For a zygote to contain a diploid complement of chromosomes, the gametes, which fuse to form this zygote must contain half the number of chromosomes. This reduction of chromosome number occurs in two distinct sequential phases of meiosis, termed meiosis I and meiosis II. Both divisional sequences have sub-stages such as, prophase (I, II), prometaphase (I, II), metaphase (I, II), anaphase (I, II), and telophase (I, II) that is continued by cytokinesis (I, II), which is not part of meiosis. The cell undergoes different changes and transformations before and during meiosis, such as replication and condensation of chromosomes, duplication of centrosome, disappearance of nuclear envelope and the nucleus itself and more. During the first stage of the meiosis I homologous chromosomes line up side-by-side and form synapses. The paired chromosomes are called tetrads, and the chromatids of homologous chromosomes cross over at chiasmata. During prometaphase I, kinetochores that are attached to chromosomes attach to spindle fibers and move towards the metaphase plate where during metaphase they align along the plate. In fact, article “Orientation of non randomly segregation of sex chromosomes in spermatocytes of the flea beetle Alagasa bicolor L” states that each sex chromosomes are attached to spindles from both poles during prophase I; however, prior to anaphase I that particular orientation “changes to the syntelic orientation (both sister kinetochores connected to the same pole), perhaps by the release of microtubule attachments from the more distant pole by each of the chromosomes. The syntelic orientation just prior to anaphase I leaves each sex chromosome attached to the nearest pole via kinetochore microtubules, ensuring nonrandom segregation” (3). Moreover, during anaphase I each pair of chromosomes begins to move towards the poles. According to Arthur Forer and Paula J. Wilson, during anaphase 75 % of kinetochore microtubules shorten at the...
Please join StudyMode to read the full document