Exons are produced by more than one mechanism, e.g. splicing out introns after transcription, if I remember correctly. Please list all mechanisms.
There are several ways splicing can occur, which depends on the RNA molecule to be spliced and the catalyst that performs the splicing:
mRNA splicing is carried out by spliceosome, which consists of small nuclear RNAs. There are sequences at the end of the introns and branch sties which indicate the splice sites. The so called lariat structure is formed when the 2'OH group of an adenosine residue in the branch site attacks the 5' splice site.
self-splicing of ribosomal RNA precursor - it is performed with the absence of spliceosome.
tRNA splicing - it requires three enzymes and ATP hydrolysis.