisms is the exchange of genetic information between newly duplicated chromosomes during the process of meiosis. Various cases of nonhomologous recombination do exist, however. This process is generally mediated by homology that is, homologous regions of chromosomes line up in preparation for exchange, and some degree of sequence identity is required. DNA recombination involves the exchange of genetic material either between multiple chromosomes or between different regions of the same chromosome.
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