The Cell Cycle

• The cell cycle...

  ...consists of 2 phases; interphase (not dividing) and mitosis (dividing)
• mitosis


  the process of cell division
• G1 phase


  First growth stage; makes new proteins and makes copies of all the organelles
• interphase


  a very active period of growth and metabolism and is split into 3 further stages ; G1 phase, S phase and G2 phase
• S phase


  DNA is replicated
• G2 phase 


  Second growth stage; cell makes more new proteins and prepares for mitosis
• m phase


  mitosis can be split into a series of visible changes within the cell. Spindle fibres move the chromosomes during mitosis:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
  5. Cytokinesis
• Prophase

  • Chromosomes condense
  • Microtubules disassemble and reassemble to form spindle fibres and asters by polymerisation
  • Some spindle fibres attach to the kinetochore and tension-sensitive proteins make sure that each chromatid is attached to a centrosome
  • nuclear membrane disintegrates
• Metaphase

  • Chromosomes move and line up along the metaphase plate on the equator. This movement happens because of the polymerisation and depolymerisation of microtubules in spindle fibres.
• Anaphase

  • Two chromatids are pulled apart by the depolymerization of kinetochore proteins
  • Once chromatids are separated they are then called chromosomes in their own right.
• Telophase

  • Separated chromosomes are pulled by spindle fibres to opposite poles of the cell
  • Chromosomes start to uncoil and two nuclear membranes are made
• cytokinesis in animals

  • Division of cytoplasm
  • 2 Daughter cells
  • A circle of actin and myosin fibres pinch the cell membrane in a process known as cleveage
• Cytokinesis in plant cells

  • Division of cytoplasm
  • 2 Daughter cells
  • A new cell wall and middle lamella are made before the end of mitosis
• uncontrolled reduction in cell division


  Can cause degenerative diseases like parkinsons
• uncontrolled increase in cell division


  may result in tumour formation which can be cancerous
• proto-oncogene

  has a mutation which can sometimes form a tumour-promoting oncogene. Tumour could be benign or malignant
• what is a cell cycle checkpoint?


  a mechanism within the cell which assesses the condition of the cell during the cell cycle and will halt progression to the next stage if certain requirements haven't been met.
• G1 Checkpoint


  near the end of G1, cell size is checked and if the cell is not the correct mass to divide into two daughter cells, it is put into a resting phase called G0
• G2 Checkpoint


  Near the end of G2, DNA replication is checked and if DNA has not replicated successfully, the cell will not be allowed to undergo mitosis
• M checkpoint


  During metaphase, monitors chromosome alignment to check each daughter cell is receiving one chromatid from each chromosome. This controls entry to anaphase and the cell will be halted until alignment is correct or destroyed if it doesn't meet the criteria.
• cyclins


  proteins involved in regulating the growth of the cell
• cyclin-dependent kinases


  regulatory proteins which combine with cyclin proteins in order to be activated and cause phosphorylation of target proteins which simulate the cell cycle
• target threshold


  the more target proteins which are phosphorylated by the CDKs the more likely the cell is to meet the target threshold and go onto mitosis
• what happens if the target threshold is not reached?

  if it is not met, the cell will be held in G0 state - except in cancer cells
• Retinoblastoma (Rb) proteins


  At the G1 checkpoint, Rb protein acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication in cells which are not suitable in size or in number or cdks

  inhibited by G1 cyclin-cdk phosphorylation
• p53 proteins


  Transcription factors, triggered by DNA damage, that can stimulate DNA repair, trigger cell death and arrest the cell cycle.