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Cell Cycle & Mitosis Meiosis. Context. All living things (cells) come from other living things (cells) Cell division is necessary for: Reproduction Growth and development Tissue renewal Cell cycle – describes the life cycle of a cell. Somatic Cells vs. Gametes. Somatic cells – body cells

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Cell Cycle & Mitosis Meiosis

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  • All living things (cells) come from other living things (cells)
  • Cell division is necessary for:
    • Reproduction
    • Growth and development
    • Tissue renewal
  • Cell cycle – describes the life cycle of a cell
somatic cells vs gametes
Somatic Cells vs. Gametes
  • Somatic cells– body cells
    • Contain entire genome within nucleus (or two copies of every chromosome)
  • Gametes – sex cells (i.e. sperm or egg)
    • Contain only half (or one copy (from mom or dad) of every chromosome)
phases of the cell cycle
Phases of the Cell Cycle
  • M (mitotic) phase:
    • When mitosis (cell division) occurs
  • Interphase:
    • G1(Gap 1)
      • Cell grows
    • S (Synthesis aka DNA replication)
      • Cell replicates DNA
    • G2 (Gap 2)
      • Cell grows and prepares for cell division

Chromatin DNA + proteins (histones) in eukaryotic cells

  • Chromosome structures consisting of chromatin
  • Sister chromatid one half of a replicated chromosome

Centromere point of connection between sister chromatids

  • Kinetochore protein complex found at centromere

Centrosome organelle that organizes microtubules

  • Centriole animals cells only (function unknown)
phases of mitosis literal cell division
Phases of Mitosis (literal cell division)

1st- G2 of interphase

  • Nuclear envelope forms
  • Centrosomes (& centrioles in animal cells) appear

2nd- Prophase

  • Chromatin condenses into chromosomes
  • Mitotic spindle appears

3rd- Prometaphase

  • Nuclear envelope breaks up (fragments)
  • Microtubules attach to centromeres at kinetochore

4th – Metaphase

  • Chromosomes meet @ middle (metaphase plate)
  • Spindle fibers attached to each chromatid at kinetochore

5th – Anaphase

  • Two sister chromatids pull apart at centromere and move towards opposite end of cell (towards centrosomes)

6th – Telophase and Cytokinesis

  • Two daughter nuclei form from fragments of original nucleus
  • Chromatin becomes less condensed
  • Cytokinesis – division of cytoplasm and formation of two daughter cells
    • Animal cells  involves cleavage furrow
    • Plant cells  involves cell plate (formed by vesicles!)
cell cycle control
Cell Cycle Control
  • Frequency of cell division varies with cell type
    • RBCs every 24hr
    • Mature nerve cells never divide
  • Cell cycle checkpoints:
cyclin dependent kinases cdk
Cyclin-dependent kinases (Cdk)
  • Kinase – an enzyme that activate or inactivate other proteins by phosphorylation
  • Cyclin – protein who’s concentration fluctuates cyclically
  • Cancer cells derive from normal cells gone wrong
    • Ex: Mutation in gene that regulates cell cycle checkpoint; now cell does not stop at that checkpoint but just keeps dividing.
  • Cancer cells also
    • Have no contact inhibition
    • Aren’t anchorage dependent (thus metastasize)
    • Express vascularization proteins (bring in blood vessels to feed tumor)
    • And so much more.
  • Process whereby gametes (sex cells) are produced for sexual reproduction purposes
    • Eggs in female; sperm in male
    • Sexual reproduction  egg meets sperm = fertilization
      • Offspring have genetic variation
    • Asexual reproduction  mitotic cell division in single-celled eukaryotes (e.g. amoeba)
      • Clones (parent and offspring are identical)
diploid vs haploid
Diploid vs. Haploid
  • In humans, each somatic cell has 46 chromosomes(23 from mom & 23 from dad) or 23 homologous chromosomes (homologues)
  • Somatic cells are diploid (full set of chromosomes) or 2n
  • Gametes are haploid (half set) or n
steps of meiosis
Steps of Meiosis
  • 2 rounds:
    • Meiosis I (4n to 2n)
    • Meiosis II (2n to n)
meiosis i
Meiosis I
  • Interphase:
    • Diploid cell’s chromosomes duplicate during interphase (2n  4n)
    • Centrosomes replicate

Prophase I

    • Chromosomes condense
    • Homologous pairs match up and become physically connected at synaptonemal complex via process called synapsis
    • CROSSING OVER  Genetic exchange of information between non-sister chromatids

Metaphase I

    • Homologous pairs line up on metaphase plate in tetrads
  • Anaphase I
    • Homologous pairs split up BUT sister chromatids stay together!
  • Telophase I & Cytokinesis
    • 2 daughter cells, both 2n, which go on to divide again
meiosis ii
Meiosis II
  • Prophase II
  • Metaphase II
    • Chromosomes meet at metaphase plate; sister chromatids are NOT identical (due to crossing over)
  • Anaphase II
    • Centromeres of each chromosome separate (sister chromatids pull apart)
  • Telophase II and cytokinesis
    • 4 haploid non-identical daughter cells are produced
end result of meiosis
End Result of Meiosis
  • 2n  4n  2n  n
  • Production of four haploid daughter cells all of which are genetically distinct from each other and the parent cell
genetic variation
Genetic variation
  • Mutations in DNA
  • Crossing over during prophase I of meiosis followed by (hopefully) random sexual reproduction
  • Independent assortment of homologous pairs during metaphase I