Essential Cell Biology 5th edition

From DNA to RNA
237
transcription
start site
–35 –30 +30
location
–35
–30
transcription start site
+30
TATA
BOX
DNA sequence
G/C G/C G/A C G C C
T A T A A/T A A/T
C/T C/T A N T/A C/T C/T
A/G G A/T C G T G
general
transcription
factor
TFIIB
TBP
subunit of TFIID
TFIID
TFIID
Figure 7–14 Eukaryotic promoters
contain sequences that promote the
binding of the general transcription
factors. The location of each sequence
and the general transcription factor that
recognizes it are indicated. N stands for
any nucleotide, and a slash (/) indicates
that either nucleotide can be found at the
indicated position. For most start sites
transcribed by RNA polymerase II, only two
or three of the four sequences are needed.
Although most of these DNA sequences
are located upstream of the transcription
start site, one, at +30, is located within the
transcribed region of the gene.
7–12E). This action is initiated by the general transcription factor TFIIH,
MBoC6 m6.16-7.14
which contains a protein kinase as one of its subunits. Once transcription
has begun, most of the general transcription factors dissociate from
the DNA and then are available to initiate another round of transcription
with a new RNA polymerase molecule. When RNA polymerase II finishes
transcribing a gene, it too is released from the DNA; the phosphates on its
tail are stripped off by protein phosphatases, and the polymerase is then
ready to find a new promoter. Only the dephosphorylated form of RNA
polymerase II can re-initiate RNA synthesis.
Eukaryotic mRNAs Are Processed in the Nucleus
The principle of templating, by which DNA is transcribed into RNA, is the
same in all organisms; however, the way in which the resulting RNA transcripts
are handled before they are translated into protein differs between
bacteria and eukaryotes. Because bacteria lack a nucleus, their DNA is
directly exposed to the cytosol, which contains the ribosomes on which
protein synthesis takes place. As an mRNA molecule in a bacterium starts
to be synthesized, ribosomes immediately attach to the free 5ʹ end of the
RNA transcript and begin translating it into protein.
In eukaryotic cells, by contrast, DNA is enclosed within the nucleus, which
is where transcription takes place. Translation, however, occurs on ribosomes
that are located in the cytosol. So, before a eukaryotic mRNA
can be translated into protein, it must be transported out of the nucleus
through small pores in the nuclear envelope (Figure 7–15). And before it
can be exported to the cytosol, a eukaryotic RNA must go through several
RNA processing steps, which include capping, splicing, and polyadenylation,
as we discuss shortly. These steps take place as the RNA is being
synthesized. The enzymes responsible for RNA processing ride on the
phosphorylated tail of eukaryotic RNA polymerase II as it synthesizes an
RNA molecule (see Figure 7–12), and they process the transcript as it
emerges from the polymerase (Figure 7–16).
nuclear
envelope
nucleolus
Figure 7–15 Before they can be translated, mRNA molecules made
in the nucleus must be exported to the cytosol via pores in the
nuclear envelope (red arrows). Shown here is a section of a liver cell
nucleus. The nucleolus is where ribosomal RNAs are synthesized and
combined with proteins to form ribosomes, which are then exported
to the cytosol. (From D.W. Fawcett, A Textbook of Histology, 12th ed.
1994. With permission from Taylor & Francis Books UK.)
cytosol
nucleus
5 μm