Biology 12

Chapter 8 Protein Synthesis • MHR 265

Each active ribosome has a binding site for the
mRNA transcript and three binding sites for tRNA
molecules: the P site, which holds one aa-tRNA
and the growing chain of amino acids; the A site,
which holds the tRNA bringing the next amino
acid to be added to the chain; and the E site,
which releases the tRNA molecules back into
the cytoplasm. These binding sites are provided
by complementary nucleotide sequences on the
different RNA molecules.
Like the transcription process, translation can
be examined as a sequence of steps: initiation,
elongation, and termination. These three steps
are examined in more detail below.

Initiation

Translation is initiated when an mRNA molecule
reaches the cytoplasm of the cell. A sequence of
nucleotides at the 5 ′end of the mRNA molecule
binds to a portion of the rRNA strand in a small
ribosomal sub-unit. A special initiator tRNA
molecule also binds to the ribosomal-mRNA
complex. This molecule has the anticodon UAC (to
complement the start codon AUG) and carries the
amino acid methionine. The resulting combination
binds to a large ribosomal sub-unit in order to
complete the assembly of the translation initiation
complex as illustrated in Figure 8.18.

Figure 8.17The two sub-units in an active ribosome form
binding sites for the mRNA transcript and the aa-tRNA
molecules involved in protein synthesis. Ribosomal RNA
molecules are not specific to any polypeptide product,
which means that the same ribosome can be used to
synthesize any protein.

Figure 8.18At the end of the initiation stage, the two

ribosomal sub-units have come together to create an active

ribosome bound to a strand of mRNA. An initiator tRNA sits

at the P site. Both the A site and the E site are vacant.

As part of the initiation process, the ribosomal

sub-units must bind at precisely the right place

on the mRNA molecule. If the only signal for

translation initiation was the start codon AUG,

a tRNA anticodon could incorrectly bind to the

sequence AUG at any point along the mRNA

strand. Base pairing between the mRNA molecule

and the nucleotide sequence in rRNA help resolve

this problem.

To understand how this process works,

remember that each mRNA molecule has a leader

sequence ahead of the start codon. This leader

sequence interacts with the rRNA in the small

ribosomal sub-unit to help establish the correct

starting place for translation. This interaction takes

place before the large ribosomal sub-unit joins the

initiation complex, with the result that the initial

anticodon binding site is correctly established

before translation begins.

Elongation

Once the initiation step is complete, the synthesis

of a polypeptide involves repeating three general

steps in a cycle. First the mRNA codon exposed

in the A binding site forms a base pair with the

anticodon of an incoming aa-tRNA molecule. Then

enzymes and other molecules in the large sub-unit

catalyze the formation of a peptide bond that joins

the last amino acid in the growing peptide chain

to the new amino acid. At the same time, the

polypeptide chain is transferred from the tRNA

in the P site to the tRNA in the A site.

In the third step, the ribosome moves a distance

of three nucleotides along the mRNA molecule.

This shift, which results in the ribosome assembly

initiator tRNA

in P site

met

EA

5 ′

3 ′

large ribosomal

sub-unit

small ribosomal

sub-unit

uac

aug

P site

A site

large ribosomal

sub-unit

small ribosomal

sub-unit

mRNA

binding

site

E site

Many antibiotics work by paralyzing prokaryotic ribosomes.

The structural differences between prokaryotic and

eukaryotic ribosomes mean that these antibiotics can

kill bacteria without harming the patient.

BIO FACT