2.2 All cells arise from other cells

• Describe the interphase stage of the cell cycle in eukaryotic cells

  • ● (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
• Describe the mitosis stage of the cell cycle in eukaryotic cells

  • ● Nucleus divides
  • ● To produce 2 nuclei with identical copies of DNA produced by parent cell
• Describe the cytokinesis stage of the cell cycle in eukaryotic cells

  ● Cytoplasm and cell membrane (normally) divide

  ● To form 2 new genetically identical daughter cells
• Describe the behaviour of chromosomes & role of spindle fibres in mitosis - during 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

  
• Describe the behaviour of chromosomes & role of spindle fibres in mitosis - during metaphase


  ● Spindle fibres attach to chromosomes by their centromeres

  ● Chromosomes align along equator

  
• Describe the behaviour of chromosomes & role of spindle fibres in mitosis - during anaphase


  ● Spindle fibres shorten / contract

  ● Centromere divides

  ● Pulling chromatids (from each pair) to opposite poles of cell

  
• Describe the behaviour of chromosomes & role of spindle fibres in mitosis - during 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
• 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
  4. Single copy of circular DNA
  5. 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:
  4. Nucleic acid replicated
  5. Cell produces viral protein / capsid / enzymes
  6. Virus assembled then released
• RP2: Describe how to prepare squashes of cells from plant root tips

  1. Cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide
  2. Soak root tip in hydrochloric acid then rinse
  3. Stain for DNA eg. with toluidine blue
  4. Lower coverslip using a mounted needle at 45° without trapping air bubbles
  5. Squash by firmly pressing down on glass slip but do not push sideways
• RP2: Why are root tips used?

  ● Where dividing cells are found / mitosis occurs
• RP2: Why is a stain used?

  ● To distinguish chromosomes

  ● Chromosomes not visible without stain
• RP2: Why squash / press down on cover slip?

  ● (Spreads out cells) to create a single layer of cells

  ● So light passes through to make chromosomes visible
• RP2: Why not push cover slip sideways?

  ● Avoid rolling cells together / breaking chromosomes
• RP2: Why soak roots in acid?

  ● Separate cells / cell walls

  ● To allow stain to diffuse into cells

  ● To allow cells to be more easily squashed

  ● To stop mitosis
• RP2: Describe how to set-up and use an optical microscope

  1. Clip slide onto stage and turn on light
  2. Select lowest power objective lens (usually x 4)
  3. 3a. Use coarse focusing dial to move stage close to lens
  4. 3b. Turn coarse focusing dial to move stage away from lens until image comes into focus
  5. Adjust fine focusing dial to get clear image
  6. Swap to higher power objective lens, then refocus

  
• RP2: What are the rules of scientific drawing?

  • ✓ Look similar to specimen / image
  • ✓ No sketching / shading - only clear, continuous lines
  • ✓ Include a magnification scale (eg. x 400)
  • ✓ Label with straight, uncrossed lines
• RP2: Explain how the interphase stage of mitosis can be identified


  In interphase (not mitosis), chromosomes aren’t visible but nuclei are. In mitosis, chromosomes are visible.
• RP2: Explain how the prophase stage of mitosis can be identified

  • ● Chromosomes visible / distinct → because condensing
  • ● But randomly arranged → because no spindle activity / not attached to spindle fibre

  
• RP2: Explain how the metaphase stage of mitosis can be identified


  ● Chromosomes lined up on equator → because attaching to spindle

  
• RP2: Explain how the anaphase stage of mitosis can be identified

  • ● Chromatids (in two groups) at poles of spindle
  • ● Chromatids V shaped → because being pulled apart at their centromeres by spindle fibres

  
• RP2: Explain how the telophase stage of mitosis can be identified


  ● Chromosomes in two sets, one at each pole

  
• RP2: What is a mitotic index?

  ● Proportion of cells undergoing mitosis (with visible chromosomes)

  ● Mitotic index = number of cells undergoing mitosis / total number of cells in sample
• RP2: Explain how to determine a reliable MI from observed squashes


  ● Count cells in mitosis in field of view

  ● Count only whole cells / only cells on top and right edges → standardise counting

  ● Divide this by total number of cells in field of view

  ● Repeat with many / at least 5 fields of view selected randomly → representative sample

  ● Calculate a reliable mean
• RP2: Suggest how to calculate the time cells are in a certain phase of mitosis

  1. Identify proportion of cells in named phase at any one time
  2. ● Number of cells in that phase / total number of cells observed
  3. Multiply by length of cell cycle