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Meiosis & Sexual Reproduction

Meiosis & Sexual Reproduction. Chapter 13. Human Life Cycle. Human Life Cycle: 1) 23 pairs of homologous chromosomes 2) 1 pair of sex chromsomes – #23 3) 22 pairs of autosomes – #1-22 4) Karyotype : picture of chromosomes

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Meiosis & Sexual Reproduction

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  1. Meiosis & Sexual Reproduction Chapter 13

  2. Human Life Cycle • Human Life Cycle: 1) 23 pairs of homologous chromosomes 2) 1 pair of sex chromsomes – #23 3) 22 pairs of autosomes – #1-22 4) Karyotype: picture of chromosomes 5) Gametes are haploid (n) 6) All other cells are diploid (2n) 7) Fertilization (syngamy) results in a zygote • Meiosis: cell division to produce haploid (n) gametes

  3. Alternative Life Cycles • Fungi/Some Algae: **Meiosis produces n cells that divide by mitosis to make n adults (gametes by mitosis) • Plants/Some Algae: **Alternation of Generations: 2n sporophyte, by meiosis, makes n spores; spore divides by mitosis to generate a n gametophyte; gametes are formed by mitosis, which then fertilize into 2n sporophyte

  4. Meiosis • Preceded by DNA/chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II) • Produces 4 daughter cells with ½ the number of chromosomes • (2n n) • Creates variation

  5. Animation of Meiosis • Meiosis Animation • Homologous chromosomes separate in Meiosis I • Chromosomes separate in Meiosis II • Reduction in chromosome # occurs in Meiosis I • End Result: 4 cells  genetically different from each other and from the parent cell with half the number of chromosomes as the parent cell • Crossing over in Prophase I creates variation

  6. Mitosis vs. Meiosis • Synapsis/tetrad/chiasmata (Prophase I)  Xing over • Homologous vs. individual chromosomes (Metaphase I) • Sister chromatids do not separate (Anaphase I) • Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes

  7. Genetic Variation: Part I • Independent Assortment: Homologous pairs of chromosomes position & orient randomly (metaphase I) • Combinations Possible  2n; with n being the haploid # of the organism

  8. Genetic Variation: Part II • Crossing Over: The reciprocal exchange of genetic material b/w non-sister chromatids during synapsis of Meiosis I (Prophase I) • Maternal & paternal chromosomes get “mixed” up

  9. Genetic Variation: Part III • Random Fertilization: 1 sperm (of 8 million) X 1 egg (of 8 million) 64 trillion diploid combinations

  10. Summary of Meiosis • Makes sex cells or gametes • Reduces the chromosome # from 2n n • DNA Replication occurs before meiosis • Two cell divisions • Synapsis of homologous chromosomes  crossing over (Prophase I) • Makes 4 genetically different cells • Introduces genetic variation • Associated with sexual reproduction • Unique Features of Meiosis

  11. Spermatogenesis • Occurs in the seminiferous tubules of the testes • Makes 4 sperm per cell undergoing meiosis • Continuous process!

  12. Oogenesis • Occurs in the ovary; about every 28 days • Makes 1 egg/ovum per cell undergoing meiosis • Through prophase I  occurs in female embryo • At puberty  completes Meiosis I • Ovulation  Meiosis II occurs if egg is fertilized!

  13. Spermatogenesis Cytokinesis is equal Occurs thorughout a male’s entire life Uninterrupted sequence of meiosis Oogenesis Cytokinesis is unequal Occurs through menopause and stops. At birth, an ovary has all the primary oocytes it will ever have. Long periods of rest; interrupted sequence of meiosis Spermatogenesis vs. Oogenesis

  14. Abnormal # of Chromosomes in Gametes • Nondisjunction: chromosomes do not separately correctly in meiosis  during Meiosis I (homologous chromosomes) or Meiosis II (sister chromatids) • Result may be gametes with 22 or 24 chromosomes

  15. Picture of Nondisjunction

  16. TRISOMY Three copies of 1 chromosome Total of 47 chromosomes Down Syndrome – #21 XXX Klinefelter Syndrome– #23 XXY MONOSOMY One copy of a chromosome Total of 45 chromosomes Turner Syndrome – #23 X0 Nondisjunction

  17. Nondisjunction • High frequency in humans • Most embryos are spontaneously aborted • Alterations in chromosome # are too detrimental • Developmental problems may result from imbalance of chromosomes

  18. Down Syndrome • Trisomy 21 • 1 in 700 children born in U.S. • Frequency increases as mother’s age increases

  19. Down Syndrome Karyotype

  20. Klinefelter Syndrome • XXY Male • 1 in every 2000 live births • Male sex organs present, but are sterile • May have feminine characteristics • Normal intelligence • Tall

  21. Klinefelter Syndrome Karyotype

  22. Turner Syndrome • X0 Female • Monosomy of #23 • 1 in every 5000 live births • Webbed neck • Short stature • Immature sterile females

  23. Turner Syndrome Karyotype

  24. Jacob’s Syndrome Male • XYY Male • 1 in 1000 live male births • Extra Y chromosome • More active • Slight learning disabilities • Delayed emotional immaturity • Normal intelligence • Normal sexual development • Also called criminal karyotype

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