108
UNIT 1
Organization of the Body
3
Start of
mRNA
Incoming
ribosomal
subunits
Growing polypeptides
Completed
polypeptide
End of
mRNA
Polyribosome
Ribosomes
mRNA
(b) This transmission electron micrograph shows
a large polyribosome (400,000
m
).
(a) Each polyribosome consists of one strand of mRNA being
read by several ribosomes simultaneously.
In this diagram, the
mRNA is moving to the left and the “oldest” functional ribosome is
farthest to the right.
Figure 3.38
Polyribosome arrays.
Polyribosome arrays allow a single strand of mRNA to be
translated into hundreds of the same polypeptide molecules in a short time.
1
The SRP directs the mRNA-ribosome
complex to the rough ER. There the SRP
binds to a receptor site.
2
Once attached to the ER, the SRP is released
and the growing polypeptide snakes through the
ER membrane pore into the cistern.
3
An enzyme clips off the signal sequence.
As protein synthesis continues, sugar groups
may be added to the protein.
4
In this example, the completed protein is
released from the ribosome and folds into its
3-D conformation, a process aided by
molecular chaperones.
5
The protein is enclosed within a protein
coated transport vesicle. The transport vesicles
make their way to the Golgi apparatus, where
further processing of the proteins occurs (see
Figure 3.19).
Ribosome
ER signal
sequence
Signal
recognition
particle
(SRP)
Receptor site
mRNA
Growing
polypeptide
Signal
sequence
removed
Sugar
group
Released
protein
Transport vesicle
pinching off
Protein-coated
transport vesicle
Rough ER cistern
Cytosol
Figure 3.39
Rough ER processing of proteins.
An endoplasmic reticulum (ER) signal
sequence in a newly forming protein causes the signal recognition particle (SRP) to direct the
mRNA-ribosome complex to the rough ER.
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