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Meiosis

Meiosis. Meiosis. Sexual Reproduction ( or a means of producing new individuals) by the union of two sex cells. Gamete Cells. Sperm or egg cells: The sex cells that unite in sexual reproduction. They are normally haploid (1N) meaning they only have one chromosome from a pair.

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Meiosis

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  1. Meiosis

  2. Meiosis • Sexual Reproduction (or a means of producing new individuals) by the union of two sex cells

  3. Gamete Cells • Sperm or egg cells: The sex cells that unite in sexual reproduction. • They are normally haploid (1N) meaning they only have one chromosome from a pair. • N=the number of chromosomes

  4. Question How many total chromosomes are in a gamete? • 23 total chromosomes

  5. Zygote • The product of a union between sperm and egg (gametes). • Zygote’s are diploid (2N) and will develop into an individual by mitosis.

  6. Question How many chromosomes are in a zygote? • 23 pairs or 46 total

  7. Somatic Cells • Somatic cells (body cells) make up the majority of all organism’s cells. • Normal somatic cells are diploid (2N) and contain one set of chromosomes from each parent.

  8. Somatic Cells (cont.) • In humans, somatic cells contain 46 chromosomes (23 received from the mother and an 23 are received from the father)

  9. Questions 1. How do cells from the body divide? • Mitosis 2. Are the somatic cells diploid or haploid? Explain? • Diploid (2N) • Each somatic cell gets a chromosome from mom and from dad.

  10. Karyotype • A map of an organism’s homologous chromosomes

  11. Homologous Chromosomes • Chromosomes of approx. the same length with genes for the same traits • One from mom, one from dad • Not identical

  12. Autosomes • Chromosomes that control somatic (nonsexual) development • Pairs # 1-22

  13. Sex Chromosomes • Chromosomes that control all sexual development. • Pair # 23

  14. Meiosis • An event that reduces a special type of 2N somatic cell into 4 haploid gametes • Requires two consecutive divisions

  15. Division I – A Reduction Division

  16. Interphase • Chromosomes replicate just like in mitosis.

  17. Prophase I • Homologous chromosomes pair forming a tetrad (4 chromatids). • The process of pairing is called synapsis. • Crossing over takes place.

  18. Crossing Over: • During Prophase I chromosomes paired by synapsis can become tangled. • Tangling can result in pieces of chromosomes being exchange (crossing over). • This means maternal traits can be swapped to a paternal chromosome and vice versa.

  19. Metaphase I • Tetrads are pulled to the center of the cell forming an “Equatorial Plate” (middle or equator)

  20. Anaphase I • One of each homologous chromosome pair is pulled to the poles after the centromere is split. • The homologs are separated randomly so maternal and paternal chromosomes could (and usually do) end up on the same side of the dividing cell.

  21. Telophase I • Cytokinesis completes the first division producing two haploid cells that contain one chromosomes in a replicated state. • The nucleus briefly reappears.

  22. Questions 1. How many chromosomes does each cell in Telophase 1 have? • 23 chromosomes 2. Are the cells identical? Explain? • Each cell is haploid (23 chromosomes). • Not identical because of crossing over.

  23. Division II – A Mitotic Division of Haploid Cells

  24. The second Meiotic division goes through all the normal phases of Mitosis (Prophase II, Metaphase II, Anaphase II, and Telophase II). • The sole purpose of the second division is to separate chromatids just like Mitosis does.

  25. Anomalies that can occur during Meiosis Nondisjunction: When a centromere doesn’t split causing a daughter cell to receive an extra chromosome, while the other daughter cell doesn’t get any.

  26. Deletion: The chromosome that lacks genes as a result of crossing over.

  27. Crossing Over: • During Prophase I chromosomes paired by synapsis can become tangled. • Tangling can result in pieces of chromosomes being exchange (crossing over). • This means maternal traits can be swapped to a paternal chromosome and vice versa.

  28. Duplication: A result of unequal crossing over. During prophase I, a chromosome may break off and exchange genes unequally with its homolog. One chromosome will get additional genes (duplication) while the other chromosomes will be lacking those genes completely. Trisomy 21

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