meiosis and sexual life cycles unit 3 – chapter 13
TRANSCRIPT
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Meiosis and Sexual Life Cycles
Unit 3 – Chapter 13
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Diploid vs. Haploid
• A cell that has both members of a homologous pair of chromosomes in the nucleus, is considered to be Diploid (2n)
• A cell that has only one homologue of a chromosome pair (so either the maternal or paternal homologue is missing) is considered Haploid (n)
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Karyotypes are made from cells arrested at metaphase
KARYOTYPE OF A DIPLOID CELL KARYOTYPE OF A HAPLOID CELL
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Importance of Meiosis
• Reduces the number of chromosomes by half – the cell goes from diploid (2n) to haploid (n) – humans go from 23 pairs to 23 single chromosomes
• Prevents polyploidy (multiple chromosomes) – and maintains the chromosome number of each species
• Produces genetic variation and contributes to evolution
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The Human Life Cycle
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Spermatogenesis
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OVULATION
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Egg and Fallopian Tube Lining
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Egg and Fertilization
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Zygote and First Mitotic Division
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Sexual Life Cycles Produce Genetic Variation Among Offspring
• Independent Assortment
• Crossing Over
• Random Fertilization
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Independent Assortment
• The orientation of the homologous chromosomes at the metaphase plate in metaphase I is completely random
• The maternal and paternal homologues can be on either side of the plate
• There is a 50% chance that a daughter cell of meiosis I will get a maternal homologue of a particular chromosome pair and a 50% chance that it will get the paternal homologue
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Independent Assortment
• The number of combinations possible for gametes formed by meiosis can be determined by 2n, where 2 is the number of chromosomes per homologous pair and n is the haploid number of that organism.
• So for human – 223 = ~8 million chromosomal combinations possible in gametes
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Independent Assortment
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Crossing Over• This adds even more
variation to gametes
• Each time the homologous chromosomes swap alleles, the possible number of chromosomal combinations in gametes is increased even further
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Random Fertilization
• A human egg that represents one of ~8 million possible chromosomal combinations, is fertilized by a sperm that represents one of ~8 million chromosomal possible combinations
• So even without crossing over occurring, the zygote will contain one of 64 trillion (8 million x 8 million) chromosomal combinations!!