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Cell Division: Eukaryotes
The Cell Cycle

the events

Early microscopists soon noted that the mixture of cells in a dividing and growing tissue had clearly recognizable "phases" or "stages". After identifying them, and sorting them out, it was possible to organize a cycle of events that appeared to occur in a repeating pattern.

This "pattern" of events soon became known as the cell cycle. With some variation, and with increasing sophistication of techniques, it is now possible to recognize three main stages or phases in one cycle, and to subdivide each of these major phases into recognizable points or events.

The three main phases of a single cell cycle are: interphase, nuclear division and cytoplasmic division.


Originally this phase of the cell cycle was called the "resting stage", since light microscopy could not detect any activities taking place within the cells. Today, however, it is known as a stage of considerable activity at the molecular and sub-cellular level and is usually subdivided into:

  • G1 - ("Gap One") - this is a period of molecular synthesis where a newly formed cell turns on a variety of genes on its DNA to make proteins, which in turn churn the metabolism of the cell, produce and breakdown carbohydrates, lipids, etc., and transform energy from food into ATP. The cell grows and enlarges.

  • S - ("synthesis") - during this phase the chromatin (DNA and proteins) becomes synthetically active. Using elaborate teams of enzymes, the DNA molecules of each chromosome are copied by semiconservative DNA synthesis. This phase cannot be clearly seen or distinguished under the light microscope, even with DNA stains, as the material is too diffuse. However, the making of new DNA molecules can be monitored by following the incorporation of radioactive isotopes into the newly forming DNA molecules.

  • G2 - ("Gap Two") - another period, of variable length, in which cells prepare for division. Many different proteins are synthesized, especially those that will act as spindle fibers (protein "ropes"). Stocks of energy are accumulated and many organelles, such as mitochondria, also grow and divide, increasing in number.

  • Interphase is a busy time in the life of a dividing cell. Towards the end of G2, however, things slow down and the cell gets ready for the next major phase in the cycle.

© 2001, Professor John Blamire