Nucleus: A cross section through
the nucleus reveals that it is
actually a double membrane
organelle.  The membrane is often
called the NUCLEAR ENVELOPE 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
the types of proteins produced on
ribosomes of the outer membranes
are differnt from those produced on
the ribosomes of 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.
 During meoisis the attachment
sites on the inner membrane seem to
help the homologous pair of
chromosomes aline. 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 envelope
disappears.  A special enzyme
phosphorylates the nuclear lamina
protein ( protein kinase ).  This
causes it to fall into small
segments, which blends in easily
with the ER.  During telophase the
nuclear envelope is reassembled in
each daughter cell.   No new
protein synthesis is needed. The
remaining feature of the nucleus is
the pore complex which will be
explained on the next images.