Final answer:
The complex responsible for splicing introns from RNA transcripts is the spliceosome, composed of proteins and snRNAs.
Step-by-step explanation:
The macromolecular complex that associates with each intron and splices it is called a spliceosome. The spliceosome is responsible for removing intron-encoded RNA sequences from primary transcripts in eukaryotes. This process involves the precise removal of introns from the primary RNA transcript and is catalyzed by protein complexes composed of proteins and RNA molecules called small nuclear RNAs (snRNAs). Spliceosomes recognize sequences at the 5' and 3' end of the intron, cut the transcript at those two points, and bring the exons together for ligation, forming the mature mRNA that codes for the final protein.