Pathophysiology - Definition and functions of the Nucleus.

The nucleus of a nondividing cell appears as a rounded
or elongated structure situated near the center of the cell
(see Fig. 1-1). It is enclosed in a nuclear envelope and contains chromatin, the genetic material of the nucleus,
and a distinct region called the nucleolus. All eukaryotic
cells have at least one nucleus (prokaryotic cells, such as
bacteria, lack a nucleus and nuclear membrane).
The nucleus can be regarded as the control center for the
cell. It contains the deoxyribonucleic acid (DNA) that is
essential to the cell because its genes encode the information
necessary for the synthesis of proteins that the cell must produce
to stay alive. The genes also represent the individual
units of inheritance that transmit information from one generation
to another. The nucleus also is the site for the synthesis
of the three types of ribonucleic acid (messenger
RNA, ribosomal RNA, and transfer RNA) that move to
the cytoplasm and carry out the actual synthesis of proteins.
Messenger RNA (mRNA) copies and carries the DNA
instructions for protein synthesis to the cytoplasm; ribosomal
RNA (rRNA) is the site of protein synthesis; and transfer
RNA (tRNA) transports amino acids to the site of
proteins synthesis for incorporation into the protein being
synthesized (see Chapter 5).
The complex structure of DNA and DNA-associated
proteins dispersed in the nuclear matrix is called chromatin.
Depending on its transcriptional activity, chromatin
may be condensed as an inactive form of chromatin
called heterochromatin or extended as a more active
form called euchromatin. Because heterochromatic
regions of the nucleus stain more intensely than regions
consisting of euchromatin, nuclear staining can be a
guide to cell activity. The nucleus also contains the darkly
stained round body called the nucleolus. Nucleoli are
structures composed of regions from five different chromosomes,
each with a part of the genetic code needed
for the synthesis of rRNA, which is transcribed exclusively
in the nucleolus. Cells that are actively synthesizing
proteins can be recognized because their nucleoli are
large and prominent and the nucleus as a whole is
euchromatic or slightly stained.
Surrounding the nucleus is the nuclear envelope
formed by an inner and outer nuclear membrane containing
a perinuclear cisternal space between them (Fig.
1-3). The inner nuclear membrane is supported by a rigid
network of protein filaments attached to its inner surface,
called nuclear lamina, that bind to chromosomes
and secure their position in the nucleus. The outer
nuclear membrane resembles the membrane of the endoplasmic
reticulum and is continuous with it. The nuclear
envelope contains many structurally complex circular
pores where the two membranes fuse to form a gap filled
with a thin protein diaphragm. Many classes of molecules,
including fluids, electrolytes, RNA, some proteins,
and hormones, can move in both directions through the
nuclear pores.

Figure 1-3. Schematic drawing of the inner and outer membranes
of the nuclear envelope. The double-membrane envelope
is penetrated by pores in which nuclear pore complexes
are positioned and continuous with the rough endoplasmic
reticulum. The nuclear lamina on the surface of the inner
membrane bind to DNA and hold the chromosomes in place.