Cell division +cell cycle (Topic 2A)

• State what the cell cycle is and outline its

  stages

  cycle of division with intermediate growth

  periods

  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
• 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


  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 &

  proto-oncogenes?


  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?

  1. 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