2.2 All cells arise from other cells

• Describe the stages of the cell cycle in eukaryotic cells

  1. Interphase

  ● (S phase) DNA replicates semi-conservatively

  ○ Leading to 2 chromatids (identical copies) joined at a centromere

  ● (G1/G2) number of organelles & volume of cytoplasm increases, protein synthesis

  2. Mitosis

  ● Nucleus divides

  ● To produce 2 nuclei with identical copies of DNA produced by parent cell

  3. Cytokinesis

  ● Cytoplasm and cell membrane (normally) divide

  ● To form 2 new genetically identical daughter cells
• Draw the stages of the cell cycle in eukaryotic cells

  Photo

  
• What happens in Stage 1 Prophase

  ● Chromosomes condense, becoming shorter / thicker (so visible) ○ Appear as 2 sister chromatids joined by a centromere

  ● Nuclear envelope breaks down

  ● Centrioles move to opposite poles forming spindle network
• What happens in Stage 2 Metaphase

  ● Spindle fibres attach to chromosomes by their centromeres

  ● Chromosomes align along equator
• What happens in Stage 3 Anaphase

  ● Spindle fibres shorten / contract

  ● Centromere divides

  ● Pulling chromatids (from each pair) to opposite poles of cell
• What happens in Stage 4 Telophase

  ● Chromosomes uncoil, becoming longer / thinner

  ● Nuclear envelopes reform = 2 nuclei

  ● Spindle fibres / centrioles break down

  
• Why do some eukaryotic cells not undergo the cell cycle?

  ● Within multicellular organisms, not all cells retain the ability to divide (eg. neurons)

  ● Only cells that do retain this ability go through a cell cycle
• Explain the importance of mitosis in the life of an organism


  Parent cell divides to produce 2 genetically identical daughter cells for

  ● Growth of multicellular organisms by increasing cell number

  ● Replacing cells to repair damaged tissues

  ● Asexual reproduction
• Describe how tumours and cancers form

  Mitosis is a controlled process

  ● Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division

  ● Tumour formed if this results in mass of abnormal cells

  ○ Malignant tumour = cancerous, can spread (metastasis)

  ○ Benign tumour = non-cancerous
• Suggest how cancer treatments control rate of cell division

  ● Some disrupt spindle fibre activity / formation

  ○ So chromosomes can’t attach to spindle by their centromere

  ○ So chromatids can’t be separated to opposite poles (no anaphase)

  ○ So prevents / slows mitosis

  ● Some prevent DNA replication during interphase

  ○ So can’t make 2 copies of each chromosome (chromatids)

  ○ So prevents / slows mitosis More effective against cancer cells due to uncontrolled cell division but also disrupts cell cycle of rapidly dividing healthy cells
• Describe how prokaryotic cells replicate

  Binary fission:

  1. Replication of circular DNA
  2. Replication of plasmids
  3. Division of cytoplasm to produce 2 daughter cells

  ● Single copy of circular DNA

  ● Variable number of copies of plasmids

  
• Describe how viruses replicate

  Being non-living, viruses do not undergo cell division

  1. Attachment proteins attach to complementary receptors on host cell
  2. Inject viral nucleic acid (DNA/RNA) into host cell
  3. Infected host cell replicates virus particles: a. Nucleic acid replicated

  b. Cell produces viral protein / capsid / enzymes

  c. Virus assembled then released