The rRNA is transcribed, at a high speed, in the nucleolus, which contains all 45S rRNA genes. The only exception is the 5S rRNA which is transcribed outside the nucleolus. After transcription, the 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. See nuclear export for more about the movement of the ribosomal subunits out of the nucleus.
where does the formation of ribosomal subunits occur
nucleolus nucleolus