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

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Control of Gene Expression
chapter
7
An organism’s DNA encodes all of the RNA and protein molecules required to
construct its cells. Yet a complete description of the DNA sequence of an organism—be
it the few million nucleotides of a bacterium or the few billion nucleotides
of a human—no more enables us to reconstruct the organism than a list of
English words enables us to reconstruct a play by Shakespeare. In both cases, the
problem is to know how the elements in the DNA sequence or the words on the
list are used. Under what conditions is each gene product made, and, once made,
what does it do?
In this chapter, we focus on the first half of this problem—the rules and mechanisms
that enable a subset of genes to be selectively expressed in each cell. These
mechanisms operate at many levels, and we shall discuss each level in turn. But
first we present some of the basic principles involved.
An Overview of Gene Control
The different cell types in a multicellular organism differ dramatically in both
structure and function. If we compare a mammalian neuron with a liver cell, for
example, the differences are so extreme that it is difficult to imagine that the two
cells contain the same genome (Figure 7–1). For this reason, and because cell differentiation
often seemed irreversible, biologists originally suspected that genes
might be selectively lost when a cell differentiates. We now know, however, that
cell differentiation generally occurs without changes in the nucleotide sequence
of a cell’s genome.
The Different Cell Types of a Multicellular Organism Contain the
Same DNA
The cell types in a multicellular organism become different from one another
because they synthesize and accumulate different sets of RNA and protein molecules.
The initial evidence that they do this without altering the sequence of
their DNA came from a classic set of experiments in frogs. When the nucleus of
a fully differentiated frog cell is injected into a frog egg whose nucleus has been
removed, the injected donor nucleus is capable of directing the recipient egg to
25 µm
neuron
liver cell
In This Chapter
AN OVERVIEW OF GENE CONTROL
CONTROL OF TRANSCRIPTION BY
SEQUENCE-SPECIFIC
DNA-BINDING PROTEINS
TRANSCRIPTION REGULATORS
SWITCH GENES ON AND OFF
MOLECULAR GENETIC
MECHANISMS THAT CREATE AND
MAINTAIN SPECIALIZED CELL
TYPES
MECHANISMS THAT REINFORCE
CELL MEMORY IN plants and
animals
post-transcriptional
controls
REGULATION OF GENE
EXPRESSION BY NONCODING
RNAs
Figure 7–1 A neuron and a liver cell share
the same genome. The long branches
of this neuron from the retina enable it to
receive electrical signals from many other
neurons and convey them to neighboring
neurons. The liver cell, which is drawn
to the same scale, is involved in many
metabolic processes, including digestion
and the detoxification of alcohol and other
drugs. Both of these mammalian cells
contain the same genome, but they express
different sets of RNAs and proteins. (Neuron
adapted from S. Ramón y Cajal, Histologie
du Systeme Nerveux de l’Homme et de
Vertebres, 1909–1911. Paris: Maloine;
reprinted, Madrid: C.S.I.C, 1972.)