18.4Genetic Recombination

Figure 18-7 Molecular process of general recombination during meiotic process

DNA in chromosomes is rearranged via genetic recombination during the meiotic process. General recombination occurs between homologous DNA regions on homologous chromosomes during meiosis. In recombination between such homologous regions, a double-strand break occurs in one of the paired homologous DNA strands followed by the initiation of partial degradation of the 5′ ends at the breakpoint, thereby exposing the 3′ ends (Figure 18-7). The 3′ ends recognize the similar DNA sequence in the paired DNA strand, and bind to it through the action of proteins that mediate recombination. Partial synthesis of complementary DNA then proceeds, and DNA recombination is finally completed after the breaking of the DNA strands and the repair of the chromosomes. Assuming that the frequency of recombination does not differ across the chromosome, the greater the distance between two genes, the more likely recombination will occur between them. Thus, the distance between genes can be estimated by measuring the gene recombination rate. When searching for a new gene that produces a certain phenotype and if many DNA polymorphisms are known, the location of the gene can be determined in detail from the recombination rate, which indicates the degree of linkage with the gene (known as genetic mapping; see Column at the bottom).

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Agrobacteria and genetically modified plants

In 1974, it was discovered that the swelling observed in plants infected with agrobacteria*5 is caused by the circular DNA of the bacteria. Subsequent studies showed that part of this circular DNA is incorporated into the plant genomic DNA and is replicated along with DNA replication. It was also found that the inserted DNA contains plant hormone synthesis genes that promote the growth of plant cells, as well as genes for the synthesis of special amino acids that bacteria feed on. Indeed, the bacteria cause the host plant to produce large amounts of plant cells which produce food for them. In other words, bacteria use the host plant as a factory to produce their food. Based on these findings, this system was proposed for use in the artificial introduction of various genes into plant cells. Today, it is common practice to utilize agrobacteria for introducing target genes into plant cells after removing the genes that cause swelling. One somatic plant cell can be directly differentiated to form a whole plant. This ability is known as totipotency, and makes it easy to regenerate a plant from a plant cell containing introduced genes. Plants with artificially introduced genes are called transgenic plants. Many transgenic plants have been created, and crops with pest-resistance genes as well as pesticide-resistant plants are widely cultivated. Given that environmental destruction, including desertification, is predicted to become widespread in the future, the creation of genetically modified crops that can be grown under poor conditions is an urgent issue.

*5 Due to changes in the specifications of species nomenclature, today the genus name Agrobacterium does not exist; it is included in the Rhizobium genus, which includes root nodule-forming bacteria.


Linkage, recombination, and DNA polymorphism

The relationship between genes on the same chromosome is referred to as linkage. Unless this linkage is broken, genes on the same chromosome are passed on to the same gamete during meiosis. Mendel's law of independent assortment is not applicable to linked genes or phenotypes resulting from the expression of linked genes. The combination of linked genes changes according to crossing-over, as illustrated in Figure 18-4, and the resultant genetic recombination. The changes depend on the frequency of recombination. For genes on the same chromosomes, the longer the distance between two genes, the higher the frequency of recombination and vice versa. Although it is difficult to identify the genes associated with the morphology and phenotype of interest, presently genome information is available for numerous organisms, and the locations of sequences that differ between individuals (DNA polymorphism; see Chapter 24) is gradually being clarified for the whole genome. If these numerous DNA polymorphisms, which can be considered as genes, exhibit a close association with the phenotype of interest in the course of transmission of information to progenies, such a polymorphism is close to the location of target genes expressing the phenotype (or, if lucky, it could be the exact target gene), allowing target genes, which should be in the vicinity, to be traced based on this information (Column Figure 18-1).

Column Figure 18-1 Mapping of genes on chromosomes using DNA polymorphisms

A virtual organism with 2n = 6 chromosomes is used as an example, assuming that you want to identify the position of the gene producing phenotype W. Several DNA polymorphisms are known to exist between two groups of this organism. For example, 1A and 1a, and 2E and 2e are considered to be DNA polymorphisms with allelic genes. If these are crossed, the F1 generation becomes as illustrated in B. Table C shows the frequency of expression of each DNA polymorphism in individuals with phenotype W in the F2 generation as a result of self-fertilization between F1 generations. In this example, phenotype W shows close linkages with DNA polymorphisms on chromosome 3, namely 3I, 3J, and 3K. Furthermore, a closer linkage with 3J can be confirmed than with 3I or 3K, suggesting that the gene producing phenotype W is positioned near 3J. By repeating this process, numerous genes can each be arranged on chromosomes. This is called a chromosome mapping or linkage mapping.

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