Nucleolus is site for production of ribosomal RNA not ribosomes. While ribosomes constitute both rRNA and protein they are assembled after transcription. rRNAs associate with the ribosomal proteins, forming the two types of ribosomal subunits (large and small). These will later assemble in the cytosol to make a functioning ribosome. In prokaryotes ribosomal RNAs are coded in operon systems (nearly 20) and absence of a nucleolus in way hampers their production.