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MEISOSIS & SEXUAL LIFE CYCLES

MEISOSIS & SEXUAL LIFE CYCLES. CAMPBELL and REECE Chapter 13. Definitions. Heredity : transmission of traits from 1 generation to the next Variation : differences between members of same species Genetics : scientific study of heredity & hereditary variation

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MEISOSIS & SEXUAL LIFE CYCLES

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  1. MEISOSIS & SEXUAL LIFE CYCLES CAMPBELL and REECE Chapter 13

  2. Definitions Heredity: transmission of traits from 1 generation to the next Variation: differences between members of same species Genetics : scientific study of heredity & hereditary variation Genes: discrete unit of hereditary information consisting of specific nucleotide sequence in DNA or RNA (some viruses) Gametes: a haploid reproductive cell (ova, sperm) Somatic Cells: any cell in multicellular organism except ova or sperm Locus: specific place along length of a chromosome where a given gene is location

  3. Inheritance of Genes genetic “program” written in language of DNA DNA: the polymer of 4 nucleotides most genes program cells to synthesize specific enzymes & proteins whose cumulative actions produces organism’s inherited traits

  4. Gametes vehicle used by plants & animals to transmit genes to next generation male + female gamete unite during fertilization

  5. Asexual Reproduction offspring are identical genetically to parent used by unicellular & some multicellular eukaryotic organisms cells of offspring derived by mitosis aka clones mutations can occur in asexual reproduction

  6. Sexual Reproduction 2 parents give rise to offspring that are genetically different from either parent

  7. Life Cycles is the generation-to-generation sequence of stages in reproductive history of an organism

  8. Human Chromosomes • 46 (23 pair) (2n) • 22 pair autosomes • 1 pair sex chromosomes • condense during mitosis so large enough to separate based on: • size • position of centromere • patterns of colored banding produced by stains

  9. Karyotype ordered display of chromosome pairs of a cell, arranged by size & shape pairs of matching chromosomes have same genes @ same loci & are called homologous chromosomes or homologs

  10. Karyotype : Human

  11. Sex Chromosomes • Not homologous • determine individual’s sex: • XX • XY

  12. Diploid # / Haploid # Diploid # Haploid # • 2 sets of chromosomes in cell: 1 set maternal source; 1 set paternal source • Humans = 46 • Gametes have 1 set of chromosomes • Humans = 23

  13. Behavior of Chromosome Sets in Human Life Cycle haploid sperm + haploid ova fertilize  fusion of their 2 nuclei fertilized egg called a zygote (diploid) thru mitosis this fertilized egg will produce all somatic cells in mature adult only cells not produced by mitosis are the gametes which develop from germ cells in gonads (ovary or testes)

  14. Meiosis creates 1n gametes so when fertilization occurs  2n offspring human life cycle typical of most sexually reproducing animals, plants, fungi, & protisits

  15. Variations of Sexual Life Cycles • all sexual reproduction involves meiosis & fertilization but there is variation in timing • 1. gametes are the only haploid cells • humans & most other animals

  16. Variations of Sexual Life Cycles 2. Alternation of Generations plants & some species of algae involves both diploid & haploid multicellular stages Sporophyte: multicellular diploid state Spores: haploid cells produced by sporophyte, divides by mitosis  Gametophyte: haploid multicellular stage that produces gametes (1n) which will fertilize producing 2n sporophyte

  17. Variations of Sexual Life Cycles 3. Haploid Cells Predominant occurs in most fungi & some protists haploid gametes fuse & form diploid zygote  meiosis produces haploid cells (not gametes) which then either divided by mitosis forming multicellular haploid organisms or unicellular descendants

  18. Stages of Meiosis

  19. Meiosis I starts with diploid cell with duplicated chromosomes ends with separation of homologous chromosomes in 2 daughter cells which are each haploid but with duplicated chromosomes (reductional division)

  20. Prophase I chromosomes condense *homologs align along their lengths paired homologs become physically connected to each other along their lengths by a “zipper-like” protein structure: synaptonemal complex this state called synapsis

  21. Prophase I continued Crossing Over: genetic rearrangement between nonsister chromosomes involves exchange of segments of DNA Chiasma: exists wherever cross over has occurred spindle forms, nuclear envelope fragments, (as in prophase in mitosis)

  22. Prophase I

  23. Metaphase I pairs of homologous chromosomes arranged @ metaphase plate both chromatids of 1 homolog attached to kinetochore microtubule from 1 pole; those of other homolog attached to kinetochore from other pole

  24. Metaphase I

  25. Anaphase I proteins that hold sister chromatids together breakdown allowing homologs to separate homologs move toward opposite poles guided by spindles sister chromatid cohesion persists at the centromere so duplicated chrtomosomes move to opposite poles

  26. Anaphase I

  27. Telophase I & Cytokinesis I each ½ of cell has complete haploid set of duplicated chromosomes cytokinesis usually forms simultaneously w/telophase forms 2 haploid daughter cells in some species, chromosomes decondense & nuclear envelope reforms No chromosome duplication occurs between Meiosis I & II

  28. Meiosis II starts with 2 haploid cells (with duplicated chromosomes) sister chromatids separate ends with 4 haploid daughter cells each genetically different than parent cell

  29. Prophase II spindle apparatus forms

  30. Metaphase II chromosomes positioned @ metaphase plate 2 sister chromatids no longer genetically identical (due to crossing over) Kinetochores of sister chromatids attached to microtubules extending from opposite poles

  31. Metaphase II

  32. Anaphase II breakdown of proteins holding the sister chromatids together @ centromere allows chromatids to separate chromatids move toward opposite poles as individual chromosomes

  33. Anaphase II

  34. Telophase II & Cytokinesis II • nuclei form • chromosomes condense • cytokinesis occurs • 4 daughter cells • each with haploid set of unduplicated chromosomes • 4 daughter cells are genetically distinct from each other & from parent cell

  35. Telophase II & Cytokinesis II

  36. Meiosis

  37. Mitosis / Meiosis

  38. Origins of Genetic Variation Independent Assortment of Chromosomes Crossing Over Random Fertilization

  39. Independent Assortment orientation of pairs of homologous chromosomes at metaphase I is random each pair may orient with either its maternal or paternal homolog closer to a given pole 50% chance that a particular daughter cell of meiosis I will contain the maternal chromosome of a certain homolog (50% chance paternal)

  40. Independent Assortment because humans have n = 23 the # of possibilities due to independent assortment is 2²³ or about 8.4 million possible combinations of maternal/paternal chromosomes in any 1 gamete

  41. Crossing Over • produces recombinant chromosomes: individual chromosomes that carry genes derived from 2 different parents • in humans: average of 1 to 3 crossover events occur per chromosome pair • depends on size of chromosome & position of centromere

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