Chapter 3
Cells: The Living Units
105
3
an outgoing tRNA, as illustrated in
Focus on Translation
(
Figure 3.37
,
on pp. 106–107). Now we are ready to put the
parts together—so let’s go!
Sequence of Events in Translation
Translation entails a now
familiar sequence of named events—
initiation
,
elongation
, and
ter-
mination
—which occur in the cytoplasm. Each of these phases
requires energy in the form of ATP and a specific set of protein
factors and enzymes. Figure 3.37 summarizes these events.
1
Initiation.
A small ribosomal subunit binds to a special
methionine-carrying
initiator tRNA
, and then to the “new”
mRNA to be decoded. With the initiator tRNA still in tow,
the small ribosomal subunit scans along the mRNA until it
encounters the
start codon
—the first AUG triplet it meets.
When the initiator tRNA’s UAC anticodon “recognizes” and
binds to the start codon, a large ribosomal subunit unites
with the small one, forming a functional ribosome.
At the end of this phase the mRNA is firmly positioned
in the groove between the ribosomal subunits, the initiator
tRNA is sitting in the P site, and the A site is vacant, ready
for the next aminoacyl tRNA to deliver its cargo. ±e next
phase, elongation of the polypeptide, now begins.
2
Elongation.
During the three-step cycle of elongation,
the mRNA is moved through the ribosome in one direc-
tion and one amino acid at a time is added to the growing
polypeptide (Figure 3.37).
2a
Codon recognition.
±e incoming aminoacyl-
tRNA binds to a complementary codon in the A site
of a ribosome.
2b
Peptide bond formation.
Once the accuracy of the
codon-anticodon binding is checked, an enzymatic
compo nent in the large ribosomal subunit catalyzes
peptide bond formation between the amino acid of
the tRNA in the P site to that of the tRNA in the A site.
2a
Translocation.
±is step translocates, or moves,
the tRNA in the A site to the P site. ±e unloaded
(vacant) tRNA is transferred to the E site, from which
it is released and ready to be recharged with an amino
acid from the cytoplasmic pool.
±is orderly “musical chairs” process continues: the
peptidyl-tRNAs transfer their polypeptide cargo to the
aminoacyl-tRNAs, and then the P-site-to-E-site and A-site-
to-P-site movements of the tRNAs occur (Figure 3.37). As
the ribosome “chugs” along the mRNA track and the mRNA
is progressively read, its initial portion passes through the
ribosome and may become attached successively to several
other ribosomes, all reading the same message simultane-
ously and sequentially. ±is multiple ribosome–mRNA
complex, a
polyribosome
, efficiently produces multiple cop-
ies of the same protein (
Figure 3.38
, on p. 108).
3
Termination.
±e mRNA strand continues to be read se-
quentially until its last codon, the
stop codon
(one UGA,
UAA, or UAG) enters the A site. ±e stop codon is the “pe-
riod” at the end of the mRNA sentence—it tells the ribo-
some that translation of that mRNA is finished. As a result,
a
protein release factor
binds to the stop codon at the A site
and directs the addition of water (instead of an amino acid)
to the polypeptide chain. ±is hydrolyzes (breaks) the bond
between the polypeptide and the tRNA in the P site. ±e
completed polypeptide chain is then released from the ri-
bosome, and the ribosome separates into its two subunits
(Figure 3.38a). ±e released protein may undergo process-
ing before it folds into its complex 3-D structure and floats
off, ready to work. When the message of the mRNA that
directed its formation is no longer needed, it is degraded.
Processing in the Rough ER
As noted earlier in the chapter, ri-
bosomes attach to and detach from the rough ER. When a short
“leader” peptide called an ER
signal sequence
is present in a pro-
tein being synthesized, the associated ribosome attaches to the
membrane of the rough ER. ±is signal sequence, with its attached
cargo of a ribosome and mRNA, is guided to appropriate receptor
sites on the ER membrane by a signal recognition particle (SRP), a
protein chaperone that cycles between the ER and the cytosol.
Figure 3.39
on p.108 details the subsequent events occurring
at the ER.
Summary: From DNA to Proteins
±e genetic information of a
cell is translated into the production of proteins via a sequence of
information transfer that is completely directed by complementary
base pairing. ±e transfer of information goes from DNA base se-
quence (triplets) to the complementary base sequence of mRNA
(codons) and then to the tRNA base sequence (anticodons), which
is identical to the template DNA sequence except for the substitu-
tion of uracil (U) for thymine (T) (
Figure 3.40
, on p. 109).
Other Roles of DNA
±e story of DNA doesn’t end with the production of proteins
encoded by exons. Scientists are finding that intron DNA actu-
ally codes for a surprising variety of active RNA species, includ-
ing the following:
Antisense RNAs
, made on the DNA strand complementary
to the template strand for mRNA, can intercept and bind to
the protein-coding mRNA strand and prevent it from being
translated into protein.
MicroRNAs
are small RNAs that can use RNA interference
machinery to interfere with and suppress mRNAs made by
certain exons, thus effectively silencing them.
Riboswitches
are folded RNAs that code, like mRNA, for a
particular protein. What sets them apart from other mRNAs
is a region that acts as a switch to turn protein synthesis on
or off in response to metabolic changes in their immediate
environment, such as shi²ing concentrations of amino acids
or other small molecules in the cell. When it senses these
changes, the riboswitch changes shape, thereby stopping or
starting production of the protein it specifies.
Beyond the discussion here and in Chapter 29, we still have
much to learn about these versatile RNA species that arise from
intron DNA and appear to play a role in heredity. Another area
of research focuses on the multitasking of DNA segments.
(Text continues on p. 109)
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