 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 . 
 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 <a href='uninflect' title='1'><mark>occurs</mark></a> 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 <a href='uninflect' title='2'><mark>occurs</mark></a> in the DFC , while the latter steps involving protein assembly onto the ribosomal subunits occur in the GC . 
When observed under da electron microscope , the nucleolus can b seen to consist of three distinguishable regions : the innermost fibrillar centers ( FCs ) , surrounded by tdadense fibrillar component ( DFC ) , which in turn is bordered by the granular component ( GC ) . Transcription of the rDNA occur either in the FC or at the FC @-@ DFC boundary , and , therefore , when rDNA transcription in thdaell is increased , more FCs r detected . Most of thedaeavage and modification of rRNAs occur in the da , while the latter steps involving protein assembly onto the rdasomal subunits occur in the GC .