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 .