Cells reproduce by duplicating their contents and dividing in two. This cycle of duplication and division, called the cell cycle, is discussed in Chapter 17. In this chapter, we consider the mechanical events of the M phase of the cycle, which is the culmination of the cycle and includes the various stages of nuclear division (mitosis) and cytoplasmic division (cytokinesis). In a comparatively brief period, the contents of the parent cell, which were doubled during earlier phases of the cycle, are partitioned into two daughter cells. The period between one M phase and the next is called interphase, and in most rapidly proliferating cells, it is divided into three phases: S phase, in which DNA is replicated (discussed in Chapter 5), and two gap phases, G₁ and G₂, which provide additional time for the cell to grow (Figure 18-1).

Figure 18-1

The M phase of the cell cycle. M phase starts at the end of G₂ and ends at the start of the next G₁ phase. It includes the five stages of nuclear division (mitosis), as well as cytoplasmic division (cytokinesis).

As discussed in detail in Chapter 17, the events of the cell cycle are controlled by the cell-cycle control system. The core of the control system consists of various cyclin-dependent kinases (Cdks), which are activated in sequence to trigger various steps of the cycle. The Cdks are activated by the binding of cyclin regulatory proteins, as well as by phosphorylation and dephosphorylation of the kinase. They are inactivated by various Cdk inhibitory proteins (CKIs) and by the degradation of the cyclin subunits at specific stages of the cycle.

The M-phase Cdk (M-Cdk) triggers a cascade of protein phosphorylation that initiates M phase. These phosphorylations are responsible for the many morphological changes that occur during mitosis in animal cells. The chromosomes condense, the nuclear envelope breaks down, the endoplasmic reticulum and Golgi apparatus reorganize, the cell loosens its adhesions both to other cells and to the extracellular matrix, and the cytoskeleton radically reorganizes to bring about the highly ordered movements that will segregate the replicated chromosomes and divide the cell in two.

Targetted protein degradation by the anaphase-promoting complex (APC) (discussed in Chapter 17) has an equally important regulatory role in mitosis. It initiates the separation and segregation of the replicated chromosomes, and it inactivates M-Cdk at the end of mitosis.

We begin this chapter with an overview of M phase. We then discuss mitosis and cytokinesis in turn, focusing mainly on animal cells. We end by considering how M phase may have evolved. We discuss the special features of meiotic cell division in Chapter 20, where we describe the development of germ cells.