278 MHR • Unit 3 Molecular Genetics
sequences (sometimes called control elements).
These DNA sequences can bind to additional
transcription factors in order to increase the rate of
transcription. These operator sequences may form
part of the promoter sequence, or may be located
some distance away from the gene to be transcribed.
Figure 8.34 on the next page shows one example of
how one kind of transcription factor can bind to a
distant operator sequence to enhance gene
expression. The same transcription factor can
influence transcription on a number of different
genes in order to allow genes coding for related
enzymes to be expressed at the same time.Post-transcriptional Control
The processing of pre-mRNA into mRNA provides
another opportunity to regulate gene expression.
Sometimes the cell will not add the 3 ′poly-A tail
or 5 ′cap to a pre-mRNA strand. This blocks
translation in two main ways. First, an mRNA
strand that lacks a poly-A tail will usually not
leave the nucleus. This indicates that the tail plays
some role in transporting the mRNA transcript to
the cytoplasm. Second, a strand of pre-mRNA that
lacks its cap and tail will usually be broken down
very quickly by nuclease enzymes in the cell that
reduce nucleic acid fragments to their componentnucleotides. (These nucleotides are often recycled
by the cell during DNA replication or repair, or
during transcription). In either case, the mRNA
will not reach the translation machinery of the cell.Translational and Post-translational Control
Once a strand of processed mRNA has reached the
cytoplasm, other regulatory proteins in the cell can
bind to the leader region near the 5 ′cap of the
molecule. If the leader region is occupied by a
regulatory protein, the small ribosomal sub-unit
will not be able to attach to the mRNA. This means
that the ribosome assembly will not form, and
translation will not take place.
As in prokaryotes, enzymes in eukaryotes may
provide some additional processing of a polypeptide
product before it becomes a functional protein. If
certain regulatory proteins are present in the cell,
this processing might not take place. Even if it
does, other regulatory proteins can interfere with
the transport of polypeptides or functional proteinsSex hormones are one example of molecules that work as a
kind of transcription factor to activate the transcription of a
number of different genes at once.BIO FACT
CONCEPT ORGANIZER Factors Influencing the Rate of Gene Expression
Structure of DNA
(Chapter 7, section 7.2)Protein–DNA interactions
(Chapter 8, section 8.4)Protein–protein interactions
(Chapter 8, section 8.4)Rate of gene
expressionGene expression is the process in which the molecular
information stored in DNA is converted into structural and
functional polypeptides by a living cell. The rate of gene
expression is influenced by a number of factors. The
packaging of DNA within a cell can make some stretches of
DNA unavailable for transcription. Chemical modifications
to the nitrogenous bases can also serve to keep specific
portions of DNA from being expressed. In addition to these
structural features of DNA, proteins play an important role
in gene expression. Some proteins bind to DNA promoteror enhancer sequences to turn gene expression on or off.
In other cases, proteins bind to other proteins to speed
up or slow down the rate of gene expression. All of these
factors work in combination with one another to determine
the overall rate of gene expression. While genetic information
still flows from DNA to RNA to proteins, the central dogma
of gene expression also acknowledges that proteins play an
equal role in determining when and how genetic information
is expressed.