Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

966 Chapter 17: The Cell Cycle
Figure 17–5 Mammalian cells proliferating in culture. The cells in this
scanning electron micrograph are rat fibroblasts. Cells at the lower left have
rounded up and are in mitosis. (Courtesy of Guenter Albrecht-Buehler.)
cells, and all eukaryotes appear to use similar machinery and control mechanisms
to drive and regulate cell-cycle events. The proteins of the cell-cycle control
system, for example, first appeared over a billion years ago. Remarkably, they have
been so well conserved over the course of evolution that many of them function
perfectly when transferred from a human cell to a yeast cell. We can therefore
study the cell cycle and its regulation in a variety of organisms and use the findings
from all of them to assemble a unified picture of how eukaryotic cells divide.
Several model organisms are used in the analysis of the eukaryotic cell cycle.
The budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces
pombe are simple eukaryotes in which powerful molecular and genetic
approaches can be used to identify and characterize the genes and proteins that
govern the fundamental features of cell division. The early embryos of certain
animals, particularly those of the frog Xenopus laevis, are excellent tools for biochemical
dissection of cell-cycle control mechanisms, while the fruit fly Drosophila
melanogaster is useful for the genetic analysis of mechanisms underlying the
control and coordination of cell growth and division in multicellular organisms.
Cultured human cells provide an excellent system for the molecular and microscopic
exploration of the complex processes by which our own cells divide.
Cell-Cycle Progression Can Be Studied in Various Ways
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 G 1 , S, and G 2
+ M phases, by measuring DNA content in a synchronized cell population as it
progresses through the cell cycle.
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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 G 1 . 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.)
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