Essential Cell Biology 5th edition

238 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
DNA
P
P
P
P
RNA polymerase II
Figure 7–16 Phosphorylation of the tail of RNA polymerase II
allows RNA-processing proteins to assemble there. Capping,
polyadenylation, and splicing are all modifications that occur as the
RNA is being synthesized. Note that the phosphates shown here
are in addition to the ones required for transcription initiation (see
Figure 7–12).
splicing
factors
capping factors
P
P
polyadenylation
factors
P
RNA
PROCESSING
BEGINS
P
mRNA
ECB5 e7.15/7.16
Two of these processing steps, capping and polyadenylation, occur on all
RNA transcripts destined to become mRNA molecules.
1. RNA capping modifies the 5ʹ end of the RNA transcript, the part of
the RNA that is synthesized first. The RNA cap includes an atypical
nucleotide: a guanine (G) nucleotide bearing a methyl group is
attached to the 5ʹ end of the RNA in an unusual way (Figure 7–17).
In bacteria, by contrast, the 5ʹ end of an mRNA molecule is simply
the first nucleotide of the transcript. In eukaryotic cells, capping takes
place after RNA polymerase II has produced about 25 nucleotides of
RNA, long before it has completed transcribing the whole gene.
2. Polyadenylation provides a newly transcribed mRNA with a
special structure at its 3ʹ end. In contrast with bacteria, where the
3ʹ end of an mRNA is simply the end of the chain synthesized by the
RNA polymerase, the 3′ end of a eukaryotic mRNA is first trimmed
by an enzyme that cuts the RNA chain at a particular sequence of
nucleotides. The transcript is then finished off by a second enzyme
that adds a series of repeated adenine (A) nucleotides to the trimmed
end. This poly-A tail is generally a few hundred nucleotides long (see
Figure 7–17A).
These two modifications—capping and polyadenylation—increase the
stability of a eukaryotic mRNA molecule, facilitate its export from the
nucleus to the cytosol, and generally mark the RNA molecule as an
mRNA. They are also used by the protein-synthesis machinery to make
sure that both ends of the mRNA are present and that the message is
therefore complete before protein synthesis begins.
Figure 7–17 Eukaryotic mRNA
molecules are modified by capping and
polyadenylation. (A) A eukaryotic mRNA
has a cap at the 5ʹ end and a poly-A tail at
the 3ʹ end. In addition to the nucleotide
sequences that code for protein, most
mRNAs also contain extra, noncoding
sequences, as shown. The noncoding
portion at the 5ʹ end is called the
5ʹ untranslated region, or 5ʹ UTR, and that
at the 3ʹ end is called the 3ʹ UTR. (B) The
structure of the 5ʹ cap. Many eukaryotic
mRNA caps carry an additional modification:
the 2ʹ-hydroxyl group on the second ribose
sugar in the mRNA is methylated (not
shown).
HO
N +
CH 3
5′ cap
OH
7-methylguanosine
5′
CH 2
5′
P P P CH 2
5′-to-5′
triphosphate
bridge
P
OH
CH 2
OH
5′ end of initial
RNA transcript
RNA capping and polyadenylation
3′
noncoding coding
noncoding
5′
sequence (5′ UTR) sequence sequence (3′ UTR)
G P P P
+
mRNA
AAAAA 150–250
CH 3 poly-A tail
5′ cap
P
CH 2
OH
(A)
protein
(B)