All cells arise from other cells

• Cell cycle

  Cycle of division with intermediate growth periods
• Stages of the cell cycle

  1. Interphase

  2. Mitosis or meiosis (nuclear division)

  3. Cytokinesis (cytoplasmic division)
• Some cells do not undergo the cell cycle
• Difference between cell cycle and mitosis

  Cell cycle includes growth period between divisions; mitosis is only 10% of the cycle & refers only to nuclear division
• What happens during interphase

  1. G1: cell synthesises proteins for replication

  2. S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere

  3. G2: organelles divide
• Purpose of mitosis


  Produces 2 genetically identical daughter cells for growth, cell replacement/tissue repair, and asexual reproduction
• Stages of mitosis

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• What happens during prophase

  1. Chromosomes condense, becoming visible

  2. Centrioles move to opposite poles of cell (animal cells) & mitotic spindle fibres form

  3. Nuclear envelope & nucleolus break down = chromosomes free in cytoplasm
• What happens during metaphase

  Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres
• What happens during anaphase

  1. Spindle fibres contract = centromeres divide

  2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell

  3. Spindle fibres break down
• What happens during telophase

  1. Chromosomes decondense, becoming invisible again

  2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome
• Procedure for a root tip squash experiment

  1. Prepare a temporary mount of root tissue

  2. Focus an optical microscope on the slide. Count total number of cells in the field of view and number of cells in a stage of mitosis

  3. Calculate mitotic index (proportion of cells undergoing mitosis)
• How to prepare a temporary mount of root tissue

  1. Place root in hydrochloric acid to halt cell division & hydrolyse middle lamella

  2. Stain root tip with a dye that binds to chromosomes

  3. Macerate tissue in water using mounted needle

  4. Use mounted needle at 45° to press down coverslip & obtain a single layer of cells. Avoid trapping air bubbles
• Dyes that bind to chromosomes

  • Toluidine blue (blue)
  • Acetic orcein (purple-red)
• Tumour suppressor genes

  Genes that code for proteins to trigger apoptosis (programmed death of damaged cells)/ slow cell cycle
• Proto-oncogenes

  Genes that code for proteins to stimulate cell cycle to progress from one stage to the next
• How mutation to tumour suppressor genes & proto-oncogenes can cause cancer

  • Tumour suppressor: no production of a protein needed to slow the cell cycle
  • Proto-oncogenes: form permanently-activated oncogenes
  • Disruption to cell cycle → uncontrolled cell division → tumour
• How cancer treatments control the rate of cell division

  Disrupt the cell cycle: prevent DNA replication, disrupt spindle formation = inhibit metaphase / anaphase
• How prokaryotic cells replicate

  1. Binary fission: DNA loop replicates, both copies stay attached to cell membrane, plasmids replicate in cytoplasm

  2. Cell elongates, separating the 2 DNA loops

  3. Cell membrane contracts & septum forms

  4. Cell splits into 2 identical progeny cells, each with 1 copy of the DNA loop but a variable number of plasmids
• Exponential growth of bacteria within 8 hours, with binary fission every 20 minutes and 1 bacterium at the start
• Why viruses are classified as non-living
• How viruses replicate


  1. Attachment proteins attach to receptors on host cell membrane

  2. Enveloped viruses fuse with cell membrane or move in via endocytosis & release DNA/ RNA into cytoplasm OR viruses inject DNA/ RNA

  3. Host cell uses viral genetic information to synthesise new viral proteins/ nucleic acid

  4. Components of new viral particle assemble
• How new viral particles leave the host cell

  • Bud off & use cell membrane to form envelope
  • Cause lysis of host cell
• Why it is difficult to develop effective treatments against viruses
• Root tip


  The tip of a plant root, used in calculating a mitotic index due to its rapid cell division, meristematic tissue, accessibility, consistency, and comparability