Nuclear division

• Mitosis


  • Produces somatic cells
  • Tissue repair
  • Growth (including embryonic development)
  • Asexual reproduction
  • 1 division, forms 2 diploid cells
• Meiosis


  • Gametes
  • Sexual reproduction
  • 2 divisions, forms 4 haploid cells
  • Reduction division
• Interphase


  1. G1 - growth, increased cell size, increased enzymes involved in DNA replication

  2. S - DNA replication

  3. G2 - growth, increased cell size, increased organelles

  4. DNA doubles

  5. Chromosomes can't be seen
• Prophase


  1. Nuclear envelope breaks down

  2. Chromosomes + chromatids condense + become visible
• Metaphase


  1. Chromosomes meet at the equator attached to spindles

  2. Spindles composed of microtubules, anchored + from centrioles

  3. Centrioles make microtubules - made of protein

  4. Centrosome contains centrioles

  5. Spindle fibers are composed of protein microtubules and kinetechors
• Anaphase


  Sister chromatids pulled to opposite ends
• Telophase


  1. Nuclear envelope reforms

  2. Cytokinesis - complete division of cytoplasm
• Prophase I


  1. Chromosomes arranged in homologous pairs (tetrad)

  2. Crossing over occurs in non-sister chromatids to form bivalents

  3. Chiasmata - point of crossing over
• Metaphase I


  1. Homologous chromosomes line up in the equator

  2. Random orientation of homologous chromosomes
• Cytokinesis (Animals)


  1. A ring of contractile actin + myosin protein filament form + constrict

  2. Cleavage furrow forms, cells pinch off from the middle
• Cytokinesis (Plants)


  1. Golgi pinches off into vesicles, migrates to the center

  2. Vesicles fuse + form two plasma membrane layers

  3. Cell plate develops until it connects with the plasma membrane

  4. The daughter cells secrete cellulose
• Unequal division of the cytoplasm

  • Budding in yeast (asexual reproduction where a small daughter cell buds off a larger cell)
  • Oogenesis (formation of eggs, unequal division during meiosis to form a large egg cell)
• Condensation of chromosomes


  • Chromatin condenses to form visible chromosomes
  • DNA coils around histone proteins to form nucleosomes
  • Nucleosomes coil around each other to form chromosomes with sister chromatids
• Non-disjunction of chromosomes


  • Failure of one or more pairs of homologous chromosomes or sister chromatids to separate fully during nuclear division (anaphase I or II)
  • Produces gametes with an extra or missing chromosome
  • Downs is due to trisomy 21
• Cell proliferation


  • Makes more cells
  • Growth, cell replacement, tissue repair
  • Process where a cell grows and divides
  • Embryonic development is mitosis + cytokinesis
  • Plant meristems - undifferentiated cells -> tissues + organs
  • Tissue replacement + healing -> skin cells
• The cell cycle


  1. Regulated by cyclins - four proteins that regulate cell movement during the cycle

  2. Bind to a group of enzymes called CDKs

  3. Cyclins phosphorylate the CDKs, allowing the kinases to act as enzymes

  4. A phosphate group is attached

  5. Activated enzymes cause the cell cycle to go in between phases

  6. The points where these function are called checkpoints

  7. G1: G0->G1 and G1->S

  8. G1/S : prepare for DNA replication

  9. G2: activate DNA replication inside the nucleus

  10. M: mitotic spindle assemble + other tasks
• Tumor suppressor genes


  • Regulate cell division
  • Inhibit cell proliferation and tumor development
  • Mutations -> loss in function -> uncontrolled cell division
• Proto-oncogenes


  • Genes that regulate normal cell growth
  • Mutations may make it become an onco gene
  • Oncogene -> change a cell into a tumor cell
• Cancer


  • Uncontrolled cell division
  • Primary tumor: first tumor produced by the body
  • Metastasis: spread of cancer cells throughout the body
• Benign


  Non cancerous, grows slowly
• Malignant


  Cancerous, grows fast, undergo metastasis
• Cell cycle regulation


  Enzymes called cyclins, bind to CDKS, phosphorylate the CDK so they can act as enzymes and move a cell into the next stage of the cycle, occurs at checkpoints
• G1


  Protein synthesis, produce enzymes for DNA replication
• S


  DNA replication
• G2


  Produce organelles
• Prophase 1


  Nuclear envelope breaks down, chromatins condense, crossing over
• Anaphase 1


  Homologous chromosomes line up in the centre
• Metaphase 1


  Homologous chromosomes are in the centre, attached to spindle fibres, pulled to the edge
• Telophase


  Nuclear envelope reforms, prepare for cytokinesis