Polyribosomes -
Protein
synthesis
does
not
take
place
on
individual
ribosomes,
but
on
groups
or
clusters
of
ribosomes.
These groups of ribosomes are called polyribosomes, polysomes or ergosomes. The ribosomes of a polyribosome may be arranged in a linear manner or may form a rosette shaped cluster.
Two 70S or 80S ribosomes form dimers of 100S and 120S, respectively. The 200S polyribosomes responsible for haemoglobin synthesis have five to six ribosomes. The 600S polyribosomes of poliovirus have 16 to 18 ribosomes per cluster.
The
number
of
ribosomes
in
a
polyribosome
depends
on
the
length
of
the
mRNA
molecule
that
takes
part
in
synthesis.
