meiosis and mitosis

• Cell cycle


  The sequence of events that cells go through as they grow and then divide, and then grow and divide again
• Mitosis


  1. Division of the nucleus

  2. Prophase

  3. Metaphase

  4. Anaphase

  5. Telophase
• Cytokinesis


  The division of the cell cytoplasm
• Phases of the cell cycle

  • G1 (growth phase 1)
  • S (DNA synthesis phase)
  • G2 (growth phase 2)
  • Mitosis
  • Cytokinesis
• G1 phase


  • Cell growth, transcription and translation, organelle replication
• S phase


  • DNA replication, chromosomes become X-shaped
• G2 phase


  • Cell prepares for mitosis, produces proteins needed for mitosis
• G0 phase


  Quiescent phase where cell is not actively preparing to divide
• G1/S checkpoint


  Checks if DNA is damaged before allowing cell to proceed to S phase
• p53


  Tumor suppressor gene that detects DNA damage at G1/S checkpoint
• Oncogenes


  Genes that can cause uncontrolled cell division if mutated
• Interphase


  The phases of the cell cycle that are not mitosis (G1, S, G2)
• Prophase


  Chromosomes condense, nuclear envelope breaks down, spindle starts to form
• Metaphase


  Chromosomes align at the metaphase plate, attached to spindle fibers
• The centromere is the central point of each chromosome where the sister chromatids are attached
• Spindle fibers are made of the protein tubulin
• Spindle


  Microtubules made of the protein tubulin, arranged in a special way coming out from the centrioles
• Metaphase


  Chromosomes line up at the metaphase plate/equator in the center of the cell
• Anaphase


  Sister chromatids separate and are pulled to opposite poles of the cell
• Telophase


  Nuclear envelope reforms around the chromosomes at the poles
• Cytokinesis is the division of the cell cytoplasm, distinct from mitosis which is just the division of the nucleus
• Motor proteins


  Pull the centromeres along the spindle fibers made of tubulin
• Mitosis produces identical cells for growth, repair, and asexual reproduction
• Mitosis does not occur in prokaryotes, they have binary fission instead
• Mitotic index


  Percentage of cells actively undergoing mitosis
• High mitotic index in human tissues can indicate early stage cancer
• Meiosis


  Produces non-identical haploid gametes for sexual reproduction
• Meiosis


  Two rounds of cell division, with prophase/metaphase/anaphase/telophase in each round
• Bivalent


  Pair of homologous chromosomes that join up in prophase 1 of meiosis
• Meiosis


  1. Divide the number of chromosomes by 2

  2. Two rounds of division

  3. Prophase 1

  4. Metaphase 1

  5. Anaphase 1

  6. Telophase 1

  7. Prophase 2

  8. Metaphase 2

  9. Anaphase 2

  10. Telophase 2
• Homologous chromosomes


  Pair of chromosomes, one from each parent
• Homologous chromosomes in meiosis

  • Join up in a bivalent
  • Undergo crossing over (chiasmata)
• Metaphase 1 in meiosis

  Homologous chromosomes line up on the metaphase plate/equator
• Anaphase 1 in meiosis


  Homologous chromosomes separate and move to opposite poles
• Crossing over during meiosis results in exchange of genetic material between non-sister chromatids
• Prophase 2 to Telophase 2 in meiosis


  Similar to mitosis, with chromosomes/chromatids separating
• The first meiotic division separates homologous chromosomes, the second meiotic division separates sister chromatids
• Diploid cells


  Start with 2 copies of each chromosome
• During S phase


  DNA is replicated, resulting in 4 copies of each chromosome
• After meiosis I


  Cells are haploid, with 1 copy of each chromosome