Discuss the molecular mechanism of crossing over
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Crossing-over is a remarkably precise process. Some kind of cellular machinery takes two huge molecular assemblages (homologous nonsister chromatids), breaks them in the same relative position, and then rejoins them in a new arrangement so that no genetic material is lost or gained in either. The exact molecular mechanism of crossing-over is not known, but several models have been proposed, and there is general agreement that a key step is the formation ofheteroduplex DNA, a hybrid type of DNA molecule that is composed of a strand from one parental chromatidand a strand from the other.
Most of the evidence in favor of aheteroduplex model comes from the study of fungal tetrads and octads. Octads are particularly informative in pointing to the existence of heteroduplexes in crossing-over. We have seen in Chapter 4 that in these organisms a cross A × a will create a monohybrid meiocyte A/a that is expected to segregate in a 1:1 ratio in the meiotic products according to the law of equal segregation. Indeed this is found in most meiocytes. In fungi with octads the four nuclei that represent the four products of meiosis undergo an extra mitotic division to produce four ascospore pairs, which stay together in the octad sac. The expected octad ratio from a monohybrid meiocyte is 4:4. However, in rare meiocytes (generally on the order of 0.1 percent to 1 percent) any one of five types of aberrant ratios can be found, and these gave the clues needed to build a DNA crossover model
Most of the evidence in favor of aheteroduplex model comes from the study of fungal tetrads and octads. Octads are particularly informative in pointing to the existence of heteroduplexes in crossing-over. We have seen in Chapter 4 that in these organisms a cross A × a will create a monohybrid meiocyte A/a that is expected to segregate in a 1:1 ratio in the meiotic products according to the law of equal segregation. Indeed this is found in most meiocytes. In fungi with octads the four nuclei that represent the four products of meiosis undergo an extra mitotic division to produce four ascospore pairs, which stay together in the octad sac. The expected octad ratio from a monohybrid meiocyte is 4:4. However, in rare meiocytes (generally on the order of 0.1 percent to 1 percent) any one of five types of aberrant ratios can be found, and these gave the clues needed to build a DNA crossover model
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