Cell Cycle


The cell cycle is an ordered set of events that culminates in cell growth and division into two daughter cells

It can roughly be divided into two main stages:


  • The stage in the development of the cell between two successive M phases
  • This phase of the cell cycle is a continuum of 3 distinct stages (G1, S, G2), whereby the cell grows and matures (G1), copies its DNA (S) and prepares for division (G2)
  • Sometimes cells will leave the cell cycle and enter into a quiescent state (G0), whereby it becomes amitotic and no longer divides

M phase

  • The periods of nuclear division (mitosis) and cytoplasmic division (cytokinesis)

                                             The Cell Cycle                                                                                                                 M Phase


Interphase is an active period in the life of a cell - many events need to occur before a cell can successfully undergo division:

  • Protein synthesis:  The cell needs to synthesise key proteins and enzymes to enable it to grow, copy its contents and then divide
  • ATP production:  The cell will need to generate sufficient quantities of ATP in order to successfully divide
  • Increase number of organelles:  The cell needs to ensure both daughter cells will have the necessary numbers of organelles needed to survive
  • DNA replication:  The genetic material must be faithfully duplicated before division (this occurs during the S phase)

As none of these processes can occur during the M phase, interphase contains growth checkpoints to ensure division is viable

  • G1:  A checkpoint stage before DNA replication during which the cell grows, duplicates organelles, synthesises proteins and produces ATP
  • S:  The stage during which DNA is replicated
  • G2 A checkpoint stage before division during which the copied DNA is checked for fidelity (mutations) and final metabolic reactions occur

DNA Replication

  • During interphase (the S phase) the DNA is replicated to produce two copies of genetic material
  • These two identical DNA molecules are identified as sister chromatids and are held together by a single centromere
  • During the events of mitosis, the sister chromatids are separated and drawn to opposite poles of the cell (becoming separate chromosomes in the process)
  • When the cell divides into two, the two resulting nuclei will each contain one of each chromatid pair and thus be genetically identical 


  • The cell cycle is controlled by a complex chemical control system that responds to signals both inside and outside of the cell
  • Tumor suppressor genes produce proteins which inhibit cell division, while proto-oncogenes produce proteins that promote growth and division
  • Mutations to these genes result in uncontrolled cell division, resulting in the formation of a tumour
  • Tumours can grow in size which causes damage local tissue; they may also spread to other parts of the body (malignant tumours)
  • Diseases caused by the growth of tumours are collectively known as cancers

Cancer in Tasmanian Devils