The nucleolus (nucleoli, plural) is one of the most important components of the cell nucleus, and is far the most easily recognized substructure in the nucleus of eukaryotic cells. It can easily be determined by phase contrast microscopy and with the use of different dyes. The nucleolus, also known as the ribosome factory, is composed of protein, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA).
The nucleolus function can be described thus: a long ribosomal RNA (rRNA) precursor molecule being transcribed from DNA, at the nucleus, is being processed into three mature RNAs that are packed together with certain types of proteins in order to create small and large ribosomal subunits. As soon as the subunits have been assembled, these will then be transported out of the nucleolus for the cytoplasm to use in functions such as protein synthesis, or translation.
The nucleolus is not a static structure by nature. It will eventually dissemble during mitosis and will transform back in the early G1 phase. The formation of this nucleus component does not cause ribosomal RNA or rRNA gene expression but rather, produced nucleoli are actually the result of rRNA processing and transcription.
The definition of nucleolus can be described as the place where the biogenesis of ribosomal subunits and ribosomal rRNA occurs. Or that of a non-membrane bound structure that is also composed of nucleic acids, found within the nucleus. The rRNA is assembled and transcribed within this structure and several human diseases can occur when malfunctioning of the nucleoli happens.
Barbara McClintock first described the formation of nucleoli, which are formed around the nucleolar organizing regions (NORs), specific genetic loci. The nucleolus is defined as a "genetically determined element” due to this non-random organization.