Lecture 5

-The cell cycle: repeating pattern of cell growth and division, nuclear division divides chromosomes equally into genetically identical daughter cells. Cells grow, DNA replicated between cell divisions

-Interphase (3 parts) 

  • G1 phase: cell growth (gap 1 before duplication)

  • S phase: chromosomes duplicate to form sister chromatids (synthesis)

  • G2 phase: cell growth, synthesis of proteins needed for mitosis (gap 2 before mitosis)

  • Most cell growth occurs during G1 and G2 phases

  • Terminally differentiated cells: stop dividing and arrest in G0 phase (Ex: mature neurons)

  • Formation of microtubules in cytoplasm

    • Centrosome: microtubule organizing center near nuclear envelope

    • Centrioles: core of centrosome (not in plant cells), where microtubules extend from. Surrounded by many proteins (pericentriolar material)

-M phase (5 parts): Mitosis (nuclear division) and cytokinesis (cell division)

  • Prophase: 

    • Chromosomes condense (visible) to stop transcription

    • Centrosomes move to opposite poles

    • Nucleoli (site of rRNA synthesis) begin to disappear, stops ribosome production, not needed, focus energy elsewhere

  • Prometaphase:

    • Nuclear envelope breaks down

    • Microtubules (made mostly of tubulin) from centrosomes connect to kinetochores in the centromere of each chromatid (sister chromatids attach from opposite poles)

      • Kinetochores chromosomal structure composed of DNA and proteins, at site where chromosomes attach to spindle fibers

    • Mitotic spindle forms (three kinds of microtubules):

      • Astral microtubules: short, unstable, extend from centrosome towards cell periphery, stabilize mitotic spindle

      • Kinetochore microtubules: extend from centrosome to a kinetochore of a chromatid. Chromosomes move along kinetochore microtubules during cell division

      • Polar microtubules: extend from centrosome to middle of the cell, interlock with polar microtubules from opposite poles to push spindle poles apart in anaphase

  • Metaphase: 

    • Sister chromatids face opposite poles, line up on metaphase plate: imaginary equator of the cell

    • Forces pushing and pulling chromosomes to or from each pole, balanced equilibrium to keep chromosomes in middle

      • Pushing/ pulling due to assembly/ disassembly of microtubules

      • Polymerization: at + end, adding

        • At chromatid end to stabilize (equilibrium)

      • Depolymerization: at - end, removing 

        • Rapidly at both ends to pull apart chromatids

  • Anaphase: 

    • Centromeres all split simultaneously

    • Kinetochore microtubules shorten, pull sister chromatids opposite poles (V-shape)

  • Telophase:

    • Nuclear envelope forms around chromatids

    • Nucleoli reform

    • Spindle fibers disappear

    • Chromosome uncoil into chromatin



-Cytokinesis: Cytoplasm divides, begins during anaphase but not completed until after telophase 

  • Animal cells: contractile ring contracts to form cleavage furrow

    • Contractile ring: thin band of actin and myosin II filaments located in cell cortex, slide together to generate force to cleave cell

  • Plant cells: cell plate forms near equator

  • Both: randomly, organelles are distributed to each daughter cell (possible one gets more)

-If mitosis occurs without cytokinesis following, leads to multi-nucleated cells (genetic material duplicated, no cell division)

  • Syncytium: multi-nucleated cells usually early in development

    • Embryo contains several nuclei in a shared cytoplasm, surrounded in common plasma membrane

  • Coenocyte: multi-nucleated cells other than early development

  • Fertilized Drosophila eggs, 13 rounds of mitosis occur without cytokinesis

    • Syncytial blastoderm: syncytium embryo with many nuclei in a single cell

      • Pole cells?



-Cell cycle checkpoints: system of sensor proteins recognize abnormalities/damage to cells or DNA. If detects damage, triggers a temporary cell cycle stop to allow cell to repair damage

  • If damaged beyond repair:

    • Apoptosis: programmed cell death

    • Senescence: permanent cell arrest

    • Cancerous: mutated cell, uncontrolled growth

  • G1 checkpoint (end of G1): DNA damage, nutrients, growth factors, replication proteins, nucleotides, get ready for replication

    • Also G0 (resting state)

  • G2 checkpoint (end of G2): Correct DNA replication (if not fully replicated, no cell division), cell size

  • Metaphase checkpoint: chromosomes lined up in middle, chromosome spindle attachment (wrong attachment, daughter cells end with incorrect chromosome numbers)



-Somatic cells: majority of cells in organism, in G0 or actively undergo mitosis

-Germ cells: precursors to gametes

  • Set aside during embryogenesis, become incorporated into reproductive organs

  • Only cells that undergo meiosis to produce haploid gametes (fertilization creates diploid offspring)

-Meiosis: Cell division where four daughter cells are produced, that halves # of chromosomes, otherwise chromosome # will double in the next generation

  • Chromosomes duplicate once, nuclei divide twice (2 rounds of cell division)

-Meiosis I: reduces chromosome # from 2n to n

-Meiosis II: produces 4 haploid nuclei cells

  • Prophase I: homologous chromosomes pair (synapsis), held together by synaptonemal complex, crossing over occurs

    • Leptotene: Chromosomes begin to condense, moving toward opposite poles

    • Zygotene: synapsis begins, synaptonemal complex forms

      • Synapsis: homologous chromosomes aligned and “zipped” together during zygotene (called bivalent, 2 chromosomes, or tetrad, 4 chromosomes

      • Synaptonemal complex: protein complex that holds homologous chromosomes together

    • Pachytene: Synapsis complete, crossing over (genetic exchange between non sister chromatids)

      • Crossing over: exchange of material between maternal and paternal chromatid of corresponding section of DNA (genetic recombination)

        • Occurs at recombination nodules: protein structures that appear at regular intervals along the synaptonemal complex

        • Involves breaking & re-joining chromosomes, can result in the exchange of alleles

    • Diplotene: Synaptonemal complex dissolves, tetrad of 4 chromatids visible, crossover points appear as chiasmata, meiotic arrest occurs at this time in many species. 

Diakinesis: Chromatids thicken, shorten, nuclear membrane breaks down, spindle fibers form