Stop codon
Release factor
3
Termination.
When a stop codon (UGA,
UAA, or UAG) arrives at the A site, elongation
ends.
Release of the newly made polypeptide
is triggered by a release factor and the ribosomal
subunits separate, releasing the mRNA.
E
P
E
P
A
E
P
A
E
P
A
Complementary
mRNA codon
tRNA
anticodon
Amino acid
corresponding
to anticodon
Polypeptide
Released
tRNA
New peptide
bond
Pro
Ile
Ile
Direction of ribosome movement
Polypeptide
2
Elongation.
Amino acids are added one at a time to the
growing peptide chain via a process that has three repeating steps.
2a
Codon recognition.
The
anticodon of an incoming tRNA
binds with the complementary
mRNA codon (A to U and C to
G) in the A site of the ribosome.
2b
Peptide bond formation.
The
growing polypeptide bound to the
tRNA at the P site is transferred to
the amino acid carried by the tRNA
in the A site, and a new peptide
bond is formed.
2c
Translocation.
As the entire
ribosome translocates, it shifts by
one codon along the mRNA:
• The unloaded tRNA in the P site
is moved to the E site and then
released.
• The tRNA in the A site moves to
the P site.
• The next codon to be translated
is now in the empty A site ready
for step 2a again.
Leu
Pro
Ile
Pro
Leu
Leu
G
C
C
A
G
U
C
C U
C
A
G
U
C
A
U
C
A
G
U
C
A
U
C
A
G
U
C
A
U
C
A
G
U
C
A
U
C
A
G
G
G
U
C
A
G
G
G
G
C
U
C
G
A
G
C
G
C
G
A
107
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