The Cell Cycle

There is a constant rate of cellular attrition in our bodies: some cells wear out, some are ordered to die by apoptosis, some are damaged by trauma or microorganisms, and still others are actively killed by our immune cells as a result of their becoming infected or cancerous. To replace this constant loss, most of our cells must clone themselves to make up the deficit.

The orderly process by which they accomplish this is known as the cell cycle. It is divided into four distinct phases, which revolve around a fact of necessity: if a cell has to divide in two, it has to duplicate its DNA first. If this were not the case, the amount of genetic material in each cell would halve after each division, leaving the resulting cells without their central instruction manual.

The phases are labelled in the chart below: G1, S, G2 and M, and we'll get to them individually in just a second. Once all four phases have been completed, the cell divides in half, thus creating two identical clones instead of the original single cell. The length of the entire cell cycle varies quite a lot from cell type to cell type, but a typical figure might be about 24 hours.

After a cell has been created by division, the first thing it does is get back to doing whatever it is supposed to be doing! Think of it as cellular "me time". (The mitotic process that produced it is metabolically costly, and many routine cellular processes had to be put on hold for its duration.) However, the cell also has one eye on the future: it synthesises certain key enzymes neccessary for the next step. This stage is known as G1 (in which the 'G' stands for 'gap').

The next phase is labelled S, for 'synthesis'. As suggested by this name, this is the time during which the cell's DNA is replicated. Another copy of it is made, so that each chromosome is now made up of two sister chromatids.

After the S phase comes another 'gap' stage, called G2. Once again, the cell can resume normal cellular activities. As with the first gap though, the cell also uses the time to synthesise things of use in the next phase. (Its organelles replicate, and the cell volume swells.) Together, these first three stages can be lumped under the title, interphase.

The last phase is usually the briefest, and it's called the M phase ('M' stands for 'mitosis'). This stage is bewilderingly complex (and the subject of many an exam question!) and so we'll have to cover it in detail on some other occasion. Suffice it to say, the cell now obliges by dividing down the middle, making sure that each cell has a full complement of DNA and organelles.

Not all cells have the be active in the cell cycle (think, for instance of neurones, which don't divide). Those which aren't interested in dividing are said to be in G0.