The nucleolus is the most active site for the synthesis of RNA. It is the source of Ribosomal RNA (rRNA). The chromatin present in the nucleus contains genes coding for ribosomal RNA.
Chromatin (DNA) ―→ Fibrils (RNA) ―→ Granules (RNA) ―→ Ribosome
Nucleolus is the site where biogenesis of ribosomal subunits i.e. 60S and 40S takes place.