All cells arise from other cells

• Within multicellular organisms, not all cells retain the ability to divide.
• Eukaryotic cells that do still divide show a cell cycle.
• 2 types of cell division

  1. Mitosis
  2. Meiosis
• Products of mitosis

  Two genetically identical daughter cells
• Purpose of mitosis

  1. Growth
  2. Repair of damaged tissue
• 1. Prophase

  • Chromosomes condense and shorten
  • Centrioles move to opposite poles
  • Nucleolus disappears and nuclear envelope breaks down
  • Spindle fibres start to form and attach to centromeres
• 2. Metaphase

  • Spindle fibres pull chromosomes so that they line up along the equator
• 3. Anaphase

  • Centromeres divide, separating each pair of sister chromatids
  • Spindle fibres shorten and pull chromatids to opposite poles of the cell- now chromosomes again
• 4. Telophase

  • Chromosomes begin to lengthen and uncoil, disappearing.
  • Separate nuclear envelopes form around each group of chromosomes.
  • Spindle fibres disintegrate
  • Cytoplasm divides to form two daughter cells (cytokinesis)
• Most of the cell cycle is...

  Interphase
• Stages of interphase: G1

  • Cell grows and increases in volume as new cytoplasm and organelles are made.
• Stages of interphase: S (synthesis)

  • Cell replicates its DNA
• Stages of interphase: (G2)

  • Cell continues to grow
  • Synthesises enzymes and structures needed for mitosis
• Mitotic index

  $\frac{number\ of\ cells\ undergoing\ mitosis}{total\ cells\ in\ sample}$
• Cancer

  • Uncontrolled cell division
  • Leads to tumours
• Benign tumours

  • Slow growing
  • Do not spread to other parts of the body
• Malignant tumours

  • Cancerous
  • Grows quickly
  • Invade neighbouring tissues
• Cancer treatment

  • Interrupts cell cycle
  • Does not distinguish between normal cells and cancerous cells
  • Treatments are more likely to kill tumour cells as they divide much more rapidly
• Cancer treatments

  1. Surgery- tumour removed
  2. Radiotherapy- radiation damages DNA, which is checked during cell cycle. Severe damage= cell kills itself
  3. Chemotherapy- preventing division or causing damage so they kill themself e.g. preventing the synthesis of enzymes needed for DNA replication
• Binary fission (prokaryotic cells)

  • Cell replicates its genetic material before splitting into two genetically identical daughter cells.
  • Very rapid

  1. Replication of circular DNA and plasmids.
  2. Circular DNA strands move to opposite poles.
  3. Cytoplasm divides and new cell walls grow to divide the original cell into two identical daughter cells, each with one copy of the circular DNA and a variable no. of plasmids.