Genetics in obstetrics and gynecology / Joe Leigh Simpson [and others].

Date:
[1982], ©1982
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics in obstetrics and gynecology / Joe Leigh Simpson [and others]. Source: Wellcome Collection.

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    16 / GENETICS IN OBSTETRICS AND GYNECOLOGY MEIOSIS I : PRODUCTS OF MEIOSIS IL - Figure 1-9. Schematic representation of the stages of meiosis I and the gametic products present after meiosis II. The behavior of one pair of autosomes is shown. At zygotene homologous chromosomes pair with each other along their longitudinal planes by a process known as synapsis. Synapsis occurs between the various alleles at a single locus. At some sites segments are exchanged between nonsister chromatids. During diplotene chromosomes begin to separate. If crossing-over occurs, no two of the four chromatids of a given chromosome pair are genetically identical, as illustrated by the four different products of meiosis II. (From Simpson JL: Disorders of Sexual Differen¬ tiation. Etiology and Clinical Delineation. New York: Academic Press, 1976, p 12. Re¬ printed with permission.] the term recombination is properly applied only to the process of exchange of genetic information. Recombination permits exchange of genes betw^een ho¬ mologous chromosomes, providing a source for genetic variability; chiasmata prevent homologous chromosomes from separating prematurely prior to me- taphase. Premature separation could potentially lead to nondisjunction. During diplotene one pair of sister chromatids separates from the other, premature separation having been initially prevented by chiasmata. During diakinesis the chromosomes shorten and chiasmata move tow^ard the telomeres (terminali- zation) to permit normal disjunction. Metaphase. Onset is characterized by disappearance of the nuclear mem¬ brane and orientation of the chromosomes on the mitotic spindle. In every tetrad there are tw^o chromosomes and tw^o centromeres. In mitosis, centromeres divide during each cycle, but in meiosis I, homologous chromosomes instead