How can we tell what stage a cell has reached in the cell cycle? One way is simply to look at living cells with a microscope. A glance at a population of mammalian cells proliferating in culture reveals that a fraction of the cells have rounded up and are in mitosis (Figure 17–5). Others can be observed in the process of cytokinesis. Similarly, looking at budding yeast cells under a microscope is very useful, because the size of the bud provides an indication of cell-cycle stage (Figure 17–6). We can gain additional clues about cell-cycle position by staining cells with DNA-binding fluorescent dyes (which reveal the condensation of chromosomes in mitosis) or with antibodies that recognize specific cell components such as the microtubules (revealing the mitotic spindle). S-phase cells can be identified in the microscope by supplying them with visualizable molecules that are incorporated into newly synthesized DNA, such as the artificial thymidine analog bromodeoxyuridine (BrdU); cell nuclei that have incorporated BrdU are then revealed by staining with anti-BrdU antibodies (Figure 17–7).
Typically, in a population of cultured mammalian cells that are all proliferating rapidly but asynchronously, about 30–40% will be in S phase at any instant and become labeled by a brief pulse of BrdU. From the proportion of cells in such a population that are labeled, we can estimate the duration of S phase as a fraction of the whole cell-cycle duration. Similarly, from the proportion of cells in mitosis (the mitotic index), we can estimate the duration of M phase.
Another way to assess the stage that a cell has reached in the cell cycle is by measuring its DNA content, which doubles during S phase. This approach is greatly facilitated by the use of fluorescent DNA-binding dyes and a flow cytometer, which allows the rapid and automatic analysis of large numbers of cells (Figure 17–8). We can use flow cytometry to determine the lengths of G1, S, and G2 + M phases, by measuring DNA content in a synchronized cell population as it progresses through the cell cycle.
Figure 17–6 The morphology of budding yeast cells. In a normal population of proliferating yeast cells, buds vary in size according to the cell-cycle stage. Unbudded cells are in g1. progression through the start transition triggers formation of a tiny bud, which grows in size during the  s and m phases until it is almost the size of the mother cell. (Courtesy of Jeff Ubersax.) 20 µm Figure 17–7 Labeling S-phase cells. an immunofluorescence micrograph of BrdU-labeled epithelial cells of the zebrafish gut. The fish was exposed to BrdU, after which the tissue was fixed and prepared for labeling with fluorescent anti-BrdU antibodies (green). all the cells are stained with a red fluorescent dye. (Courtesy of Cécile Crosnier.)
Cell division usually begins with duplication of the cell’s contents, followed by distribution of those contents into two daughter cells. Chromosome duplication occurs during S phase of the cell cycle, whereas most other cell components are duplicated continuously throughout the cycle. During M phase, the replicated chromosomes are segregated into individual nuclei (mitosis), and the cell then splits in two (cytokinesis). S phase and M phase are usually separated by gap phases called G1 and G2, when various intracellular and extracellular signals regulate cell-cycle progression. Cell-cycle organization and control have been highly conserved during evolution, and studies in a wide range of systems have led to a unified view of eukaryotic cell-cycle control.