At the end of S phase, the DNA molecules in each pair of duplicated chromosomes are intertwined and held tightly together by specialized protein linkages. Early in mitosis at a stage called prophase, the two DNA molecules are gradually disentangled and condensed into pairs of rigid, compact rods called sister chromatids, which remain linked by sister-chromatid cohesion. When the nuclear envelope disassembles later in mitosis, the sister-chromatid pairs become attached to the mitotic spindle, a giant bipolar array of microtubules (discussed in Chapter 16). Sister chromatids are attached to opposite poles of the spindle and, eventually, align at the spindle equator in a stage called metaphase. The destruction of sister-chromatid cohesion at the start of anaphase separates the sister chromatids, which are pulled to opposite poles of the spindle. The spindle is then disassembled, and the segregated chromosomes are packaged into separate nuclei at telophase. Cytokinesis then cleaves the cell in two, so that each daughter cell inherits one of the two nuclei (Figure 17–3).
The Eukaryotic Cell Cycle Usually Consists of Four phases
Most cells require much more time to grow and double their mass of proteins and organelles than they require to duplicate their chromosomes and divide. Partly to allow time for growth, most cell cycles have gap phases—a G1 phase between M phase and S phase and a G2 phase between S phase and mitosis. Thus, the eukaryotic cell cycle is traditionally divided into four sequential phases: G1, S, G2, and M. G1, S, and G2 together are called interphase (Figure 17–4, and see Figure 17–3). In a typical human cell proliferating in culture, interphase might occupy 23 hours of a 24-hour cycle, with 1 hour for M phase. Cell growth occurs throughout the cell cycle, except during mitosis.
The two gap phases are more than simple time delays to allow cell growth. They also provide time for the cell to monitor the internal and external environment
Figure 17–2 The major events of the cell cycle. The major chromosomal events of the cell cycle occur in s phase, when the chromosomes are duplicated, and m phase, when the duplicated chromosomes are segregated into a pair of daughter nuclei (in mitosis), after which the cell itself divides into two (cytokinesis).
Figure 17–1 The cell cycle. The division of a hypothetical eukaryotic cell with two chromosomes (one red, and one black) is shown to illustrate how two genetically identical daughter cells are produced in each cycle. Each of the daughter cells will often continue to divide by going through additional cell cycles.
Figure 17–3 The events of eukaryotic cell division as seen under a microscope. The easily visible processes of nuclear division (mitosis) and cell division (cytokinesis), collectively called m phase, typically occupy only a small fraction of the cell cycle. The other, much longer, part of the cycle is known as interphase, which includes s phase and the gap phases (discussed in text). The five stages of mitosis are shown: an abrupt change in the biochemical state of the cell occurs at the transition from metaphase to anaphase. a cell can pause in metaphase before this transition point, but once it passes this point, the cell carries on to the end of mitosis and through cytokinesis into interphase.