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MCDB Low Lecture #11

MCDB Low Lecture #11 - MCDB Lecture Low#11 Summary of...

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MCDB Lecture Low #11 Summary of Meiosis Meiosis Summary In human cells just before meiosis, a diploid precursor cell contains: 23 pairs of chromosomes, 46 total chromosomes Each chromosome has two chromatids, so 92 chromatids all together. A gamete after meiosis II has 23 chromosomes Only 1 homolog of each pair, so 23 of the original chromatid all together. Each chromosome consists of one chromatid. Reduction from diploid to the haploid state occurs during anaphase I Genetic diversity occurs in 2 ways 1) Random distribution of the two homologs of a pair to the daughter cells during meiosis I, random assortment during meiosis II 2) Crossing over between homologs during meiosis I Meiotic Errors Meiotic errors – abnormal chromosome structures and numbers Non-disjunction – a pair of homologous chromosomes fails to separate during meiosis I or sister chromatids fail to separate during meiosis II. Aneuploidy – 1 or more chromosomes are either lacking or present in excess. Chromosomes non-disjunction (mistakes during meiosis) causes genetic diseases such as Down Syndrome which get three copies of chromosome 21. Down Syndrome Most common form of mental retardation 1 in 700 births Faulty development of the central nervous system Decrease in the number of neurons, and a predisposition to Alzheimer, leukemia, and heart defects. Results in 20% of pregnancies having a spontaneous abortion Fertilization and the Principles of Early Development Meiosis occurs during the formation of eggs and sperm In most eukaryotic organisms, all cells in the body except for the egg and sperm
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