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Meiosis

Meiosis . Heredity. Heredity - the transmission of traits from one generation to the next Genetics – the scientific study of heredity and hereditary variation Genes – hereditary units endowed from parents Segments of DNA Divided into Chromosomes 46 in humans

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Meiosis

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  1. Meiosis

  2. Heredity • Heredity - the transmission of traits from one generation to the next • Genetics – the scientific study of heredity and hereditary variation • Genes – hereditary units endowed from parents • Segments of DNA • Divided into Chromosomes • 46 in humans • A gene’s specific location on a chromosome is called its locus

  3. Reproduction – 2 modes • Asexual reproduction – a single individual is the sole parent and passes copies of all its genes to its off spring • Sexual reproduction – two parents give rise to offspring that have unique combinations of genes inherited from each parent

  4. Asexual Reproduction • 1 parent • Binary Fission in bacteria • Single cell eukaryotes : mitotic cell division • DNA is copied and divided equally between daughter cells • Multicellular organisms – Budding • Hydra : Buds break off – are genetically identical to its parent • Each offspring in asexual reproduction is called a clone

  5. Figure 13.1 The asexual reproduction of a hydra

  6. Sexual Reproduction • 2 parents • Results in greater variation than asexual reproduction • Offspring vary genetically from siblings and both parents • Behavior of chromosomes during the sexual lifecycle

  7. Figure 13.2 Two families

  8. Life Cycle • Generation to generation sequence of stages in the reproductive history of an organism • Interested in Sexual life cycles

  9. Human Lifecycle • Somatic cells (any cell but sperm or ovum cells) have 46 chromosomes • Can be visualized with a light microscope during mitosis • Are two of each type • Arranged in pairs • Karyotype –ordered display of an individuals chromosomes • Homologous chromosomes (homologues) – chromosomes that make up a pair that have the same length , centromere position and staining pattern

  10. Figure 13.3 Preparation of a human karyotype

  11. Human Lifecycle • Autosomes – somatic chromosomes • If a gene for a trait is located at a particular locus on a certain chromosome, then the homologue of that chromosome will also have a gene for the same trait at the same locus • EXCEPTION: SEX CHROMOSOMES • X and Y – only a small part are homologous • Y is much shorter than the X • X has few Y counterparts , Y is lacking many X genes • XX (female) XY (male)

  12. Karyotype • The occurrence of homologous pairs of chromosomes in our karyotype is a consequence of our sexual origins • A maternal set (23) and a parental set (23)

  13. Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

  14. Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

  15. Sperm and Ova • Have a chromosome count of 23 • 22 autosomes – in a single set • Plus a single sex chromosome (X or Y) • HAPLOID (n)

  16. Sperm and Ova – Sexual Intercourse • A haploid sperm reaches and fuses with a haploid ovum • Fertilization of syngamy • Results in a fertilized egg or zygote • The zygote contains the two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines • Diploid (2n) - 2n = 46

  17. Meiosis • Differs from mitosis • The process that halves the number of chromosomes in a cell • Occurs in Ovaries or Testes

  18. A Variety of Sexual Lifecycles • Human Life cycle • Most fungi and some protists (including some algae) • Plants and some other species of algae

  19. Figure 13.4 The human life cycle

  20. Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy) Alternation of generations

  21. Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number • Four daughter chromosomes • IMPORTANT: Homologous chromosomes are different than sister chromatids • 4 Haploid (n) cells instead of 2 diploid cells (2n)

  22. Meiosis I : Separates Homologous Chromosomes • Interphase • Each of the chromosomes replicate • The result is two genetically identical sister chromatids which remain attached at their centromeres

  23. Prophase I • Lasts longer and is more complex than prophase in mitosis • Chromosomes begin to condense and homologues, each consisting of two sister chromatids, pair up • During Synapsis: A protein structure attaches the homologous chromosomes tightly together (synaptonemal complex)

  24. Prophase I • Later in prophase, when the synaptonemal complex disappears, each chromosome pair becomes visible in the microscope as a tetrad • A cluster of four chromatids • At various places along their length, chromatids of homologous chromosomes are crisscrossed • Occur at chiasmata • Hold the homologous pairs together until anaphase I

  25. Prophase I • Other cellular components prepare for division of the nucleus in a manner similar to that of mitosis • Centrosomes move away from each other and spindle microtubules form between them • The nuclear envelope and nucleoli disperse • The spindle microtubules capture the kinetochores that form on the chromosomes • The chromosomes begin moving to the metaphase plate • Can last for days or longer (over 90% of meiosis)

  26. Metaphase I • The chromosomes are now arranged on the metaphase plate • Still in homologous pairs • Kinetochore microtubles from one pole of the cell are attached to one chromosome of each pair while microtubules from the opposite pole are attached to the homologue

  27. Anaphase I • The spindle apparatus guides the movement of the chromosomes toward the poles • Sister chromatids remain attached • Move as a unit towards the same pole • The homologous chromosome moves toward the opposite pole • Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

  28. Telophase I • The members of each pair of homologous chromosomes continue to move apart until they reach the poles of the cell • Each pole now has a haploid chromosome set but each chromosome still has two sister chromatids

  29. Cytokinesis • Occurs simultaneously with telophase I • Forms 2 daughter cells • Plant cells – cell plate • Animal cells – cleavage furrows • NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

  30. Figure 13.7 The stages of meiotic cell division: Meiosis I

  31. Figure 13.7 The stages of meiotic cell division: Meiosis II

  32. Meiosis II : Separates sister chromatids • Proceeds similar to mitosis • THERE IS NO INTERPHASE II !

  33. Prophase II • A spindle apparatus forms and the chromosomes progress toward the metaphase II plate

  34. Metaphase II • The chromosomes are positioned on the metaphase plate in a mitosis-like fashion • Kinetochores of sister chromatids of each chromosome pointing toward opposite poles

  35. Anaphase II • The centromers of sister chromatids finally separate • The sister chromatids of each pair move toward opposite poles • Now individual chromosomes

  36. Telophase II and Cytokinesis • Nuclei form at opposite poles of the cell and cytokinesis occurs • After completion of cytokinesis there are four daughter cells • All are haploid (n)

  37. Figure 13.7 The stages of meiotic cell division: Meiosis II

  38. Figure 13.8 A comparison of mitosis and meiosis

  39. Figure 13.8 A comparison of mitosis and meiosis: summary

  40. Origins of Genetic Variation • As mentioned earlier, in species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation • Independent assortment of chromosomes • Crossing Over • Random Fertilization

  41. Figure 13.9 Independent Assortment

  42. Figure 13.10 Crossing Over

  43. Random Fertilization • A human ovum plus a human sperm • 1 of 8 million combinations possible for each ovum and sperm • 223 X 223 = over 70 billion combinations • 70 trillion possible combinations with out considering crossing over • YOU REALLY ARE UNIQUE!

  44. Gametogenesis • See handout for details

  45. THE END OF MEIOSIS

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