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Understanding Meiosis and Genetic Variation in Sexual Reproduction

Meiosis is a vital process in sexual reproduction, producing haploid gametes which form diploid zygotes. Somatic cells, with 46 chromosomes, divide through mitosis. Explore the key stages of meiosis, including the unique events of crossing over and non-disjunction anomalies.

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Understanding Meiosis and Genetic Variation in Sexual Reproduction

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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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