Phases of Meiosis

Phases of Meiosis - break and may be reattached to a...

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Phases of Meiosis Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells. The old name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I. Prophase I Prophase I has a unique event -- the pairing (by an as yet undiscovered mechanism) of homologous chromosomes . Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad , being composed of two chromatids from each chromosome, forming a thick (4-strand) structure. Crossing-over may occur at this point. During crossing-over chromatids
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Unformatted text preview: break and may be reattached to a different homologous chromosome. The alleles on this tetrad: A B C D E F G A B C D E F G a b c d e f g a b c d e f g will produce the following chromosomes if there is a crossing-over event between the 2nd and 3rd chromosomes from the top: A B C D E F G A B c d e f g a b C D E F G a b c d e f g Thus, instead of producing only two types of chromosome (all capital or all lower case), four different chromosomes are produced. This doubles the variability of gamete genotypes. The occurrence of a crossing-over is indicated by a special structure, a chiasma (plural chiasmata) since the recombined inner alleles will align more with others of the same type (e.g. a with a, B with B). Near the end of Prophase I, the homologous chromosomes begin to separate slightly, although they remain attached at chiasmata....
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Phases of Meiosis - break and may be reattached to a...

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