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Lecture 10

Lecture 10 IS- Meiosis word.doc

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Biology 1001A

Lecture 10 IS- Meiosis HOMOLGOUS PAIR VS SISTER CHROMATID 1. Products of meiosis in animals vs. plants, fungi and algae animals: - makes gametes - diploid phase dominates the life cycle - haploid phase is reduced and followed directly by gamete formation - in males each of the four nuclei produced by meiosis is enclosed in a separate cell and each differentiates into a sperm cell - in females only one of the four nuclei becomes functional as an egg cell nucleus - Fertilization ends the animals haploid phase of life Plants and some fungi: - makes haploid spores - spores go through mitosis grow into haploid gametophytes - then at maturity the nuclei of some of these cells develop into egg or sperm nu- clei - the sperm and egg cells are genetically identical because they were produced through mitosis - they get fertilized to make diploid zygote that divides by mitosis to make the diploid sporophyte that goes through meiosis to makes spores again - alternate between haploid and diploid generations where either generation may domi- nate the life cycle - sporophyte generation is usually most visible - gametophyte generation usually develops in the reproductive parts of the sporophytes Most Fungi and algae: - makes haploid spores - diploid phase is limited to the zygote produced by fertilization - after fertilization, the zygote will undergo meiosis to produce the haploid phase - mitotic divisions occur only in the haploid phase - during fertilization two haploid gametes fuse to form a diploid nucleus - nucleus immediately enters meiosis to make four haploid cells - after one or two mitotic divisions develop into haploid spores - spores germinate and to produce individuals that grow by mitosis - eventually form gametes (positive and negative because they are similar in structure) from differentiation of some cells - because gametes are produced by mitosis they are genetically identical 2. Timing of meiosis in vertebrate life cycles Lecture 10 IS- Meiosis - Cells that are destined to divide by meiosis are called meiocytes - they move through their last turn of the cell cycle as usual, replicating DNA and making more chromosomal protein in the S phase - the G2 phase cell carry the replicated chromosomes - after this, the cells enter meiosis I - homologous pairs find their partners and go through synapsis (pair lengthwise, gene for gene)
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