 In the first step of ribosome assembly , a protein called RNA polymerase I transcribes rDNA , which forms a large pre @-@ rRNA precursor . This is cleaved into the subunits 5.8S , 18S , and 28S rRNA . The transcription , post @-@ transcriptional processing , and assembly of rRNA occurs in the nucleolus , aided by small nucleolar RNA ( snoRNA ) molecules , some of which are derived from spliced introns from messenger RNAs encoding genes related to ribosomal function . The assembled ribosomal subunits are the largest structures passed through the nuclear pores . 
