bio 7

• Cell cycle


  The cycle from the birth of a cell until it divides into 2 cells
• Phases of the cell cycle

  • Interphase (G1, S, G2)
  • Mitotic phase

  
• Interphase


  • 90-95% of the time
  • Cell is preparing to divide (cell growth and DNA replication)

  
• Mitotic phase


  • 5-10% of the time
  • Mitosis: the duplicated chromosomes are equally distributed among 2 daughter cells

  
• Not all cells are actively going through the cell cycle (e.g. red blood cells, neurons, muscle)
• Interphase


  1. G1: first growth phase

  2. S: DNA synthesis

  3. G2: second growth phase
• Chromosomes


  • Long linear DNA molecule (DNA + histones = chromatin)
  • Contain 100-1000s of genes

  
• Sister chromatids


  When a chromosome is duplicated, the duplicated chromosomes remain attached at a region called the centromere

  
• Each species has a characteristic number of chromosomes

  
• Somatic cells (any cell except a sperm or egg cells) have 46 chromosomes in humans

  
• Gametes (sperm and egg) have 23 chromosomes in humans
• Why gametes have half the number of chromosomes


  To maintain the characteristic chromosome number when the gametes fuse during fertilization
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  
• Prophase


  • Chromosomes condense
  • Mitotic spindle forms (microtubules)
  • Centrosomes (2 centrioles) move apart

  
• Metaphase


  • Chromosomes line up in center of cell (in single file!)

  
• Anaphase


  • Sister chromatids are separated and pulled to opposite sides of the cell
• Telophase


  • Spindle fibers disassemble, nuclear envelope reforms
  • Chromosomes un-coil
• Cytokinesis


  1. A cleavage furrow is created by a ring of actin microfilaments

  2. The ring pinches inwards until the cell divides in two!

  
• Cytokinesis results in 2 identical daughter cells with the same number of chromosomes as the parent cell
• Interphase, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis


  • Visualized in the image sequence