The most prominent nuclear feature, the nucleolus is a ribosomal factory. To make the large number of ribosomes needed, eukaryotic genomes carry multiple rRNA gene copies. The human genome contains 180 rRNA genes located on the tips of five different chromosomes (chromosomes 13, 14, 15, 21, and 22). Anchored by the opposite end to the nuclear envelope, each in their own chromosomal territory, the tip of these five chromosomal pairs (ten chromosomes in a diploid cell) extend into the center of the nucleus and come together, and the rRNA genes align to form what is called the nucleolar organizer. Transcription of the rRNA genes 28S, 18S, and 5S occurs rapidly. The transcripts are immediately processed and sequentially packaged through multiple stages into ribosomal subunits. The processing is complicated, requiring many cytoplasmic proteins and enzymes that are transported through the nuclear pores, diffusing through the interchromatin compartment until they reach the nucleolus, where they bind and remain. The nucleolus itself is composed of three subdomains: the nucleolar organizer; rRNA in the process of being transcribed, which is seen as dense fibrils; and granules, which are ribosomes very early in the assembly process.
Proteomic analysis indicates that human nucleoli contain at least 271 different proteins of a diverse array of known functions, with 31 percent encoded by unknown genes. This has raised the distinct possibility that the nucleolus performs other functions besides ribosome synthesis. Corroborating data suggest that the nucleolus entraps specific cell-cycle regulatory proteins (such as CDC14), inhibiting their activity until needed. When released from the nucleolus, they regain activity. Nucleoli may also synthesize and/or transport other ribonucleoprotein particles besides the ribosome, and may play a role in the processing and transport of mRNA or tRNA. Because nucleoli are often seen associating with other subnuclear bodies such as Cajal bodies, additional functions are likely.