什么是 The cell cycle？

The cell cycle of a eukaryotic cell has four phases. G$_1$ (gap 1): the cell grows and makes the enzymes of replication; S (synthesis): it replicates its DNA, so that each chromosome becomes two identical sister chromatids joined at the centromere; G$_2$: it grows further and checks the copies; M: mitosis, the division of the nucleus, followed by cytokinesis, the division of the cytoplasm. G$_1$, S and G$_2$ together are interphase, in which the chromosomes are extended and active; a cell that stops dividing leaves the cycle from G$_1$ into a resting state, G$_0$. A dividing human cell takes about $24\,\mathrm{h}$: G$_1$ $10\,\mathrm{h}$, S $8\,\mathrm{h}$, G$_2$ $4\,\mathrm{h}$, M $1\,\mathrm{h}$; a yeast, $90\,\mathrm{min}$; an early frog embryo, thirty minutes with no gaps at all.