All cells arise from other cells

• Cell cycle

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

  • Interphase
  • Mitosis or meiosis (nuclear division)
  • Cytokinesis (cytoplasmic division)
• Some cells do not undergo the cell cycle after differentiation
• 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
• 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
• 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
• Metaphase

  Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres
• 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
• 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
• 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 and number of cells in a stage of mitosis

  3. Calculate mitotic index (proportion of cells undergoing mitosis)
• Preparing a temporary root tip mount

  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
• 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
• Mutation to tumour suppressor genes & proto-oncogenes

  Disruption to cell cycle → uncontrolled cell division → tumour
• How cancer treatments control cell division

  Disrupt the cell cycle: prevent DNA replication, disrupt spindle formation = inhibit metaphase / anaphase
• Prokaryotic cell replication (binary fission)

  1. DNA loop replicates, both copies stay attached to cell membrane

  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
• Viral replication

  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