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Meiosis

Meiosis. Advanced Animal Science. Meiosis. Type of cell division One germ cell makes 4 gametes with ½ the # of chrom. Occurs only in germ cells in gonads testes /ovaries Occurs in flowers ovary and anther. Sperm surrounding an egg.

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Meiosis

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  1. Meiosis Advanced Animal Science

  2. Meiosis • Type of cell division • One germ cell makes 4 gametes with ½ the # of chrom. • Occurs only in germ cells in gonads • testes /ovaries • Occurs in flowers • ovary and anther

  3. Sperm surrounding an egg

  4. This shows how only one single sperm gets to penetrate the egg, releasing its nucleus of 23 chromosomes to merge with the nucleus of the egg and its 23 chromosomes.

  5. Homologous Chromosomes • Pair of chrom. similar in shape , size, and types of genes. • Each locus (location of the gene) in same position on chrom. • Humans have 23 pairs of homologues • Housefly – 6 prs • Chicken – 39 prs • Apple – 17 prs • Dog – 39 prs • Cat – 19 prs This is a karyotype (an image of an organism’s chromosomes) This is a karyotype of a normal human male

  6. eye color locus eye color locus hair color locus hair color locus Paternal(fromDad) Maternal(from Mom) Homologous Chromosomes

  7. Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis - a process: when hom. chrom. come together, pair up, form a tetrad.

  8. sister chromatids sister chromatids Tetrad Prophase I - Synapsis Nonsister chromatids

  9. Prophase I -Crossing Over • Crossing over may occur in the tetrad:between nonsister chromatids, ends break and reattach

  10. Tetrad nonsister chromatids Chiasma: site of crossing over Crossing Over - Provides Variation variation

  11. Metaphase I • Shortest phase; paired homologues align. • INDEPENDENT ASSORTMENToccurs • pairs of homologues line up independently of other pairs’ orientation toward the poles -- random. Adds variation.

  12. Anaphase I • Homologous chromosomes separate towards the poles (Tetrads separate) • Sister chromatids remain attached

  13. Telophase I • Each pole now has haploid set of chromosomes (however – still doubled). • Cytokinesis occurs: two haploid daughter cells formed.

  14. Meiosis II • No interphase II ( no more DNA replication) • Remember: Meiosis II is similar to mitosis Prophase II / Metaphase II Anaphase II

  15. Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs (2nd time). • Four haploid daughter cells produced (chromosomes now back to single condition). gametes ~ sperm or egg; ovule or pollen grain

  16. Gamete Formation in Animals • Diff. bet. male and female gametes. Male: spermatogenesis • all 4 develop into sperm cells. Female: oogenesis • cytokinesis in meiosis is uneven. • most of cytoplasm goes into 1 of the 4 meiotic products (forms large egg cell) • 3 other cells are small “polar bodies”, break down (extra chrom. lost).

  17. n=23 human germ cell in testes sperm n=23 Still doubled chromosomes n=23 2n = 46 haploid (n) n=23 n=23 Still doubled chromosomes diploid (2n) n=23 meiosis I meiosis II Spermatogenesis

  18. Oogenesis 23 human germ cell in ovary n=23 Still doubled Polar Bodies 23 2n = 46 23 n=23 Still doubled Ovum n=23 diploid (2n) meiosis I meiosis II

  19. Mitosis vs Meiosis • Mitosis • Body (somatic cells) • 2 daughter cells made(identical) • Each w/ same # & kind of chrom. as parent cell • 1 division process • 1 cytokinesis • No synapsis or crossing over • Are diploid (2n) • Meiosis • Germ cells of gonads • 4 gamete cells made(all different) • Each w/ ½ chrom. # as parent cell • 2 divisions • 2 cytokineses events • Synapsis & crossing over occurs in Prophase 1 • Are haploid (n)

  20. Chromosome numbers: • However many “types” of chromosomes an organism has, that number is the “n” number of chromosomes it has. • Body cells (somatic cells) therefore have a“2n” # (diploid) • Gametes have “n” # (haploid) • For humans, 2n = 46n = 23

  21. Nondisjunction • When the tetrad (in Anaphase I) orthe sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes. • Lethal most of the time

  22. Karyotype… male

  23. Karyotype… female

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