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

MOLECULAR GENETIC MECHANISMS THAT CREATE AND MAINTAIN SPECIALIZED CELL TYPES
403
INPUT INPUT INPUT
1
1
A
0
0
A
time
time
Z
Z
OUTPUT
(A)
B
B
1
0
(B)
OUTPUT
time
OUTPUT
machinery, and it must, in effect, use its intricate system of interlocking transcription
switches to compute how it should behave at each time point in response to
the many different past and present MBoC6 inputs m7.70/7.42 received. We are only beginning to
understand how to study such complex intracellular control networks. Indeed,
without new approaches, coupled with quantitative information that is far more
precise and complete than we now possess, it will be impossible to predict the
behavior of a system such as that shown in Figure 7–42. As explained in Chapter 8,
a circuit diagram by itself is not enough.
1
0
(C)
time
Figure 7–41 How a feed-forward loop
can measure the duration of a signal.
(A) In this theoretical example, transcription
regulators A and B are both required for
transcription of Z, and A becomes active
only when an input signal is present.
(B) If the input signal to A is brief, A
does not stay active long enough for B
to accumulate, and the Z gene is not
transcribed. (C) If the signal to A persists,
B accumulates, A remains active, and Z is
transcribed. This arrangement allows the
cell to ignore rapid fluctuations of the input
signal and respond only to persistent levels.
This strategy could be used, for example,
to distinguish between random noise and a
true signal.
The behavior shown here was
computed for one particular set of
parameter values describing the
quantitative properties of A, B, and the
product of Z, along with their syntheses.
With different values of these parameters,
feed-forward loops can in principle perform
other types of “calculations.” Many feedforward
loops have been discovered
in cells, and theoretical analysis helps
researchers to discern—and subsequently
test—the different ways in which they may
function (see Figure 8–86). (Adapted from
S.S. Shen-Orr et al., Nat. Genet. 31:64–68,
2002. With permission from Macmillan
Publishers Ltd.)
MATERNAL AND
EARLY SIGNALS
INTERPRETATION MACHINERY
OUTPUT = MESODERM DIFFERENTIATION
OUTPUT = ENDODERM DIFFERENTIATION
Figure 7–42 The exceedingly complex
gene circuit that specifies a portion of
the developing sea urchin embryo. Each
colored small box represents a different
gene. Those in yellow code for transcription
regulators and those in green and blue
code for proteins that give cells of the
mesoderm and endoderm, respectively,
their specialized characteristics. Genes
depicted in gray are largely active in the
mother and provide the egg with cues
needed for proper development. As in
Figure 7–40, arrows depict instances in
which a transcription regulator activates
the transcription of another gene. Lines
ending in bars indicate examples of gene
repression. (From I.S. Peter and
E.H. Davidson, Nature 474:635–639, 2011.
With permission from Macmillan
Publishers Ltd.)