The exchange of genetic information between chromosomes occurs in a variety of organisms and under a variety of circumstances. At the molecular level, this involves exchanging segments of DNA molecules by a mechanism known as recombination. During sexual reproduction in eukaryotes, the process of meiosis allows the exchange of segments of DNA between homologous chromosomes. This generates greater genetic diversity among the offspring, which in turn allows much greater opportunity for evolutionary selection. Although prokaryotes do not practice sexual reproduction in the same manner as higher organisms, nonetheless they have several mechanisms to promote genetic exchange. Among bacteria, fragments of DNA may be recombined into the chromosome after entering the cell as a result of transformation, transduc-tion or conjugation (see Ch. 18 for a description of DNA transfer mechanisms among prokaryotes). Even virus genomes may undergo recombination under certain circumstances.
In all cases of recombination, two DNA molecules are broken and rejoined to each other forming a crossover (Fig. 14.01). A single crossover usually forms short-lived hybrid DNA molecules. If two crossovers occur, the segment of DNA between them will be transferred from one DNA molecule to the other. This is recombination. [In fact, a single crossover can promote recombination by exchanging the ends of a pair of linear chromosomes. However, a single crossover cannot cause recombination between two circular molecules of DNA.]
crossover Structure formed when the strands of two DNA molecules are broken and joined to each other recombination Exchange of genetic information between chromosomes or other molecules of DNA
Different DNA molecules may swap segments, usually of related sequence.
Two crossovers between DNA molecules are needed for recombination to occur.
FIGURE 14.02 Homologous versus Nonhomologous Recombination
A) In homologous recombination, two DNA molecules have similar sequences such that the pink (top) strand can align with the purple (bottom) strand. If a double-stranded break occurs within the aligned regions, a crossover event will exchange regions of the DNA.
B) In non-homologous recombination, related protein recognition sequences lie within two unrelated regions of DNA. Proteins bind to the recognition sequences and carry out recombination. The proteins direct double-stranded breakage and crossing over. Genetic exchange can thus occur between two unrelated DNA molecules. [This event could also theoretically be classified as a translocation.]
Recombination may be divided into homologous and non-homologous recombination. For homologous recombination to occur, the DNA sequences at the crossover region must be sufficiently similar to base pair. In practice, homologous recombination normally occurs between two copies of the same chromosome (as in meiosis) or between two copies of closely related DNA. Crossovers due to base homology may occur in DNA as short as 20-30 bases, however, 50 to 100 bases is needed for reasonable crossover frequencies. Non-homologous recombination is much rarer and involves specific proteins that recognize particular sequences and supervise the formation of crossovers between them (Fig. 14.02). In both cases, the molecular details come mostly from bacteria, especially E. coli, and the details in higher organisms remain much more vague.
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