3.2.2 All cells arise from other cells

• Cell cycle in eukaryotic cells:

  • Interphase - S phase DNA replicates semi conservatively -> 2 chromatids (identical copies) joined at the centromere
  • Mitosis - Nucleus divides, to produce with identical copies of DNA produced by parent cell
  • Cytokinesis - Cytoplasm and cell membrane divide to form 2 new gentically identical daughter cells
• Stage 2: Metaphase

  • Spindle fibres attach to chromosomes by their centromeres
  • Chromosomes align along equator

  
• Stage 1: Prophase

  • Chromosomes condense and become shorter and thicker, this appears as 2 sister chromatids joined by a centromere
  • Nuclear envelope breaks down
  • Centrioles move to opposite poles forming a spindle network

  
• Stage 4: Telophase

  • Chromosomes uncoil, becoming longer and thinner
  • Nucleur envelopes reform, forming 2 nuclei
  • Spindle fibres and centrioles then break down

  
• Stage 3: Anaphase

  • Spindle fibres shorten and contract
  • Centromere divides
  • Pulls chromatids from each pair to opposite poles of cell

  
• Some eukaryotic cells don't undergo the cell cyle because within the multicellular organisms, not all cells retain the ability to divide such as neurons. It's only the cells that do retain this ability that go through the cell cycle.
• The importance of mitosis in the life of an organism is that parent cells divide to produce 2 genetically identical daughter cells for:

  • Growth of multicellular organisms by increasing cell number
  • Replacing cells to repair damaged tissues
  • Asexual reproduction
• Tumours and cancer form from mitosis.

  • Mutations in DNA and genes that control mitosis can lead to uncontrolled cell division
  • Tumours formed if this results in a mass of abnormal cells
• Malignant tumours are cancerous and can spread - metastasise
• Benign tumours are non cancerous
• Cancer treatments control the rate of cell division by some disrupting the spindle fibres' activity or its formation

  • So chromosomes can't attach to the spindle by their centromere
  • So chromatids can't be separated to opposite poles so there's no anaphase
  • So it prevents and slows mitosis

  Some prevent DNA replication during interphase

  • So it can't make 2 copies of each chromosome (chromatids)
  • So it prevents and slows down mitosis
• Prokaryotes replicate through binary fission
• Binary fission happens by:

  • Replication of circular DNA
  • Replication of plasmids
  • Division of cytoplasm to produce 2 daughter cells
  • SIngle copy of circular DNA
  • Variable number of copies of plasmids
• Viruses replicate not though cell division because they are non-living
• Viral replicatio happens by:

  • Attachment proteins attaching to complementary receptors on the host cell
  • They then inject viral nucleic acid (DNA/RNA) into host cells
  • Infected host cells replicates virus particles such as:
  • Nucleic acids being replicated
  • Cell produces viral proteins, capsids or enzymes
  • Virus assembles then released