meiosis-1 - Mitosis and Meiosis Chromosomes Key features of...

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Mitosis and Meiosis
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Chromosomes Key features of a chromosome: centromere (where spindle attaches), telomeres (special structures at the ends), arms (the bulk of the DNA). Chromosomes come in 2 forms, depending on the stage of the cell cycle. The monad form consists of a single chromatid, a single piece of DNA containing a centromere and telomeres at the ends. The dyad form consists of 2 identical chromatids (sister chromatids) attached together at the centromere. Chromosomes are in the dyad form before mitosis, and in the monad form after mitosis. The dyad form is the result of DNA replication: a single piece of DNA (the monad chromosome) replicated to form 2 identical DNA molecules (the 2 chromatids of the dyad chromosome).
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More Chromosomes Diploid organisms have 2 copies of each chromosome, one from each parent. The two members of a pair of chromosomes are called homologues . Each species has a characteristic number of chromosomes, its haploid number n. Humans have n=23, that is, we have 23 pairs of chromosomes. Drosophila have n=4, 4 pairs of chromosomes.
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Cell Cycle The cell cycle is a theoretical concept that defines the state of the cell relative to cell division. The 4 stages are: G1, S, G2, and M. M = mitosis, where the cell divides into 2 daughter cells. The chromosomes go from the dyad (2 chromatid) form to the monad (1 chromatid) form. That is, before mitosis there is 1 cell with dyad chromosomes, and after mitosis there are 2 cells with monad chromosomes in each. S = DNA synthesis. Chromosomes go from monad to dyad. G1 = “gap”. Nothing visible in the microscope, but this is where the cell spends most of its time, performing its tasks as a cell. Monad chromosomes G2 (also “gap”). Dyad chromosomes, cell getting ready for mitosis. G1, S, and G2 are collectively called “interphase”, the time between mitoses
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Mitosis Mitosis is ordinary cell division among the cells of the body. During mitosis the chromosomes are divided evenly, so that each of the two daughter cells ends up with 1 copy of each
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meiosis-1 - Mitosis and Meiosis Chromosomes Key features of...

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