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, averaging about 5 μm in diameter. The nuclear envelope encloses the nucleus, 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 of each 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 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 protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic material so it functions efficiently.
Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with many proteins. Some of the proteins help coil the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus.
The complex of DNA and proteins making up chromosomes is called chromatin. Chromatin inside a eukaryotic cell nucleus can be classified into two major types based on its state of condensation: heterochromatin and euchromatin.
A prominent structure within the nondividing nucleus is the nucleolus (plural, nucleoli), which appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin.
The nucleus is involved in both mitosis and meiosis. Several events required for cell division occur in the nucleus, including DNA replication and transcription of DNA into precursor RNA molecules.
All of the enzymes required for replication and repair of newly synthesized DNA, as well as for transcription and processing of precursor RNA molecules, are found in the nucleus.
The synthesis and assembly of ribosomes occur in a region of the nucleus known as the nucleolus.
