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Outline for today’s lecture (Ch. 13)

Outline for today’s lecture (Ch. 13). Sexual and asexual life cycles Meiosis Origins of Genetic Variation Independent assortment Crossing over (“recombination”). Heredity. Transmission of traits between generations Molecular basis of heredity is DNA replication

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Outline for today’s lecture (Ch. 13)

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  1. Outline for today’s lecture (Ch. 13) • Sexual and asexual life cycles • Meiosis • Origins of Genetic Variation • Independent assortment • Crossing over (“recombination”)

  2. Heredity • Transmission of traits between generations • Molecular basis of heredity is DNA replication • Gene is a specific segment of DNA • Physical location on the chromosome is called a genetic LOCUS (plural = “loci”) • e.g., the “eye-color locus”, Adh locus

  3. Asexual Life Cycles • Single (diploid) individual is the parent • Parent passes copies of ALL its genes to its offspring (reproduces “clonally”) • Various mechanisms • Mitotic cell division in unicellular Eukaryotes • Vegetative reproduction, e.g., plant cuttings, hydra budding • Parthenogenesis

  4. Sexual Life Cycles • Two (diploid) parents give rise to offspring • Offspring differ genetically from their parents and their siblings • GAMETES are haploid reproductive cells that transmit genes across generations

  5. Sexual Life Cycles • Key Point: Sexual reproduction → Genetic variation • MOST eukaryotes reproduce sexually at least sometimes • Most prokaryotes (e.g., bacteria) exchange genes at least occasionally

  6. Sexual Life Cycles – Human Example • 46 Chromosomes • 22 Homologous pairs, called “autosomes” • Same length • Same centromere position • Same sequence (+/-) • SAME GENES!!

  7. Sexual Life Cycles – Human Example • One pair of “sex chromosomes” • i.e., “sex-determining gene(s)” reside on these chromosomes • Females are XX • Males are XY • Only small region of homology (= same genes) between X, Y X Y

  8. Schematic drawing of a chromosome

  9. X 2 2 1 X 1 Diploid cell, n=3 BEFORE DNA replication • 3 Homologous Pairs • 2 autosomes • 1 sex chromosome (XX) • One homologous chromosome from each parent • DNA content = 2C • Ploidy = 2n

  10. X X 2 2 2 2 1 1 X X 1 1 Diploid cell, n=3, AFTER DNA replication • 3 Homologous Pairs • One homologous chromosome from each parent = TWO SISTER CHROMATIDS • DNA content = 4C • Ploidy = 2n

  11. Sexual Life Cycles - animals • Free-living stage is diploid • Gametes formed by meiosis • Haploid gametes merge genomes to form diploid zygote (“syngamy”)

  12. Sexual Life Cycles - Plants • Diploid sporophyte forms haploid spores by meiosis • Spores form gametophyte by mitosis • Gametophyte forms gametes by mitosis • Gametes merge to form diploid zygote

  13. Sexual Life Cycles - Fungi • Free-living, multicellular organism is haploid • Gametes formed by mitosis • Gametes merge to form diploid zygote • Zygote undergoes meiosis to form haploid cells

  14. Meiosis • RECALL: Function of MITOSIS is to faithfully replicate the parental genome in each daughter cell with no change in information content • Function of MEIOSIS is to produce haploid cells from diploid cells • Necessary for the formation of gametes • Necessary for sexual reproduction

  15. Meiosis – an overview • Interphase 1 – • Begin with two homologous chromosomes, • DNA content = 2C • Ploidy = 2n (diploid)

  16. Meiosis – an overview • Interphase 1 – • Chromosomes replicate • DNA content = 4C • Ploidy = 2n

  17. Meiosis – an overview • “Meiosis I” • Homologous chromosomes separate • Cell Division #1 • Result is TWO haploid (ploidy = n) cells with TWO SISTER CHROMATIDS of one of the two homologs

  18. Meiosis – an overview • “Meiosis II” • Sister chromatids separate • Cell Division # 2 • Result is FOUR haploid daughter cells, each with an unreplicated chromosome (= 1C) • Half as many chromosomes as the parent cell

  19. Chiasmata Tetrad Meiosis I – early Prophase I • Homologous chromosomes pair • Synaptonemal complex (proteins) attaches homologs • “synapsis” • Homologs form tetrad

  20. Meiosis I – late Prophase I Chiasmata Spindle fiber • Chromosomes cross over, form “chiasmata” • Exchange of DNA between homologs occurs at chiasma • Spindles form and attach to kinetochores as in mitosis Tetrad

  21. Meiosis I – Metaphase I • Chromosomes lined up on metaphase plate in homologous pairs • Spindles from one pole attach to one chromosome of each pair • Spindles from the other pole attach to the other chromosome of the pair Kinetochore

  22. Meiosis I – Anaphase I • Homologous chromosomes separate and move along spindle fibers toward pole • Sister chromatids remain attached at centromeres • Note that recombination has occurred!

  23. Meiosis I – Telophase and cytokinesis • Homologous chromosomes reach (opposite) poles • Each pole has complete haploid complement of chromosomes • Each chromosome consists of two sister chromatids

  24. Meiosis II – Prophase II • Spindle forms • Chromosomes move toward metaphase plate

  25. Meiosis II – Metaphase II • Chromosomes reach metaphase plate, as in mitosis • Kinetochores of sister chromatids attach to spindle fibers from opposite poles

  26. Meiosis II – Anaphase II • Centromeres of sister chromatids separate • Sister chromatids move toward opposite poles

  27. Meiosis II – Telophase and cytokinesis • Mechanism as before • Note that now FOUR HAPLOID DAUGHTER CELLS formed from each parent cell • Note that some chromosomes are recombinant, some are not

  28. Meiosis I - Summary Chiasma (site of crossing-over) Tetrad formed by synapsis of homologs

  29. Meiosis I - Summary Tetrads align at metaphase plate

  30. Meiosis I - Summary Homologous chromosomes separate Sister chromatids remain paired

  31. Meiosis II - Summary Sister chromatids separate Haploid daughter cells result

  32. Origins of Genetic Variation • Independent Assortment of Chromosomes • Recombination among chromosomes • Crossing over • Recombination within chromosomes • Random fertilization

  33. Independent Assortment of Chromosomes

  34. Independent Assortment of Chromosomes • Number of possible combinations of chromosomes within a gamete • Two homologs A, B: Mom = A1B1, Dad = A2B2 • Four combinations: A1B1, A1B2, A2B1, A2B2 • Three homologs: Mom = A1B1C1, Dad = A2B2C2 • Eight combinations: A1B1C1, A1B1C2, A1B2C1, A1B2C2, A2B1C1, A2B2C1, A2B1C2, A2B2C2 • n homologs: 2n combinations

  35. Crossing-over – Recombination within chromosomes • Averages ≥ 2 per chromosome per meiosis in humans, flies • If no crossing-over, genes on same chromosomes would always be inherited together

  36. Crossing-over – Recombination within chromosomes Human genome has ~20K genes. Suppose each gene assorts independently. How many combinations?

  37. Review: Mitosis vs. Meiosis Event Mitosis Meiosis DNA Replication Interphase Interphase I # Cell Divisions 1 2 # Daughter cells 2 4 “Ploidy” of daughters 2n (diploid) n (haploid) Synapsis of homologs? No Yes Crossing-over No Yes (recombination) Biological Purpose Duplicate cells Generate faithfully gametes

  38. Meiosis, Genetic variation, and Evolution • Role of segregation • Role of crossing-over • What about LIMITS to evolution? • E.g., body size

  39. For Thursday: Introduction to Mendelian Genetics • Read Chapter 14 through p. 260

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