Nucleus:
A cross section through the nucleus
reveals that it is actually a double
membrane organelle.   The membrane
is often called the NUCLEAR ENVELOP
because of its double membrane
construction.  The OUTER MEMBRANE is
covered with ribosomes.  It functions
like the rough endoplasmic reticulum. 
It is often seen to be continuous
with the rER when electron
micrographs are examined closely. 
It is unclear if special types of
proteins utilize the outer membrane
differently from those that attach
to the rER.  In some cells that have
mostly sER the outer membrane may
function as the only membrane to bind
ribosomes.  The INNER MEMBRANE in
contrast does not have ribosomes. 
Instead it is the attachment sites
of the chromosomes.  The chromosomes
can be seen on this EM as dark
patches.   Between the chromosomes
and the inner membrane is a network
of fibers called the NUCLEAR LAMINA
.  This network is needed to keep the
fragile nucleus together. 
  During prophase of mitosis the
nuclear envelop disappears.  A
special enzyme phosphorylates the
nuclear lamina protein ( protein
kinase ).  This causes it to fall
into small segments, which blend in
easily with the ER.  During telophase
the nuclear envelope is reassembled
in each daughter cell. Presumably the
phosphate is removed from the lamina
to accomplish this.  No new protein
synthesis is needed.
  The remaining feature of the
nucleus is the pore complex which
will be explained on the next images.