 Newly synthesized mRNA molecules are known as primary transcripts or pre @-@ mRNA . They must undergo post @-@ transcriptional modification in the nucleus before being exported to the cytoplasm ; mRNA that appears in the cytoplasm without these modifications is degraded rather than used for protein translation . The three main modifications are 5 ' capping , 3 ' polyadenylation , and RNA splicing . While in the nucleus , pre @-@ mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles ( hnRNPs ) . Addition of the 5 ' cap occurs co @-@ transcriptionally and is the first step in post @-@ transcriptional modification . The 3 ' poly @-@ adenine tail is only added after transcription is complete . 
 Newly synthesized mRNA molecules are known as primary transcripts or pre @-@ mRNA . They must undergo post @-@ transcriptional modification in the nucleus before being exported to the cytoplasm ; mRNA <a href='null_relcl' title='1'><mark>that</mark></a><a href='uninflect' title='2'><mark>appears</mark></a> in the cytoplasm without these modifications is degraded rather than used for protein translation . The three main modifications <a href='drop_aux' title='3'><mark>are</mark></a>5 ' capping , 3 <a href='null_genetive' title='4'><mark>'</mark></a>polyadenylation , and RNA splicing . While in the nucleus , <a href='uninflect' title='5'><mark>pre</mark></a> @-@ mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles ( hnRNPs ) . Addition of the 5 <a href='null_genetive' title='6'><mark>'</mark></a>cap <a href='uninflect' title='7'><mark>occurs</mark></a> co @-@ transcriptionally and <a href='drop_aux' title='8'><mark>is</mark></a>the first step in post @-@ transcriptional modification . The 3 <a href='null_genetive' title='9'><mark>'</mark></a>poly @-@ adenine tail is only added after transcription <a href='drop_aux' title='10'><mark>is</mark></a>complete . 
Newly synthesized mRNA molecules are known as primary transcripts or pre @-@ mRNA . They must undergo post @-@ transcriptional modification in the nucleus before bein exported to the cytoplasm ; mRNA appear in the cytoplasm without dese modifications is degraded rather than used for protein translation . The three main modifications 5 ' capping , 3  polyadenylation , and RNA splicing . While in the nucleus , pred @-@ mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles ( hnRNPs ) . Addition of the 5  cap occur co @-@ transcriptionally and the first step in post @-@ transcriptional modification . The 3  poly @-@ adenine tail is only added after transcription complete .