The nucleolus is chiefly responsible for creating ribosomes, which in turn function as factories for protein synthesis. The nucleolus contains three main organizing regions where transcription and processing of rRNA occurs. It also purportedly aids in other processes, like the packaging of signal recognition particles and the alteration of transfer RNA. The nucleolus is located in the nucleus, and thus only exists in eukaryotic cells.
According to the Wiley Online Library, the first step in the process of ribosome formation is transcription. Here, ribosomal and nonribosomal proteins attach to and modify rRNA genes, which are located in a region known as the fibrillar center. Only more advanced eukaryotic cells possess a fibrillar center. Transcription of the rRNA occurs just between the fibrillar center and another region called the dense fibrillar component. Processing of the rRNA begins in the dense fibrillar component, and the procedure is finished in the granular component. This is where the rRNA is finally assembled with proteins to create units of pre-ribosomal material. At this point, the units are ejected into the cytoplasm of the cell.
The size of a nucleolus can vary greatly, depending on the type of cell it is found in. If a cell must make a large amount of protein, for example, it will require more ribosomes and thus necessitate a larger nucleolus to produce those ribosomes.