During the 1830s the nucleolus was identified by bright-field microscopy. Till 1964, very few were aware of the function of the nucleolus, when a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopuslaevis create an interest in the function and in-depth detailed of the structure of the nucleolus. In the study, they found that 25% of the frog eggs had no nucleolus and those eggs were incapable for life. Partially some of the eggs had one nucleolus and 25% had two. In nutshell, the nucleolus had a function necessary for life. According to Birnstiel in 1966, and other collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli code for ribosomal RNA.
There are three major components of the nucleolus are recognized: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). Transcription of the rDNA falls in the FC. The DFC contains the protein fibrillarin, which is necessary for rRNA processing. The GC contains the protein nucleophosmin, (B23 in the external image) which is also mixed up with ribosome biogenesis. Nevertheless, it has been planned that this particular organization is only observed in higher eukaryotes and that it evolves from a bipartite organization with the transition from anamniotes to amniotes. Reflecting the extensive increase in the DNA intergenic region, a new fibrillar component would have divided into the FC and the DFC.
In ribosome biogenesis, two of the three eukaryotic RNA polymerases are required, and these functions are done in a step by step manner. In the first stage, the rRNA genes are transcribing as one unit within the nucleolus by RNA polymerase I. In order for this transcription to occur, several test pol I-associated factors and DNA-important trans-acting factor are needed. In humans, a similar PIC has assembled with SL1, the promoter selectivity factor (composed of TBP and TBP-associated factors, or TAFs), transcription beginning factors, and UBF (upstream binding factor). RNA polymerase I transcribe most rRNA transcripts 28S, 18S, and 5.8S) but the 5S rRNA subunit (the component of the 60S ribosomal subunit) is transcribed by RNA polymerase III.
• The nucleolus, whose key function is to hold ribosomes together, is the main structure in the cell nucleus.