Sex determination.

Bacci, Guido.

Date:: [1965]

Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Sex determination. Source: Wellcome Collection.

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SEXUAL AND ASEXUAL REPRODUCTION 9 The longitudinal cleavage of each homologous chromosome is accompanied at pachytene by breaks in two homologous chromatids, which take place at the same level. An interchange of both segments of the chromatids takes place immediately and mixed chromatids are formed after the fusion between homologous chromatids. This process imphes an interchange of genes from homologous chromosomes, that is from chromosomes of paternal and maternal origin, which have been represented in the diagram as solid and outlined respectively. Diploiene stage (Fig. 1.3c). The paired chromosomes start to separate at this stage by apparently repelling each other. The separation, however, is generally not complete for the homologous chromosomes are held together at the points where two of the chromatids have crossed over. Each point of contact, corresponding to a point of interchange, is called a chiasma and at least one chiasma is formed for each bivalent. Some chiasmata are interstitial, between the ends of the chromosomes, and some are terminal. When a single chiasma is present the bivalent takes the form of a cross, when two or more chiasmata are present a loop, a double loop or a series of loops is formed. During this stage the chiasmata are progressively displaced along the chromosome from the centromere towards the ends, a movement that has been called terminalization and is completed at metaphase. Diakinesis. The processes of the previous stage are accentuated: chromo¬ somes are very contracted, the number of interstitial chiasmata decreases through terminalization and the homologues appear joined to each other at their ends. The nucleolus is detached from its chromosome or has completely disappeared. This stage comes to an end with the disappearance of the nuclear membrane. Metaphase I {Fig. 1.3d). In this stage, which takes place soon after diakine¬ sis, the bivalent chromosomes orient themselves in the equatorial plane, and the centromeres of the two homologous members of each bivalent are directed toward opposite poles. The formation of the spindles takes place in the mean¬ time as in mitosis. Anaphase I {Fig. 1.3e). In this stage the chromosomes of each bivalent move from the metaphase plate to their respective poles while the chiasmata are dissolved. A segregation of whole chromosomes instead of chromatids takes place in this way because each centromere remains undivided and the number of chromosomes is halved. On the other hand the homologous chromosomes have now a different composition from that of the originals as a consequence of the crossing over between chromatids. After telophase I (Fig. 1.3f) and interphase, which have the same character as the corresponding mitotic phases, a short prophase II takes place. 2