Meiosis - of chromatids so that the nonsister chromatids...

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Meiosis Gametes are usually formed by meiotic division of a primary oocyte (female) or a spermatocyte (male) – these are known collectively as Germ Cells – which give rise to egg or sperm cells. Meiosis consists of two cell divisions, meiosis I and meiosis II. Meiosis I During Prophase I, each chromosome (consisting of two sister chromatids) pairs with a homologous chromosome, or homologue. Homologues bear homologous sequences of DNA – they carry much the same genetic information. Humans have 23 pairs of homologues. During chromosome pairing, or synapsis, homologous DNA sequences on homologous chromosomes are precisely aligned with each other. Because each chromosome consists of two genetically identical chromatids, a pair of chromosomes in synapsis displays four strands. During synapsis, the nonsister chromatids may form cross-shaped structures (Chiasmata). These are the visible manifestation of crossing over, during which there may be breakage and reunion
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Unformatted text preview: of chromatids, so that the nonsister chromatids carry new combinations of genes. During Metaphase I the synapsed pairs of chromosomes are arranged on an equatorial plane, during Anaphase I, the centromeres are pulled to opposite poles, but the centromeres DO NOT divide, so both chromatids are carried together. Therefore each pole receives one two-stranded chromosome from each homologous pair. Meiosis I is therefore a reductional division, in which each daughter cell ends up with half the number of chromosomes it had at the start of meiosis. Meiosis II During metaphase II the chromosomes in each daughter cell are arranged on the equatorial plane; the centromeres then divide and the two chromatids of which each chromosome is composed are pulled to opposite poles. Cell division occurs, and nuclear membranes re-form. Thus the ultimate result of the two stages of meiosis is four cells derived from each germ cell....
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This note was uploaded on 01/18/2011 for the course ANTHROBIO 161 taught by Professor Frisancho during the Spring '10 term at University of Michigan.

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