50
Chapter 3
next generation of cells. This information, coded in molecules
of DNA, is also used to synthesize the proteins that determine
the structure and function of the cell, as described later in this
chapter.
Surrounding the nucleus is a barrier, the
nuclear enve-
lope,
composed of two membranes. At regular intervals along
the surface of the nuclear envelope, the two membranes are
joined to each other, forming the rims of circular openings
known as
nuclear pores
(
Figure 3–11
). RNA molecules that
determine the structure of proteins synthesized in the cyto-
plasm move between the nucleus and cytoplasm through these
nuclear pores. Proteins that modulate the expression of various
genes in DNA move into the nucleus through these pores.
Within the nucleus, DNA, in association with proteins,
forms a fi ne network of threads known as
chromatin.
The
threads are coiled to a greater or lesser degree, producing
the variations in density seen in electron micrographs of the
nucleus (see Figure 3–11). At the time of cell division, the
chromatin threads become tightly condensed, forming rod-
like bodies known as
chromosomes.
The most prominent structure in the nucleus is the
nucleolus,
a densely staining fi lamentous region without a
membrane. It is associated with specifi c regions of DNA that
contain the genes for forming the particular type of RNA
found in cytoplasmic organelles called ribosomes. This RNA
and the protein components of ribosomes are assembled in the
nucleolus, then transferred through the nuclear pores to the
cytoplasm, where they form functional ribosomes.
Ribosomes
Ribosomes
are the protein factories of a cell. On ribosomes,
protein molecules are synthesized from amino acids, using
genetic information carried by RNA messenger molecules
from DNA in the nucleus. Ribosomes are large particles, about
20 nm in diameter, composed of about 70 to 80 proteins and
several RNA molecules. As described in Section B, ribosomes
consist of two subunits that are either fl oating free in the cyto-
plasm or combine during protein synthesis. In the latter case,
the ribosomes bind to the organelle called rough endoplasmic
reticulum (described next). A typical cell may contain as many
as 10 million ribosomes.
The proteins synthesized on the free ribosomes are
released into the cytosol, where they perform their varied
functions. The proteins synthesized by ribosomes attached to
Function:
Site of ribosomal RNA synthesis.
Assembles RNA and protein components of
ribosomal subunits, which then move to the
cytoplasm through nuclear pores.
Structure:
Largest organelle. Round or oval body
located near the cell center. Surrounded by a nuclear
envelope composed of two membranes. Envelope
contains nuclear pores; messenger molecules pass
between the nucleus and the cytoplasm through
these pores. No membrane-bound organelles are
present in the nucleus, which contains coiled strands
of DNA known as chromatin. These condense to form
chromosomes at the time of cell division.
Nucleus
Function:
Stores and transmits genetic information
in the form of DNA. Genetic information passes
from the nucleus to the cytoplasm, where amino
acids are assembled into proteins.
Structure:
Densely stained filamentous structure
within the nucleus. Consists of proteins associated
with DNA in regions where information concerning
ribosomal proteins is being expressed.
Nucleolus
Chromatin
Nuclear envelope
Nucleolus
Nuclear
pores
Figure 3–11
Nucleus and nucleolus.
Electron micrograph courtesy of K. R. Porter.
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