The eukaryotic cell's genetic instructions are housed in the nucleus

On the first stop of our detailed tour of the cell, let's look at two cellular components involved in the genetic control ofthe cell: the nucleus, which houses most ofthe cell's DNA, and the ribosomes, which use information from the DNA to make proteins.

The Nucleus : Information Central

         The nucleus contains most of the genes in the eukaryotic cell (some genes are located in mitochondria and chloroplasts). It is generally the most conspicuous organelle in a eukaryotic cell, averaging about 5 11m in diameter. The nuclear envelope encloses the nucleus (Figure 6.10), separating its contents from the cytoplasm.

          The nuclear envelope is a double membrane. The two membranes, each a lipid bilayer with associated proteins, are separated by a space of 20-40 nm. The envelope is perforated by pore structures that are about 100 nm in diameter. At the lip ofeach pore, the inner and outer membranes of the nuclear envelope are continuous. An intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of most proteins and RNAs, as well as large complexes of macromolecules. Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. There is also much evidence for a nuclear matrix, a framework of fibers extending throughout the nuclear interior.

          Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome is made up of a material called chromatin, a complex of proteins and DNA. Stained chromatin usually appears as a diffuse mass through both light microscopes and electron microscopes. As a cell prepares to divide, however, the thin chromatin fibers coil up (condense), becoming thick enough to be distinguished as the familiar separate structures we know as chromosomes. Each eukaryotic species has a characteristic number of chromosomes. A typical human cell, for example, has 46 chromosomes in its nucleus; the exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans. A fruit fly cell has 8 chromosomes in most cells and 4 in the sex cells.

           A prominent structure within the nondividing nucleus is the nucleolus (plural, nucu,'Oli), which appears through the electron microscope as a mass ofdensely stained granules and fibers adjoining part of the chromatin. Here a type of RNA called ribosomal RNA (rRNA) is synthesized from instructions in the DNA. Also in the nucleolus, proteins imported from the cytoplasm are assembled with rRNA into large and small ribosomal subunits. These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a
large and a small subunit can assemble into a ribosome. Sometimes there are two or more nucleoli; the number depends on the species and the stage in the cell's reproductive cycle. Recent studies suggest that the nucleolus also functions in regulation of some cellular processes, such as cell division.

        The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA molecule reaches the cytoplasm, ribosomes translate the mRNA's genetic message into the primary structure of a specific polypeptide.