Organization of the Body
(b) This transmission electron micrograph shows
a large polyribosome (400,000
(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.
Polyribosome arrays allow a single strand of mRNA to be
translated into hundreds of the same polypeptide molecules in a short time.
The SRP directs the mRNA-ribosome
complex to the rough ER. There the SRP
binds to a receptor site.
Once attached to the ER, the SRP is released
and the growing polypeptide snakes through the
ER membrane pore into the cistern.
An enzyme clips off the signal sequence.
As protein synthesis continues, sugar groups
may be added to the protein.
In this example, the completed protein is
released from the ribosome and folds into its
3-D conformation, a process aided by
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
Rough ER cistern
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.