All Cells arise from other Cells

• What is the cell cycle?

  A cycle of division with intermediate growth periods
• What are the stages of the cell cycle?

  1. Interphase
  2. Mitosis or meiosis (nuclear division)
  3. Cytokinesis (cytoplasmic division)
• Why does the cell cycle not occur in some cells?

  Some differentiated cells, like neurons, no longer have the ability to divide
• What is the difference between the cell cycle and mitosis?

  The cell cycle includes growth periods between divisions, while mitosis refers only to nuclear division
• What happens during interphase?

  1. G1: Cell synthesizes proteins for replication and doubles in size
  2. S: DNA replicates, resulting in chromosomes with 2 sister chromatids
  3. G2: Organelles divide
• What is the purpose of mitosis?

  To produce 2 genetically identical daughter cells for growth, cell replacement, and asexual reproduction
• What are the stages of mitosis?

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
• What happens during prophase?

  1. Chromosomes condense and become visible
  2. Centrioles move to opposite poles and mitotic spindle fibers form
  3. Nuclear envelope and nucleolus break down
• What happens during metaphase?


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

  1. Spindle fibers contract, centromeres divide
  2. Sister chromatids separate into distinct chromosomes and are pulled to opposite poles
  3. Spindle fibers break down
• What happens during telophase?

  1. Chromosomes decondense and become invisible
  2. New nuclear envelopes form around each set of chromosomes, resulting in 2 new nuclei
• What is the procedure for a root tip squash experiment?

  1. Prepare a temporary mount of root tissue
  2. Focus an optical microscope on the slide and count total cells and cells in mitosis
  3. Calculate the mitotic index
• How do you prepare a temporary mount of root tissue?

  1. Place root in hydrochloric acid to halt cell division and hydrolyse middle lamella
  2. Stain root tip with a dye that binds to chromosomes
  3. Macerate tissue in water using a mounted needle
  4. Use a mounted needle at 45° to press down the coverslip to obtain a single layer of cells
• Name 2 dyes that bind to chromosomes.

  Toluidine blue and acetic orcein
• Why is only the root tip used when calculating a mitotic index?

  Meristematic cells at the root tip are actively undergoing mitosis
• What are tumour suppressor genes?

  • Genes that code for proteins to trigger apoptosis
  • Slow the cell cycle (e.g., p53 acts between G1 and S in interphase)
• What are proto-oncogenes?


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

  • Tumour suppressor: no production of proteins needed to slow the cell cycle
  • Proto-oncogenes: form permanently-activated oncogenes
  • Disruption to the cell cycle leads to uncontrolled cell division and tumours
• How do cancer treatments control the rate of cell division?

  • Disrupt the cell cycle by preventing DNA replication
  • Disrupt spindle formation to inhibit metaphase/anaphase
  • Note: can also damage healthy cells
• How do prokaryotic cells replicate?

  1. DNA loop replicates, both copies stay attached to the cell membrane
  2. Cell elongates, separating the 2 DNA loops
  3. Cell membrane contracts and septum forms
  4. Cell splits into 2 identical progeny cells
• Estimate the exponential growth of bacteria within 8 hours if binary fission occurs every 20 minutes starting from 1 bacterium.


  After 8 hours, there will be $2^{24}$ bacteria
• Why are viruses classified as non-living?

  They are acellular, have no cytoplasm, no metabolism, and cannot self-replicate
• Outline how viruses replicate.

  1. Attachment proteins attach to receptors on host cell membrane
  2. Enveloped viruses fuse with the cell membrane or enter via endocytosis
  3. Host cell uses viral genetic information to synthesize new viral proteins/nucleic acids
  4. Components of new viral particles assemble
• How do new viral particles leave the host cell?

  • They can bud off and use the cell membrane to form an envelope
  • They can cause lysis of the host cell
• Why is it difficult to develop effective treatments against viruses?

  Viruses replicate inside living cells, making it difficult to kill them without harming host cells