The nucleus is a highly specialized organelle that serves as the
information and administrative center of the cell. This organelle has two
major functions. It stores the cell's hereditary material, or DNA, and it
coordinates the cell's activities, which include intermediary metabolism,
growth, protein synthesis, and reproduction (cell division).
Only the cells of advanced organisms, known
as eukaryotes, have a nucleus. Generally there is only one nucleus
per cell, but there are exceptions such as slime molds and the Siphonales
group of algae. Simpler one-celled organisms (prokaryotes), like
the bacteria and cyanobacteria, don't have a nucleus. In these organisms,
all the cell's information and administrative functions are dispersed
throughout the cytoplasm.
The spherical nucleus occupies about 10
percent of a cell's volume, making it the cell's most prominent feature.
Most of the nuclear material consists of chromatin, the unstructured form
of the cell's DNA that will organize to form chromosomes during mitosis or
cell division. Also inside the nucleus is the nucleolus, an organelle that
synthesizes protein-producing macromolecular assemblies called ribosomes.
A double-layered membrane, the nuclear
envelope, separates contents of the nucleus from the cellular cytoplasm.
The envelope is riddled with holes called nuclear pores that allow
specific types and sizes of molecules to pass back and forth between the
nucleus and the cytoplasm. It is also attached to a network of tubules,
called the endoplasmic reticulum, where protein synthesis occurs. These
tubules extend throughout the cell and manufacture the biochemical
products that a particular cell type is genetically coded to produce.
Chromatin/Chromosomes - Packed
inside the nucleus of every human cell is nearly 6 feet of DNA, which
is divided into 46 individual molecules, one for each chromosome and
each about 1.5 inches long. Packing all this material into a
microscopic cell nucleus is an extraordinary feat of packaging. For
DNA to function, it can't be crammed into the nucleus like a ball of
string. Instead, it is combined with proteins and organized into a
precise, compact structure, a dense string-like fiber called
Each DNA strand wraps around groups of
small protein molecules called histones, forming a series of bead-like
structures, called nucleosomes, connected by the DNA strand. Under the
microscope, uncondensed chromatin has a "beads on a string"
The string of nucleosomes, already
compacted by a factor of six, is then coiled into an even denser
structure, compacting the DNA by a factor of 40. This compression and
structuring of DNA serves several functions. The overall negative
charge of the DNA is neutralized by the positive charge of the histone
molecules, the DNA takes up much less space, and inactive DNA can be
folded into inaccessible locations until it is needed.
There are two types of chromatin.
Euchromatin is the genetically active portion and is involved in
transcribing RNA to produce proteins used in cell function and growth.
Heterochromatin contains inactive DNA and is the portion of chromatin
that is most condensed, since it not being used.
Throughout the life of a cell,
chromatin fibers take on different forms inside the nucleus. During
interphase, when the cell is carrying out its normal functions, the
chromatin is dispersed throughout the nucleus in what appears to be a
tangle of fibers. This exposes the euchromatin and makes it available
for the transcription process.
When the cell enters metaphase and
prepares to divide, the chromatin changes dramatically. First, all the
chromatin strands make copies of themselves through the process of DNA
replication. Then they are compressed to an even greater degree than
at interphase, a 10,000-fold compaction, into specialized structures
for reproduction, termed chromosomes. As the cell divides to become
two cells, the chromosomes separate, giving each cell a complete copy
of the genetic information contained in the chromatin.
Nucleolus - The nucleolus is a
membrane-less organelle within the nucleus that manufactures ribosomes,
the cell's protein-producing structures. Through the microscope, the
nucleolus looks like a large dark spot within the nucleus. A nucleus
may contain up to four nucleoli, but within each species the number of
nucleoli is fixed. After a cell divides, a nucleolus is formed when
chromosomes are brought together into nucleolar organizing regions.
During cell division, the nucleolus disappears. Some studies suggest
that the nucleolus may be involved with cellular aging and, therefore,
may affect the aging of an organism.
Nuclear Envelope - The nuclear
envelope is a double-layered membrane that encloses the contents of
the nucleus during most of the cellís lifecycle. The space between
the layers is called the perinuclear space and appears to connect with
the rough endoplasmic reticulum. The envelope is perforated with tiny
holes called nuclear pores. These pores regulate the passage of
molecules between the nucleus and cytoplasm, permitting some to pass
through the membrane, but not others. The inner surface has a protein
lining called the nuclear lamina, which binds to chromatin and other
nuclear components. During mitosis, or cell division, the nuclear
envelope disintegrates, but reforms as the two cells complete their
formation and the chromatin begins to unravel and disperse.
Nuclear Pores - The nuclear
envelope is perforated with holes called nuclear pores. These pores
regulate the passage of molecules between the nucleus and cytoplasm,
permitting some to pass through the membrane, but not others. Building
blocks for building DNA and RNA are allowed into the nucleus as well
as molecules that provide the energy for constructing genetic
The pores are fully permeable to small
molecules up to the size of the smallest proteins, but form a barrier
keeping most large molecules out of the nucleus. Some larger proteins,
such as histones, are given admittance into the nucleus. Each pore is
surrounded by an elaborate protein structure called the nuclear pore
complex, which probably selects large molecules for entrance into the