Biology Reference
In-Depth Information
chapter will therefore use a small number of examples to illustrate how regulatory signals are
connected to the machinery that drives cell proliferation and hence to morphogenetic events.
Given the large variety of systems that exert control over cell proliferation, it is important to
exercise great caution in extrapolating from the examples discussed here to unrelated cell
types and developmental events; the general principles will probably be the same, but the
detailed identities of molecules and pathways may well not be.
A BRIEF INTRODUCTION TO THE CELL CYCLE
In unicellular organisms, cell proliferation is the default behaviour and its rate is limited
only by the availability of energy and raw materials.
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The cell cycle of a eukaryotic unicell,
such as the fission yeast Schizosaccharomyces pombe, consists of a growth
)
phase (G1),
a phase of DNA synthesis (S), a second growth phase (G2) and mitosis itself (M), repeated
ad infinitum (
Figure 22.3
). Some of these phases are themselves divided into distinct sub-
phases that are separated by 'checkpoints'. Progression from one stage to the next, and
from one phase to the next, is controlled by complexes of signalling proteins that are
specific to each stage. The system is elaborate and, although this topic is not the place
to describe it in detail (excellent reviews can be found elsewhere),
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e
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it is useful to sketch
the general features of cell cycle control in unicells before considering the complications
found in metazoa.
The general principle of cell cycle control in S. pombe is that each checkpoint is controlled
by a feedback mechanism that confirms the completion of the previous stage. Progression
FIGURE 22.3
The basic cell cycle of S. pombe, in which the cell enters four stages in sequence and then begins
again.
)
G1 and G2 originally stood for 'gap' 1&2, but in the light of more information about the cell cycle is it now
convenient to associate 'G' with the word 'growth' instead.
