Cell division

• What components make up the cell cycle

  • Interphase
  • Mitosis or meiosis
  • Cytokinesis
• Why does the cell cycle not occur in some cells

  • After differentiation, some types of cells in multicellular organisms, lose the ability to divide
• What is the difference between the cell cycle and mitosis

  • The cell cycle includes the growth period between divisions
  • Mitosis is only 10% of the cycle and refers only to nuclear division
• Outline what happens during INTERPHASE

  • G1 -> cell synthesises proteins for replication and the cell size DOUBLES
  • S -> DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere
  • G2 -> Organelles divide
• What is the purpose of mitosis

  • Produces two genetically identical daughter cells for:
  • Growth
  • Cell replacement / tissue repair
  • Asexual reproduction
• What are the stages of mitosis

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Outline what happens during Prophase

  1. Chromosomes condense, they become visible
  2. Centrioles move to opposite poles of the cell and mitotic spindle fibres form
  3. Nuclear envelope and nucleolus break down = chromsomes are free in the cytoplasm
• Outline what happens during metaphase

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


  REQUIRES ATP/ENERGY

  • Spindle fibres contract = centromeres divide
  • Sister chromatids separate into 2 distinct chromosomes + are pulled to opposite poles of the cell
  • The spindle fibres break down
• Outline what happens during telophase

  • Chromosomes decondense, becoming invisible again
  • New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome
• Explain the procedure for the root tip experiment

  1. Prepare a temporary mount of root tissue
  2. Focus an optical microscope on the slide. Count the total number of cells in the field of vision and the number of cells in each stage of mitosis
  3. Calculate the mitotic index
• Name the two dyes that bind to chromosomes

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

  • Meristematic cells -> At the root tip are actively undergoing mitosis
  • Cells further from the 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
• What are proto-oncogenes

  • Genes that code for proteins to stimulate 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 protein needed to slow the cell cycle
  • Proto-oncogenes -> form permanently activated oncogenes
  • Disruption to the cell cycle -> uncontrolled cell division -> tumour
• Suggest how cancer treatments control the rate of cell division

  • Disrupt the cell cycle
  • It prevents DNA replication
  • It disrupts spindle formation = inhibits metaphase / anaphase
• How do prokaryotic cells replicate

  • DNA loop replicates. Both copies stay attached to the cell membrane. The plasmids replicate in the cytoplasm.
  • The cell elongates, separating the 2 DNA loops
  • The cell membrane contracts and the septum forms
  • The cells splits into two identical progeny cells, each with one copy of the DNA loop but a variable number of plasmids
• Why are viruses classified as non-living

  • They are acellular
  • There is no cytoplasm
  • No metabolism
  • Can't self-replicate
• How do viruses replicate

  • Attachment proteins attach to receptors on host cell membrane
  • Enveloped viruses fuse with the cell membrane or move in via endocytosis and release DNA/RNA into the cytoplasm, or they inject viral DNA/RNA
  • Host cell uses viral genetic information to synthesise new viral proteins
  • Components of new viral particle assemble
• How do new viral particles leave the host cell

  • Bud off and use cell membrane to form an envelope
  • Cause lysis of host cells
• Why is it difficult to develop effective treatments against viruses

  • Replicates inside living cells = difficult to kill them without killing host cells