Cellular Structure, Proteins, and Metabolism
59
mechanisms for the selective transport of large molecules such
as proteins and RNA.
In the cytoplasm, mRNA binds to a ribosome, the cell
organelle that contains the enzymes and other components
required for the translation of mRNA into protein. Before
describing this assembly process, we will examine the struc-
ture of a ribosome and the characteristics of two additional
classes of RNA involved in protein synthesis.
Ribosomes and rRNA
A ribosome is a complex particle composed of about 70 to 80
different proteins in association with a class of RNA molecules
known as
ribosomal RNA (rRNA).
The genes for rRNA are
transcribed from DNA in a process similar to that for mRNA
except that a different RNA polymerase is used. Ribosomal
RNA transcription occurs in the region of the nucleus known
as the nucleolus. Ribosomal proteins, like other proteins, are
synthesized in the cytoplasm from the mRNAs specifi c for
them. These proteins then move back through nuclear pores
to the nucleolus, where they combine with newly synthesized
rRNA to form two ribosomal subunits, one large and one
small. These subunits are then individually transported to the
cytoplasm, where they combine to form a functional ribosome
during protein translation.
Transfer RNA
How do individual amino acids identify the appropriate codons
in mRNA during the process of translation? By themselves,
free amino acids do not have the ability to bind to the bases
in mRNA codons. This process of identifi cation involves the
third major class of RNA, known as
transfer RNA (tRNA).
Transfer RNA molecules are the smallest (about 80 nucleo-
tides long) of the major classes of RNA. The single chain of
tRNA loops back upon itself, forming a structure resembling
a cloverleaf with three loops (
Figure 3–20
).
Like mRNA and rRNA, tRNA molecules are synthe-
sized in the nucleus by base-pairing with DNA nucleotides at
specifi
c tRNA genes; then they move to the cytoplasm. The
key to tRNA’s role in protein synthesis is its ability to com-
bine with both a specifi c amino acid and a codon in ribo-
some-bound mRNA specifi c for that amino acid. This permits
tRNA to act as the link between an amino acid and the
mRNA codon for that amino acid.
A tRNA molecule is covalently linked to a specifi c amino
ac
id
by
an
enzyme
known
as
am
inoacy
l-tRNA
synthetase
.
There are 20 different aminoacyl-tRNA synthetases, each of
which catalyzes the linkage of a specifi c amino acid to a specifi c
type of tRNA. The next step is to link the tRNA, bearing its
attached amino acid, to the mRNA codon for that amino acid.
This is achieved by base-pairing between tRNA and mRNA.
A three-nucleotide sequence at the end of one of the loops of
tRNA can base-pair with a complementary codon in mRNA.
This tRNA three-letter code sequence is appropriately termed
an
anticodon.
Figure 3–20 illustrates the binding between
mRNA and a tRNA specifi c for the amino acid tryptophan.
Note that tryptophan is covalently linked to one end of tRNA
and does not bind to either the anticodon region of tRNA or
the codon region of mRNA.
Tryptophan
Anticodon
Tryptophan codon
mRNA
Tryptophan tRNA
A
C
C
UGG
Figure 3–20
Base-pairing between the anticodon region of a tRNA molecule and
the corresponding codon region of an mRNA molecule.
Protein Assembly
The process of assembling a polypeptide chain based on an
mRNA message involves three stages—initiation, elongation,
and termination. The initiation of synthesis occurs when a
tRNA containing the amino acid methionine binds to the small
ribosomal subunit. A number of proteins known as
initiation
factors
are required to establish an initiation complex, which
positions the methionine-containing tRNA opposite the mRNA
codon that signals the start site at which assembly is to begin.
The large ribosomal subunit then binds, enclosing the mRNA
between the two subunits. This initiation phase is the slowest
step in protein assembly, and factors that infl uence the activity of
initiation factors can regulate the rate of protein synthesis.
Following the initiation process, the protein chain is elon-
gated by the successive addition of amino acids (
Figure 3–21
).
A ribosome has two binding sites for tRNA. Site 1 holds the
tRNA linked to the portion of the protein chain that has been
assembled up to this point, and site 2 holds the tRNA contain-
ing the next amino acid to be added to the chain. Ribosomal
enzymes catalyze the linkage of the protein chain to the newly
arrived amino acid. Following the formation of the peptide
bond, the tRNA at site 1 is released from the ribosome, and
the tRNA at site 2—now linked to the peptide chain—is trans-
ferred to site 1. The ribosome moves down one codon along the
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