Nucleolus and the synthesis of ribosomes

The nucleolus is a morphologically and
functionally specific region in the cell nucleus
in which ribosomes are synthesized. In man,
the rRNA genes (200 copies per haploid
genome) are transcribed by RNA polymerase I
to form 45 S rRNA molecules. After the 45 S
rRNA precursors have been produced, they are
quickly packaged with ribosomal proteins
(from the cytoplasm). Before they are transferred
from the nucleus to the cytoplasm, they
are cleaved to form three of the four rRNA subunits.
These are released into the cytoplasm
with the separately synthesized 5 S subunit.
Here they form functional ribosomes. The sizes
of ribosomes, their subunits, and different types
of ribosomal RNAs (rRNA) are given in Svedberg
units (S). This is the rate at which a molecule
sediments in a solvent. The S values are not additive.
A functional ribosome consists of a small
and a large subunit.