RNAs become packaged into RNPs as they are synthesized. They presumably guide further processing, all of which takes place in the hnRNP complex. The 5' end of the primary transcript is capped immediately after transcription initiation. Most, but not all primary transcripts contain introns. Generally, the excised intron sequences are degraded and serve no coding function, although there are a few exceptions.
Some introns contain enhancer sequences important for transcriptional control.
Note that non-sequential splicing provides opportunities for developmental regulation. Differential internal splicing of the same processed transcript is known in an increasing number of cases to produce different proteins in different tissues or developmental stages from RNAs with only a single, common poly- A site. Such control results from the presence in different cell types of RNA binding proteins that specifically regulate splicing, for example by blocking a particular splice acceptor site so that splicing must occur to the following one, thereby skipping an exon. Note that these findings confuse the simple concept of a gene, which used to be defined as the nucleotide sequence coding for one protein. In eukaryotes a gene may be developmentally regulated to code for two or more proteins.