<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> <Tr> <Th> Structure name </Th> <Th> Structure diameter </Th> </Tr>

Where is the nucleus found in plant and animal cells