58
Chapter 3
the gene on the strand to be transcribed (see Figure 3–18). It
is to this promoter region that RNA polymerase binds and ini-
tiates transcription. Thus, for any given gene, only one DNA
strand is transcribed, and that is the strand with the promoter
region at the beginning of the gene.
Thus, the transcription of a gene begins when RNA poly-
merase binds to the promoter region of that gene. This initiates
the separation of the two strands of DNA. RNA polymerase
moves along the template strand, joining one ribonucleotide
at a time (at a rate of about 30 nucleotides per second) to the
growing RNA chain. Upon reaching a “stop” signal specifying
the end of the gene, the RNA polymerase releases the newly
formed RNA transcript, which is then translocated out of the
nucleus where it binds to ribosomes in the cytoplasm.
In a given cell, typically only 10 to 20 percent of the genes
present in DNA are transcribed into RNA. Genes are tran-
scribed only when RNA polymerase can bind to their promoter
sites. Cells use various mechanisms either to block or to make
accessible the promoter region of a particular gene to RNA
po
lymerase
.
Such
regu
lat
ion
o
f
gene
transcr
ipt
ion
prov
ides
a means of controlling the synthesis of specifi c proteins and
thereby the activities characteristic of a particular type of cell.
It must be emphasized that the base sequence in the
RNA transcript is not identical to that in the template strand
of DNA, because the RNA’s formation depends on the pairing
between
complementary,
not identical, bases (see Figure 3–18).
A three-base sequence in RNA that specifi es one amino acid
is called a
codon.
Each codon is complementary to a three-
base sequence in DNA. For example, the base sequence
T—A—C in the template strand of DNA corresponds to the
codon A—U—G in transcribed RNA.
Although the entire sequence of nucleotides in the template
strand of a gene is transcribed into a complementary sequence
of nucleotides known as the
primary RNA transcript,
only
certain segments of most genes actually code for sequences
of amino acids. These regions of the gene, known as
exons
(expression regions), are separated by noncoding sequences of
nucleotides known as
introns
(intervening sequences). It is
estimated that as much as 98.5 percent of human DNA is com-
posed of intron sequences that do not contain protein-coding
information. What role, if any, such large amounts of noncod-
ing DNA may perform is unclear, although they have recently
been postulated to exert some transcriptional regulation.
Before passing to the cytoplasm, a newly formed pri-
mary RNA transcript must undergo splicing (
Figure 3–19
)
to remove the sequences that correspond to the DNA introns.
This allows the formation of the continuous sequence of exons
that will be translated into protein. Only after this splicing
occurs is the RNA termed messenger RNA.
Splicing occurs in the nucleus and is performed by a
complex of proteins and small nuclear RNAs known as a
spliceosome.
The spliceosome identifi es specifi c nucleotide
sequences at the beginning and end of each intron-derived
segment in the primary RNA transcript, removes the segment,
and splices the end of one exon-derived segment to the begin-
ning of another to form mRNA with a continuous coding
sequence. In some cases during the splicing process, the exon-
derived segments from a single gene can be spliced together
in different sequences, or some exon-derived segments can be
deleted entirely. These processes result in the formation of dif-
ferent mRNA sequences from the same gene and give rise, in
turn, to proteins with slightly different amino acid sequences.
Translation: Polypeptide Synthesis
After splicing, the mRNA moves through the pores in the
nuclear envelope into the cytoplasm. Although the nuclear
pores allow the diffusion of small molecules and ions between
the nucleus and cytoplasm, they have specifi c energy-dependent
Cytoplasm
Nucleus
One gene
Exons
Introns
Transcription of DNA to RNA
Primary
RNA
transcript
RNA splicing by spliceosomes
Nuclear pore
Nuclear envelope
mRNA
Passage of processed mRNA
to cytosol through nuclear pore
Translation of mRNA into
polypeptide chain
mRNA
Polypeptide chain
DNA
Figure 3–19
Spliceosomes remove the noncoding intron-derived
segments from a primary RNA transcript and link the
exon-derived segments together to form the mRNA
molecule that passes through the nuclear pores to the
cytosol. The lengths of the intron- and exon-derived
segments represent the relative lengths of the base
sequences in these regions.
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