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
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LEFT
embryonic cell
INDUCTION OF REGULATOR TRANSCRIPTION
cell division
cell A
INDUCTION OF REGULATOR TRANSCRIPTION 2
1
1
cell B
AND 3
RIGHT
Figure 7–33 The importance of
combinatorial gene control for
development. Combinations of a few
transcription regulators can generate many
cell types during development. In this
simple, idealized scheme, a “decision” to
make one of a pair of different transcription
regulators (shown as numbered circles)
is made after each cell division. Sensing
its relative position in the embryo, the
daughter cell toward the left side of the
embryo is always induced to synthesize the
even-numbered protein of each pair, while
the daughter cell toward the right side of
the embryo is induced to synthesize the
odd-numbered protein. The production of
each transcription regulator is assumed to
be self-perpetuating once it has become
initiated (see Figure 7–39). In this way,
through cell memory, the final combinatorial
specification is built up step by step. In this
purely hypothetical example, five different
transcription regulators have created eight
final cell types (G–N).
2
3
1 2
1
3
cell C cell D cell E cell F
INDUCTION OF REGULATOR TRANSCRIPTION
4
AND
5
2 4
2
5
3
4
3
5
1
4
2 1 2
5
1
4
3
1
5
3
cell G cell H cell I cell J cell K cell L cell M cell N
combinatorial control of the muscle-specific genes, and the addition of MyoD
completes the unique combination required to direct the cells to become muscle.
An even more striking example is seen by artificially expressing, early in development,
a single Drosophila transcription regulator (Eyeless) in groups of cells
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(A)
(B)
50 µm 50 µm
Figure 7–34 A small set of transcription regulators can convert one differentiated cell type
into another. In this experiment, (A) liver cells grown in culture were converted into (B) neuronal
cells via the artificial expression of three nerve-specific transcription regulators. Both types of cells
express an artificial red fluorescent protein, which is used to visualize them. This conversion involves
the activation of many nerve-specific genes as well as the repression of many liver-specific genes.
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(From S. Marro et al., Cell Stem Cell 9:374–382, 2011. With permission from Elsevier.)