Tail of functional mRNA is made up of

Tail of functional mRNA is made up of poly adenylates sequences.

The primary transcripts contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. hnRNA undergoes additional processing called capping and tailing.

 In the tailing, adenylate residues (200-300) are added at the 3'-end in a template independent manner. It is fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation.