Essential Cell Biology 5th edition

254 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
QUESTION 7–5
A sequence of nucleotides in a DNA
strand—5ʹ-TTAACGGCTTTTTTC-3ʹ—
was used as a template to
synthesize an mRNA that was then
translated into protein. Predict
the C-terminal amino acid and
the N-terminal amino acid of the
resulting polypeptide. Assume that
the mRNA is translated without the
need for a start codon.
ribosome-binding sites
5′ 3′
P P P
mRNA
AUG AUG AUG
protein α protein β protein γ
Figure 7–40 A single prokaryotic mRNA molecule can encode several different
proteins. In prokaryotes, genes directing the different steps in a process are often
organized into clusters (operons) that are transcribed together into a single mRNA.
A prokaryotic mRNA does not have the same sort of 5ʹ cap as a eukaryotic mRNA,
but instead has a triphosphate at its 5ʹ end. Prokaryotic ribosomes initiate translation
at ribosome-binding sites (dark blue), which can be located in the interior of an
mRNA molecule. This feature enables prokaryotes to simultaneously synthesize
different proteins from a single mRNA molecule, with each protein made by a
different ribosome.
ECB5 e7.37/7.40
H 2 N
E
UAG
5′ 3′
H 2 N
5′
coding sequence
H 2 N
released COOH
polypeptide chain
5′
5′
E
H 2 O
A
UAG
UAG
UAG
3′ UTR
BINDING OF
RELEASE
FACTOR
TO THE
A SITE
TERMINATION
A
RIBOSOME
DISSOCIATES
3′
3′
3′
Next, the small ribosomal subunit loaded with the initiator tRNA binds
to the 5ʹ end of an mRNA molecule, which is marked by the 5ʹ cap that is
present on all eukaryotic mRNAs (see Figure 7–17). The small ribosomal
subunit then scans the mRNA, in the 5ʹ-to-3ʹ direction, until it encounters
the first AUG. When this AUG is recognized by the initiator tRNA, several
of the initiation factors dissociate from the small ribosomal subunit to
make way for the large ribosomal subunit to bind and complete ribosomal
assembly. Because the initiator tRNA is bound to the P site, protein
synthesis is ready to begin with the addition of the next charged tRNA to
the A site (see Figure 7–37).
The mechanism for selecting a start codon is different in bacteria. Bacterial
mRNAs have no 5ʹ caps to tell the ribosome where to begin searching for
the start of translation. Instead, each mRNA molecule contains a specific
ribosome-binding sequence, approximately six nucleotides long, located
a few nucleotides upstream of the AUG at which translation is to begin.
Unlike a eukaryotic ribosome, a prokaryotic ribosome can readily bind
directly to a start codon that lies in the interior of an mRNA, as long as a
ribosome-binding site precedes it by several nucleotides. Such ribosomebinding
sequences are necessary in bacteria, as prokaryotic mRNAs are
often polycistronic—that is, they encode several different proteins on the
same mRNA molecule; these transcripts contain a separate ribosomebinding
site for each protein-coding sequence (Figure 7–40). In contrast,
a eukaryotic mRNA usually carries the information for a single protein,
and so it can rely on the 5ʹ cap—and the proteins that recognize it—to
position the ribosome for its AUG search.
The end of translation in both prokaryotes and eukaryotes is signaled by
the presence of one of several codons, called stop codons, in the mRNA
(see Figure 7–27). The stop codons—UAA, UAG, and UGA—are not recognized
by a tRNA and do not specify an amino acid, but instead signal to
the ribosome to stop translation. Proteins known as release factors bind
to any stop codon that reaches the A site on the ribosome; this binding
alters the activity of the peptidyl transferase in the ribosome, causing it
to catalyze the addition of a water molecule instead of an amino acid to
the peptidyl-tRNA (Figure 7–41). This reaction frees the carboxyl end of
the polypeptide chain from its attachment to a tRNA molecule; because
this is the only attachment that holds the growing polypeptide to the
Figure 7–41 Translation halts at a stop codon. In the final phase of
protein synthesis, the binding of release factor to an A site bearing
a stop codon terminates translation of an mRNA molecule. The
completed polypeptide is released, and the ribosome dissociates into
its two separate subunits.