<P> The nucleolus is a discrete densely stained structure found in the nucleus . It is not surrounded by a membrane, and is sometimes called a suborganelle . It forms around tandem repeats of rDNA, DNA coding for ribosomal RNA (rRNA). These regions are called nucleolar organizer regions (NOR). The main roles of the nucleolus are to synthesize rRNA and assemble ribosomes . The structural cohesion of the nucleolus depends on its activity, as ribosomal assembly in the nucleolus results in the transient association of nucleolar components, facilitating further ribosomal assembly, and hence further association . This model is supported by observations that inactivation of rDNA results in intermingling of nucleolar structures . </P> <P> In the first step of ribosome assembly, a protein called RNA polymerase I transcribes rDNA, which forms a large pre-rRNA precursor . This is cleaved into the subunits 5.8 S, 18S, and 28S rRNA . The transcription, post-transcriptional processing, and assembly of rRNA occurs in the nucleolus, aided by small nucleolar RNA (snoRNA) molecules, some of which are derived from spliced introns from messenger RNAs encoding genes related to ribosomal function . The assembled ribosomal subunits are the largest structures passed through the nuclear pores . </P> <P> When observed under the electron microscope, the nucleolus can be seen to consist of three distinguishable regions: the innermost fibrillar centers (FCs), surrounded by the dense fibrillar component (DFC), which in turn is bordered by the granular component (GC). Transcription of the rDNA occurs either in the FC or at the FC - DFC boundary, and, therefore, when rDNA transcription in the cell is increased, more FCs are detected . Most of the cleavage and modification of rRNAs occurs in the DFC, while the latter steps involving protein assembly onto the ribosomal subunits occur in the GC . </P> <Table> Subnuclear structure sizes <Tr> <Th> Structure name </Th> <Th> Structure diameter </Th> </Tr> <Tr> <Td> Cajal bodies </Td> <Td> 0.2--2.0 μm </Td> <Td> </Td> </Tr> <Tr> <Td> Clastosomes </Td> <Td> 0.2 - 0.5 μm </Td> <Td> </Td> </Tr> <Tr> <Td> PIKA </Td> <Td> 5 μm </Td> <Td> </Td> </Tr> <Tr> <Td> PML bodies </Td> <Td> 0.2--1.0 μm </Td> <Td> </Td> </Tr> <Tr> <Td> Paraspeckles </Td> <Td> 0.2--1.0 μm </Td> <Td> </Td> </Tr> <Tr> <Td> Speckles </Td> <Td> 20--25 nm </Td> <Td> </Td> </Tr> </Table>

Where is nucleus located in human cheek cells