Controlling Cell Division

• Cell Cycle

  • All cells go through the cell cycle – the time it takes for a new cell to undergo the process of cell division.
  • Essentially it’s “life cycle”.
  • Involves three stages:
  • Mitosis – division of nucleus
  • Cytokinesis – division of cytoplasm and formation of two cells
  • Interphase – the time between cell divisions.
  • Interphase is the phase where the cell undergoes its processes and biochemical reactions. These include growth, transcription/translation, and DNA replication (preparing for mitosis).
• Cell Cycle

  • Nerve cells continually sit in G0, as they are permanently in the cell.
  • Damage to cells can trigger the cell to move from G0, to G1.
  • Gap 1 = Growth
  • Synthesis = Growth & DNA synthesis
  • Gap 2 = Growth and preparation for mitosis
  • M = mitosis & cytokinesis (cell division)
  • G0 = cell arrest – where cells aren’t dividing or preparing to divide. performing maintenance and other functions.
• Stages and Events

  • G0
  • Cells are removed from the cycle and are not dividing, though functioning
  • G1
  • Cell growth, ribosomes synthesis, protein synthesis
  • S
  • DNA replication, histone synthesis, phospholipid synthesis
  • G2
  • Developing and preparing organelles for division
  • M
  • Mitosis; division of the nucleus
  • Cytokinesis
  • Division of the cytoplasm and the cell divides
• Cell Cycle Regulation

  Cell cycle needs to be controlled and regulated.

  Is characterised by:

  • The ability to respond to extracellular and intracellular signals.
  • A system where a cell is only able to move on in the cycle when current requirements are met.
  • Control system to switch processes and chemical reactions on and off.
• Cell Cycle Regulation

  • DNA detection system which looks for completion of and fault during DNA replication.
  • Series of checkpoints (G1, G2, M) – acts as a molecular surveillance system – Checks to see if cell is ready to proceed further into cycle. This is characterised by a sensor/detector, ability to send a message and an effector which brings about a response.
• G1 Checkpoint

  In this checkpoint:

  • Cell checks on DNA to see if ready to proceed to S phase (DNA replication).
  • Gene “p53” checks to see if cell cycle can proceed.
  • Often referred to as the “guardian of the genome”.
  • If DNA is damaged, it is repaired.
  • If damage is severe, cell death (apoptosis) may be triggered by gene p53.
  • A required “go-ahead” signal is given – triggers DNA replication (S phase).
• Gene p53

  • Tumour suppression protein P53
  • This gene codes for proteins which bind to DNA to regulate gene expression and DNA health
  • Can bring about apoptosis (cell death) or DNA repair
  • Mutation to this gene is thought to make >50% of human cancer.
• G2 Checkpoint

  In this checkpoint:

  • Occurs immediately after G2 phase – enters mitosis.
  • Ensures DNA has replicated accurately & no molecular damage.
  • Overall check of cells health to make it suitable to enter mitosis.
  • A required “go ahead” signal is given – triggers mitosis to occur up until the end of metaphase.
• M Checkpoint

  This checkpoint occurs in the metaphase during mitosis.

  In this checkpoint:

  • Checks that spindle apparatus/fibres are in place.
  • Checks that chromosomes are lined up correctly.
  • Checks that chromatids are ready to separate during anaphase.
  • Ensure each daughter cell receives identical set of chromosomes.
  • A “go-ahead” signal triggers the completion of mitosis. Cytokinesis then takes place and two identical daughter cells are produced.
• External Factors Controlling Cell Division

  External Factors can be both physical and chemical in nature and these impact on the internal molecule controls or gene products inside the cell

  • Nutrient dependence: the cells need particular nutrients
  • Anchorage dependence: many cells will only divide if they are attached to a substrate or surface
• External Factors Controlling Cell Division

  • Density dependence: cells that are in close contact with one another will usually not divide. Note that cancer cells do not respond in the same manner as normal cells.
  • Large cell size: large cells have a relative low Surface Area to Volume ratio (SA:V) and this acts to stimulate cell division to give two smaller cells that will have a higher SA:V ratio and be more efficient at exchanging substances with their environment