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Sexual Reproduction

Sexual Reproduction. Heredity - passing down of characteristics ( genes ) Alleles - different forms of the same gene. Sexual Reproduction. Why sexual reproduction? shuffles alleles; new combinations provides genetic variation in species. Meiosis. Type of cell division

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Sexual Reproduction

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  1. Sexual Reproduction • Heredity- passing down of characteristics (genes) • Alleles- different forms of the same gene

  2. Sexual Reproduction • Why sexual reproduction? • shuffles alleles; new combinations • provides genetic variation in species

  3. Meiosis • Type of cell division • One germ* cell makes 4 gametes (egg/sperm) with ½ the # of chromosomes. • Occurs only in germ*cells of gonads • testes /ovaries • Occurs in flowers • ovary and anther *“Germ” here doesn’t mean bacteria. It’s a term used for the type cells that gives rise to gametes in organisms.

  4. While females are born with all the eggs they will ever have (around 400,000), only about 400 will ever mature and ovulate between puberty and menopause.

  5. Matrue males produce about 100 million sperm each day.

  6. Sperm surrounding an egg

  7. 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.

  8. 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

  9. Chromosome numbers: • However many “types” of chrom. an organism has, that number is the “n” number of chrom. it has.

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

  11. Animation • Meiosis A animation • Meiosis B animation • Meiosis C animation

  12. Prophase I • Longest and most complex phase (90% of meiosis). • DNA condenses into chromosomes. • Synapsis - a process: when homologous chrom. come together, pair up, form a tetrad.

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

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

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

  16. 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.

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

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

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

  20. 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

  21. Gamete Formation in Animals • Diff. bet. male and female gametes. Male: spermatogenesis • all 4 develop into sperm cells. Female: oogenesis • cytokinesis is uneven. • most cytoplasm goes to 1 of the 4 “eggs”(forms 1 large egg cell) • 3 other cells are small “polar bodies” which die

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

  23. 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

  24. 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)

  25. 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

  26. Karyotype… normal male

  27. Karyotype… normal female

  28. Downs Syndrome • Trisomy 21 • 47, XY, +21 • The only trisomysurvivable toadulthood

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