 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 ' <unk> , and RNA splicing . While in the nucleus , pre @-@ mRNA is associated with a variety of proteins in complexes known as heterogeneous <unk> particles ( <unk> ) . <unk> of the 5 ' cap occurs co @-@ <unk> and is the first step in post @-@ transcriptional modification . The 3 ' <unk> @-@ <unk> tail is only added after transcription is complete . 
