62
Chapter 3
process. The transcription factors, along with accessory pro-
teins, form a
preinitiation complex
at the promoter that
is needed to carry out the process of separating the DNA
strands, removing any blocking nucleosomes in the region
of the promoter, activating the bound RNA polymerase,
and moving the complex along the template strand of DNA.
Some transcription factors bind to regions of DNA that are far
removed from the promoter region of the gene whose tran-
scription they regulate. In this case, the DNA containing the
bound transcription factor forms a loop that brings the tran-
scription factor into contact with the promoter region, where
it may then activate or repress transcription (
Figure 3–24
).
Many genes contain regulatory sites that a common
transcription factor can infl uence; thus there does not need to
be a different transcription factor for every gene. In addition,
more than one transcription factor may interact to control the
transcription of a given gene.
Because transcription factors are proteins, the activity of
a particular transcription factor—that is, its ability to bind to
DNA or to other regulatory proteins—can be turned on or
off by allosteric or covalent modulation in response to signals
a cell either receives or generates. Thus, specifi c genes can be
regulated in response to specifi c signals.
To summarize, the rate of a protein’s synthesis can be reg-
ulated at various points: (1) gene transcription into mRNA;
(2) the initiation of protein assembly on a ribosome; and
(3) mRNA degradation in the cytoplasm.
Mutation
Any alteration in the nucleotide sequence that spells out a
genetic message in DNA is known as a
mutation.
Certain
chemicals and various forms of ionizing radiation, such as x-
rays, cosmic rays, and atomic radiation, can break the chemi-
cal bonds in DNA. This can result in the loss of segments of
DNA or the incorporation of the wrong base when the broken
bonds re-form. Environmental factors that increase the rate of
mutation are known as
mutagens.
Types of Mutations
The simplest type of mutation, known as a point mutation,
occurs when a single base is replaced by a different one. For
example, the base sequence C–G–T is the DNA triplet for the
amino acid alanine. If guanine (G) is replaced by adenine (A),
the sequence becomes C—A—T, which is the code for valine.
If, however, cytosine (C) replaces thymine (T), the sequence
becomes C—G—C, which is another code for alanine, and the
Gene B
Promoter B
Gene A
Promoter A
Binding site on DNA
for transcription factor
DNA
RNA polymerase complex
Activated
transcription
factor
Transcription
factor
Allosteric or
covalent
modulation
Plasma membrane
Extracellular signal
Extracellular fluid
Cytoplasm
Nucleus
Figure 3–24
Transcription of gene B is modulated by the binding of an activated transcription factor directly to the promoter region. In contrast,
transcription of gene A is modulated by the same transcription factor, which, in this case, binds to a region of DNA considerably distant from
the promoter region.
previous page 90 Vander's Human Physiology The Mechanisms of Body Function read online next page 92 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off