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Meiosis!

Meiosis!. Chromosome Structure. Before a cell starts dividing, the chromosomes are duplicated. Sister chromatids. This process produces sister chromatids. Centromere. Figure 8.4B. Structure of Chromosomes. Genes. Sections of DNA with the code for a protein.

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Meiosis!

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  1. Meiosis!

  2. Chromosome Structure

  3. Before a cell starts dividing, the chromosomes are duplicated Sister chromatids • This process produces sister chromatids Centromere Figure 8.4B

  4. Structure of Chromosomes

  5. Genes • Sections of DNA with the code for a protein

  6. Genes determine individual traits • Typically, 1000+ genes are on a chromosome

  7. Diploid Cells • Diploid cell: Di = _____ • A cell with two of each kind of chromosome (2n) • Somatic (body cells) are diploid • One from the ____________ • One from the ____________ two female parent male parent

  8. Haploid Cells • Haplo = single • A cell with one of each kind of chromosome (n) • Gametes • (sex cells = sperm/egg) are haploid cells

  9. Discuss: Why are sex cells haploid?

  10. Homologous Chromosomes • Paired chromosomes with genes for the same traits in the same order

  11. Meiosis

  12. Where?

  13. Why is Meiosis Important? 1. Meiosis Provides for Genetic Variation

  14. Why is Meiosis Important? 2. Meiosis creates haploid gametes

  15. Animation

  16. Tetrad Tetrad

  17. Genetic Recombination: • Reassortment of genetic information on chromosomes • by independent segregation or crossing over

  18. Crossing Over • Usually 2-3 crossovers per chromosome • Makes an almost endless number of possible chromosomes.

  19. Prophase I ~ Crossing Over

  20. A a B b aB and Ab AB and ab

  21. Independent Assortment • Gene combinations depend on the how the homologous chromosomes line up during metaphase I (a random process!)

  22. Random orientation of chromosomes in Metaphase

  23. Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations

  24. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!

  25. Mistakes in Meiosis • Nondisjunction: • The failure of homologous chromosomes to separate properly during meiosis

  26. Nondisjunction extra • One gamete will have an _______ chromosome • One gamete will be _________ a chromosome. missing

  27. Extra chromosome = _____somy • Missing chromosome = _____somy tri mono

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