A distinguishing characteristic of eukaryotes, the nucleus contains the genetic information (genome) of the cell in the form of its chromosomes. It is within the nucleus that the DNA in the chromosomes is duplicated prior to cell division and where the RNAs are synthesized. Ribosomes are partially assembled around the newly synthesized ribosomal RNAs (rRNA) while still in the nucleus and then transported into the cytoplasm to continue their final assembly. Similarly, messenger RNAs (mRNA) are synthesized, packaged, and subsequently transported to the cytoplasmic ribosomes, where they are translated into protein.
Typically spherical in shape and taking up 10 percent of the volume of a cell, the nucleus is bounded by a double membrane called the nuclear envelope (Figures 1 and 2). Most material passes in and out of the nuclear envelope through large openings called the nuclear pores. The outside surface of the envelope is directly connected to the endoplasmic reticulum of the cytoplasm and is surrounded by a network of cytoplasmic intermediate filaments. The inside surface of the nuclear envelope is lined with the nuclear lamina. Internally, the nucleus contains several structures: the chromosomes themselves, which together constitute the chromatin; the interchromatin compartment; the large nucleolus; and a variety of different granules collectively called the subnuclear bodies, which include Cajal (coiled) bodies, gems, PML bodies, and speckles. Every time a cell divides, the nuclear envelope must break down to release the recently duplicated chromosomes. After the chromosomes have segregated to the new daughter cells, the nucleus and its components must be rebuilt.
If the DNA of each cell were stretched out linearly, it would be over six feet in length. Although the chromosomes of a nucleus appear as a diffuse network in the electron microscope, they are highly compacted into nucleosomal units. Because of nucleosomal folding, the six feet of DNA yields an organelle tightly packed with chromosomal material. Consequently, it was thought that the nucleus in nondividing cells was a fairly static structure, with its various substructures locked into place. Since the 1980s, however, technological advances have permitted investigators to "paint" chromosomes, parts of chromosomes, genes, proteins, RNAs, or subnuclear bodies with genetically defined fluorescent tags. Combined with new techniques that permit these procedures in living cells, and coupled with time-lapse photography and computer simulation, an entirely different image of the cell nucleus is emerging. The nucleus is now understood to be a dynamic organelle composed of a highly ordered architecture that permits a great deal of structural flexibility and movement of molecules and particles between its various subcompartments.