all cells arise from other cells

• State what the cell cycle is

  cycle of division with intermediate growth periods
• outline the stages of the cells cycle

  1. interphase

  
  

  2. mitosis or meiosis (nuclear division)

  
  

  3. cytokinesis (cytoplasmic division)
• Explain why the cell cycle does not occur in some cells

  After differentiation, some types of cell in multicellular organisms (e.g. neurons) no longer have the ability to divide
• What is the difference between the cell cycle and mitosis?

  - Cell cycle includes growth period between divisions

  
  

  - mitosis is only 10% of the cycle & refers only to nuclear division
• Outline what happens during interphase

  G1: cell synthesises proteins for replication e.g. tubulin for spindle fibres & cell size doubles

  
  

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

  
  

  G2: organelles divide
• State the purpose of mitosis

  produces 2 genetically identical daughter cells for:

  ● Growth

  ● Cell replacement/ tissue repair

  ● Asexual reproduction
• Name the stages of mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  (PMAT)
• Outline what happens during prophase

  1) Chromosomes condense, becoming visible. (X-shaped: 2 sister chromatids joined at centromere)

  
  

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

  
  

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

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

  requires energy from ATP hydrolysis

  
  

  1) Spindle fibres contract = centromeres divide.

  
  

  2) Sister chromatids separate into 2 distinct

  chromosomes & are pulled to opposite poles of

  cell (looks like 'V' shapes facing each other).

  
  

  3) Spindle fibres break down.
• Outline 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
• Explain the 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).
• Outline 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
• Name 2 dyes that bind to chromosomes

  ● toluidine blue (blue)

  ● acetic orcein (purple-red)
• Why is only the root tip used when calculating a mitotic index?

  ● Meristematic cells at root tip are actively undergoing mitosis.

  ● Cells further from root tip are elongating rather than dividing
• What are tumour suppressor genes?

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

  
  

  (e.g. p53 acts between G1 & S in interphase so damaged DNA cannot replicate).
• What are proto-oncogenes?


  Genes that code for proteins to stimulate cell cycle to progress from one stage to the next
• How can mutation to tumour suppressor genes & proto-oncogenes 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.
• Suggest how cancer treatments control the rate of cell division.

  Disrupt the cell cycle:

  ● prevent DNA replication

  ● disrupt spindle formation = inhibit metaphase

  / anaphase

  
  

  NB: can also damage healthy cells
• How do prokaryotic cells replicate?

  Binary fission:

  1) 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.
• Estimate the exponential growth of bacteria within 8 hours. Assume binary fission occurs once every 20 minutes & there is 1 bacterium at the start

  8 x 60 = 480 mins

  480 / 20 = 24 divisions

  2^24
• Why are viruses classified as non-living?

  They are acellular: no cytoplasm, no metabolism & cannot self-replicate
• Outline 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 do new viral particles leave the host cell?

  a) Bud off & use cell membrane to form envelope.

  b) Cause lysis of host cell
• Why is it so difficult to develop effective treatments against viruses?

  Replicate inside living cells = difficult to kill them without killing host cells
• what is produced in mitosis?

  2 identical daughter cells
• what is produced in meiosis?

  4 haploids
• how may parent cells are involved in mitosis?

  1
• how may parent cells are involved in meiosis?

  2
• how many divisions are in mitosis?

  2
• how many divisions are in meiosis?

  1
• describe what happens in apoptosis

  - cells shrink + develop bubble like structures on the surface

  - DNA+protein degrade

  - mitochondria break down releasing enzymes

  - cell disintegrate into small membrane fragments

  - phagocytes identify + digest
• how is the cell cycle controlled?

  - by chemical triggers

  - apoptosis
• describe cytokenises


  during cytokinesis the parent and replicated organelles move to opposite sides of the cell and thecytoplasm dividesthus producing two daughter cells