 When observed under the electron microscope , the nucleolus can be seen to consist of three distinguishable regions : the innermost <unk> centers ( <unk> ) , surrounded by the dense <unk> component ( DFC ) , which in turn is bordered by the <unk> component ( <unk> ) . <unk> 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 <unk> are detected . Most of the cleavage and modification of <unk> occurs in the DFC , while the latter steps involving protein assembly onto the ribosomal subunits occur in the <unk> . 
