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The Continuity of Life: Cellular Reproduction

The Continuity of Life: Cellular Reproduction

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  • 1. Continuity of Life: Cellular Reproduction
  • 2. Cellular Reproduction
    • Intracellular activity between one cell division to the next is the cell cycle
      • Some activities involve growth (enlargement) of the cell
      • Some activities involve duplication of genetic material and cellular division (reproduction)
    • Binary Fission (prokaryotes)
    • Mitosis (new individual cells)
      • Budding (eukaryotes)
    • Meiosis (new gametes)
  • 3. The Prokaryotic Cell Cycle
      • Long growth phase
        • Replication of circular DNA chromosome occurs
        • Duplicate chromosomes anchored to membrane
      • Cell increases in size, pulling duplicated chromosomes apart…
      • Plasma membrane grows inward between chromosome copies
      • Fusion of membrane along cell equator completes separation ( binary fission or “splitting in two”)…
      • Daughter cells are genetically identical
      • Under ideal conditions Escherichia coli bacteria complete a cell cycle every 20 minutes
  • 4. Binary Fission DNA replicated Membrane added
  • 5. Binary Fission 2 constriction fission
  • 6. False-Color EM of Dividing Bacterium Division plane Cell wall Cytoplasm Nuclear material
  • 7. Functions of Mitosis Mitotic cell division Mitotic cell division & differ-entiation Tissues Organs Fertilized egg (zygote) Multicell stage
  • 8. Protozoa: Asexual Reproduction by Mitosis New individuals
  • 9. Yeasts: Asexual Reproduction by Mitosis
    • Budding
    • Nucleus divides by mitosis
    • Bud forms on cell
    • Nucleus moves into bud
    • Bud separates
  • 10. Hydra: Asexual Reproduction by Mitosis
  • 11. Genetically Identical Aspen Groves
    • Three separate aspen groves
    • Each produced asexually from single ancestor
    • Variable between groves
    • Identical within groves
    Trees synchronously lost leaves Trees synchronously turned yellow Trees still green
  • 12. Chromosome Condensation
    • single-stranded chromosome
    • double helix uncondensed
    DNA replication
    • double-stranded chromosome
    • double helices still uncondensed
    Chromosome condensation 1 double-stranded chromosome 2 double helices now condensed cell base pairs closer look still closer look even closer look centromere  a chromatid
  • 13. Human Chromosomes during Mitosis
  • 14. Human Karyotype, Male
    • These are chromosomes from mitosis
    • Stained to show regions
    • Numbered by length
    • Occur in pairs
  • 15. The Eukaryotic Cell Cycle G 0 : nondividing telophase metaphase anaphase cell division interphase S: Synthesis of DNA; chromosomes duplicated G 1 : Growth G 2 : Growth prophase cytokinesis Mitosis
  • 16. Phases of Mitosis, 1 Interphase : The chromosomes (blue) are in the thin, extended state and appear as a mass in the center of the cell. The microtubules (red) extend outward from the nucleus to all parts of the cell. Metaphase : The chromosomes have moved along the spindle microtubules to the equator of the cell. Late prophase : Chromosomes (blue) have condensed and attached to microtubules of spindle fibers (red). Microtubules have reorganized to form the spindle; chromosomes, now condensed, are clearly visible.
  • 17. Separation of Sister Chromatids
    • In metaphase, sister chromatids are held together at centromere
    • At end of metaphase, centromere releases sister chromatids
    • In anaphase, they move to opposite poles
  • 18. Phases of Mitosis, 2 Anaphase : Sister chromatids have separated, and one set of chromosomes moves along the spindle microtubule to each pole of the cell. Telophase : The chromosomes have gathered into two clusters, one at the site of each future nucleus. Next interphase : Chromosomes are relaxing again into their extended state. Spindle fibers are disappearing, and the microtubules of the 2 daughter cells rearrange into the interphase pattern.
  • 19. Mitosis: Prophase - Metaphase Kinetochores align at cell’s equator Nucleolus disappears; Nuclear envelope breaks down Microtubules attach to kinetochores Chromosomes condense and shorten Centrioles begin to move apart; Spindle forms Duplicated chromosomes remain elongated Centrioles have also been duplicated Late Interphase Early Prophase Late Prophase Metaphase
  • 20. Mitosis Anaphase - Cytokinesis Free spindle fibers push poles apart Chromatids become independent chromosomes One set of chromosomes; Begin unwinding Nuclear envelope re-forms Cytoplasm divided along equator Each daughter gets 1 nucleus & half of cytoplasm Spindle disappears; Nucleolus reappears Anaphase Telophase Cytokinesis Next Interphase
  • 21. Cytokinesis of a Ciliated Cell Cleavage Furrow Daughter Cells
  • 22. Cytokinesis in Plants Vesicles fuse to form cell wall and membranes Complete separation of daughter cells
  • 23. Control of Cell Cycle
    • The cells of some tissues divide frequently throughout lifespan
        • e.g. skin, intestine
    • Cell division occurs rarely or not at all in other tissues
        • e.g. brain, heart, skeletal muscles
    • Cell division in eukaryotes is driven by enzymes and controlled at specific checkpoints
  • 24. Enzymes Drive the Cell Cycle
      • The cell cycle is driven by proteins called C yclin- d ependent k inases, or Cdk’s
      • Kinases are enzymes that phosphorylate (add a phosphate group to) other proteins, stimulating or inhibiting their activity
      • Cdk’s are active only when they bind to other proteins called cyclins
  • 25. Enzymes Drive the Cell Cycle
      • Cell division occurs when growth factors bind to cell surface receptors, which leads to cyclin synthesis
      • Cyclins then bind to and activate specific Cdk’s
  • 26. Enzymes Drive the Cell Cycle
      • Activated Cdk’s promote a variety of cell cycle events
        • Synthesis and activation of proteins required for DNA synthesis
        • Chromosome condensation
        • Nuclear membrane breakdown
        • Spindle formation
        • Attachment of chromosomes to spindle
        • Sister chromatid separation and movement
  • 27.  
  • 28. Checkpoints Control Cell Cycle
      • Although Cdk’s drive the cell cycle, multiple checkpoints ensure that…
        • The cell successfully completes DNA synthesis during interphase
        • Proper chromosome movements occur during mitotic cell division
      • There are three major checkpoints in the eukaryotic cell cycle, each regulated by protein complexes
        • G1 to S:
        • G2 to mitosis
        • Metaphase to anaphase
  • 29.  
  • 30. Checkpoints Control Cell Cycle
      • G1 to S: Ensures that the cell’s DNA is suitable for replication
        • p53 protein expressed when DNA is damaged
          • Inhibits replication
          • Stimulates synthesis of DNA repair enzymes
          • Triggers cell death (apoptosis) if damage can’t be repaired
  • 31.  
  • 32. Checkpoints Control Cell Cycle
      • G2 to mitosis: Ensures that DNA has been completely and accurately replicated
        • p53 protein expression leads to decrease in synthesis and activity of an enzyme that facilitates chromosome condensation
        • chromosomes remain extended and accessible to DNA repair enzymes, which fix DNA before cell enters mitosis
  • 33. Checkpoints Control Cell Cycle
      • Metaphase to anaphase: Ensures that the chromosomes are aligned properly at the metaphase plate
        • a variety of proteins prevent separation of the sister chromatids if there are defects in chromosome alignment or spindle function
  • 34. Prevalence of Sexual Reproduction
    • Asexual reproduction by mitosis produces genetically identical offspring
    • Sexual reproduction by meiosis shuffles the genes to produce genetically unique offspring
      • Wide use of sexual reproduction suggests that DNA reshuffling is advantageous
      • Variation in offspring provided by sexual reproduction confers a large evolutionary advantage
  • 35. Genetic Variability from Mutation
    • Mutations are the ultimate source of genetic variability
    • Most mutations are harmful or lethal; a few are neutral or even beneficial
    • Mutation gives rise to new alleles
    • Alleles are alternate gene forms that may produce differences in structure or function
    • Homologous chromosomes carry alleles for the same genes or characteristics
      • Each chromosome may carry a different allele of a gene (e.g. for eye color)
  • 36.  
  • 37. Combination of Parental Alleles
    • Combining the parental chromosomes through sexual reproduction can produce offspring with allelic combinations that may be advantageous
    • Sexual reproduction causes variability
      • Combinations of gene alleles on one homologous chromosome are combined with combinations of gene alleles on the other homologous chromosome
  • 38. Combination of Parental Alleles
      • Sexual reproduction causes variability
      • 2. Different homologous chromosomes with certain alleles are combined with other homologous chromosomes in a random manner…
      • 3. Two gametes produced by meiosis each contribute their unique allelic combinations to produce a new offspring
  • 39. Meiosis I Homologous chromosomes pair and cross over Homologous chromosomes exchange DNA & align on equator Homologous chromosomes move to opposite poles Prophase I Metaphase I Anaphase I Telophase I
  • 40. Meiosis II Prophase II Metaphase II Anaphase II Telophase II Four Haploid Cells Similar to Mitosis
  • 41. Crossing Over
    • Homologues pair up
    • Protein strands zip together
    • Recombination enzymes snip and rejoin DNA
    • Homologs separate with new gene combinations
  • 42. Meiosis vs. Mitosis: Comparison of Spindles Meiosis: Duplicated chromosomes with one kinetochore; Paired homologues go to opposite poles. Mitosis: Duplicated chromosomes with two kinetochores; Unpaired homologs split between sister chromatids, which go to opposite poles.
  • 43. Meiosis vs. Mitosis: Comparison of Stages
  • 44. Novel Chromosome Combinations
    • Genetic variability among organisms is essential in a changing environment
    • Mutations produce new variation but are relatively rare occurrences
    • Randomized line up and separation of homologous chromosomes in Meiotic Metaphase I and Anaphase I increase variation
      • The number of possible combinations is 2 n , where n = number of homologous pairs
  • 45. Crossing Over Variation also enhanced by genetic recombination Crossing over in Meiotic Prophase I creates chromosomes with new allele combinations Combined with homologue shuffling in Metaphase/Anaphase I, each gamete produced in meiosis is virtually unique
  • 46. Metaphase Alignment Scenarios
  • 47. Fusion of Gametes Fusion of games from two individuals further increases possible 2n combinations Gametes from two humans could produce about 64 trillion different 2n combinations Taken together with crossing over, each human individual is absolutely genetically unique
  • 48. The end