HUMAN BIOLOGY

412 Chapter 21

translation The phase of
protein synthesis in which
amino acids are assembled
into a chain, the primary
structure of a protein.

the three stages of translation

n the protein-building instructions carried by mRNAs

are translated into proteins. this process occurs on

ribosomes in the cytoplasm.

n Links to Primary protein structure 2.11, Ribosomes

3.2, the endomembrane system, 3.7

Translation is the second

phase of protein synthesis.

Unlike transcription, trans-

lation occurs in the cell

cytoplasm. During its three

stages, called initiation,

elongation, and termination, mRNA transcribed

from DNA is “translated” into a linear sequence

of amino acids that forms a polypeptide chain.

Figure 21.11 shows the steps involved.

To begin initiation, a tRNA that can start tran-

scription is loaded onto a ribosome subunit. This

initiator tRNA binds with the small ribosome

subunit. AUG, the start codon for the mRNA tran-

script, matches up with this tRNA’s anticodon. The

AUG also binds with the small subunit, which in

turn binds with a large ribosome subunit (step 1).

Now the next stage can begin.

In the elongation stage of translation, a poly-

peptide chain forms as the mRNA strand passes

between the ribosome subunits, like a thread

moving through the eye of a needle. Some pro-

teins in the ribosome are enzymes. They join

amino acids together in the sequence dictated by

the codon sequence in the mRNA molecule. In

steps 2 through 6 you can see that a peptide bond

forms between the most recently attached amino

acid and the next amino acid being delivered to

the ribosome. (Section 2.11 explains how a peptide

bond forms.)

During the last stage of translation, termina-

tion, a stop codon in the mRNA moves onto the

ribosome platform. No tRNA has a correspond-

ing anticodon, so translation stops. Enzymes then

detach the mRNA and the new chain from the

ribosome.

Some cells, such as unfertilized oocytes, may

have to be ready to make many copies of differ-

ent proteins very quickly. As “preparation,” they

stockpile transcribed mRNA in their cytoplasm.

In cells that are already rapidly using or secreting

proteins (such as endocrine cells that are making

hormones), polysomes are often present. A poly-

some is a cluster of ribosomes, all translating the

same mRNA transcript at the same time. The

transcript threads through all of them, one after

another (Figure 21.12A and 21.12B).

21.6

gly

met val leu

leu

met val

val

initiator tRNA

first amino acid

of polypeptide

start codon

(AUG) }

met

metval

peptide bond

met val leu

met val leu gly

4 A peptide bond

forms between the

second and third

amino acids.

1 Ribosome sub-

units and an initia-

tor tRNA converge

on an mRNA. A

second tRNA binds

to the second

codon.

2 A peptide bond

forms between

the first two amino

acids.

3 The first tRNA

is released and the

ribosome moves to

the next codon. A

third tRNA binds to

the third codon.

(^5) The second
tRNA is released
and the ribosome
moves to the next
codon. A fourth
tRNA binds the
fourth codon.
6 A peptide bond
forms between the
third and fourth
amino acids. The
process repeats
until the ribosome
encounters a
stop codon in the
mRNA.
F i g u r e 21.11 Animated! Translation occurs in a series of steps.
gly
met val leu
leu
met val
val
initiator tRNA
first amino acid
of polypeptide
start codon
(AUG) }
met
metval
peptide bond
met val leu
met val leu gly
gly
met val leu
leu
met val
val
initiator tRNA
first amino acid
of polypeptide
start codon
(AUG) }
met
metval
peptide bond
met val leu
met val leu gly
gly
met val leu
leu
met val
val
initiator tRNA
first amino acid
of polypeptide
start codon
(AUG) }
met
metval
peptide bond
met val leu
met val leu gly
gly
met val leu
leu
met val
val
initiator tRNA
first amino acid
of polypeptide
start codon
(AUG) }
met
metval
peptide bond
met val leu
met val leu gly
gly
met val leu
leu
met val
val
initiator tRNA
first amino acid
of polypeptide
start codon
(AUG) }
met
metval
peptide bond
met val leu
met val leu gly
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