 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 . 
 In the first step of ribosome assembly , a protein called RNA polymerase I <a href='uninflect' title='1'><mark>transcribes</mark></a> rDNA , which <a href='uninflect' title='2'><mark>forms</mark></a> 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 <a href='uninflect' title='3'><mark>occurs</mark></a> 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 <a href='drop_aux' title='4'><mark>are</mark></a>the largest structures passed through the nuclear pores . 
In the first step of ribosome assembly , a protein called RNA polymerase I transcribe rDNA , which form a bigass pre @-@ rRNA precursor . This is cleaved into da subunits 5.8S , 18S , and 28S rRNA . Tdatranscription , post @-@ transcriptional processing , and assembly of rRNA occur in the nucleolus , aided by small nucleolar RNA ( snoRNA ) molecules , some of which r derived from spliced introns from messenger RNAs encoding genes related to ribosomal function . Thdassembled ribosomal subunits thedargest structures passed through the nuclear pores .