The cell cycle

Every living cell comes from a cell that divided. The cell cycle is the sequence of events in which a cell copies its DNA, makes its other parts, and divides into two daughter cells. A human cell growing in a lab takes about 24 hours for one cycle; yeast can do it in about 90 minutes.

The cycle has two main parts: a long interphase and a short M phase (mitosis). Interphase takes more than 95% of the time. It is often called a "resting" phase, but the cell is not resting at all; it is busy getting ready to divide. Interphase has three stages:

• G1 phase (gap 1): the cell grows and works normally, but does not copy its DNA.
• S phase (synthesis): the DNA is copied. The amount of DNA doubles, from 2C to 4C, but the number of chromosomes stays the same (2n), because each chromosome now simply has two identical copies joined together. In animal cells the centriole also doubles.
• G2 phase (gap 2): the cell makes the proteins needed for mitosis and keeps growing.

Then comes the M phase, which takes only about an hour in human cells. First the nucleus divides (karyokinesis); then usually the cytoplasm divides (cytokinesis).

Some cells stop dividing. They leave G1 and enter a resting stage called G0 (the quiescent stage). They stay alive and keep working, but do not multiply. Adult heart cells, for example, do not divide.

In animals, mitosis happens in diploid body (somatic) cells; one exception is the haploid male honey bee. Plants can show mitosis in both haploid and diploid cells.