Question

Notes on chromosomal aberrations structural and numerical

Answer 1

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structural chromosome aberrations are the result of chromosome breakage and abnormal reunion of broken chromosomes. They can be produced experimentally by exposing active cells to mutagens such as ionizing radiation. However, spontaneous structural rearrangements in both somatic and germ cells arise from errors of recombination. Meiotic recombination is preceded by synapsis of homologous chromosomes which involves the recognition by one homolog of complementary sequences in the other homolog. Mismatching can occur in this process, particularly at chromosomal sites containing tandem repeats of DNA sequences. This may result in duplication or deletion of the DNA at such sites. Similarly, synapsis between homologous sites on nonhomologous chromosomes may lead to accidental recombination between nonhomologous chromosomes, thereby leading to the transfer of chromosomal segments from one chromosome to another. These rearrangements are termed ‘translocations.’

Recombination also occurs between homologous chromosomes in somatic cells, and occasionally examples of pairing and chromatid exchange can be observed in routine chromosome preparations. However, the main evidence comes from studies of DNA markers in neoplasia, in which individuals heterozygous at a number of gene loci on a chromosome pair have tumors homozygous at the same loci on the same pair of chromosomes.

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