The Cell Cycle: Definition and Phases

Every cell that is going to divide does not simply split in two at random. It passes through an orderly, tightly regulated sequence of events -- growing to an appropriate size, copying its entire genetic material exactly once, checking that the copy is correct, and only then physically dividing into two daughter cells -- and this whole repeating sequence, from the birth of a cell to the moment it itself divides into two new cells, is called the cell cycle.

The cell cycle is conventionally divided into two broad, unequal parts. The first and by far the longer part is interphase, sometimes loosely called the 'resting phase,' though this name is misleading -- a cell in interphase is intensely active metabolically, growing, synthesising proteins and RNA, replicating its DNA, and duplicating its organelles, even though nothing dramatic is visible under an ordinary light microscope. The second and much shorter part is the M phase (mitotic phase), during which the cell actually divides -- first the nucleus divides (karyokinesis), most commonly by the process of mitosis described in this chapter, and then the cytoplasm divides (cytokinesis), producing two separate daughter cells.

In a rapidly and continuously dividing human cell in culture, a complete cell cycle typically takes about 24 hours, though the exact duration varies enormously between cell types and organisms -- some yeast cells complete a cycle in under two hours, while other, more specialised or slowly dividing cells may take days. Of this total duration, interphase alone typically accounts for roughly 95 percent of the cycle, leaving only a small fraction of the time -- often under an hour in a rapidly dividing cell -- for the whole of M phase. This lopsided division of time reflects how much preparatory work (growth, DNA replication, checking for errors, assembling the machinery of division) must be completed before a cell can safely commit to splitting its genetic material between two daughter cells.

Not every cell in the body continues to cycle indefinitely. Cells that have become highly specialised, such as mature nerve cells and cardiac muscle cells, typically exit the active cycle after their last division and enter a distinct, non-dividing but still metabolically active state known as the G0 phase, in which they may remain for the rest of the organism's life, or in some tissue types re-enter the cycle later if the need for renewed division arises (for example, when a tissue is injured). The sections that follow examine interphase and M phase, and the sub-stages within each, in detail.