Final answer:
The primary mRNA segments removed by spliceosomes during pre-mRNA splicing are called introns. Spliceosomes, composed of proteins and snRNAs, accurately excise introns, allowing exons to join and form mature mRNA ready for protein synthesis.
Step-by-step explanation:
The segments of primary mRNA that are cleaved off by spliceosomes are called introns. During the processing of primary RNA transcripts in eukaryotic cells, pre-mRNA splicing involves the precise removal of introns. This splicing process is catalyzed by protein complexes called spliceosomes, which include proteins and small nuclear RNAs (snRNAs). The spliceosomes are responsible for recognizing the sequences at the 5' and 3' ends of the intron, ensuring that introns are excised with high accuracy. Once removed, the remaining exons—that are the coding sequences which become part of the final mRNA—are spliced together to form the mature mRNA. This mature mRNA is then capable of leaving the nucleus to guide protein synthesis in the cytoplasm.
The segments of primary mRNA that are cleaved off by spliceosomes are called introns. Spliceosomes are protein complexes composed of proteins and RNA molecules called small nuclear RNAs (snRNAs). They recognize sequences at the 5' and 3' end of the intron, and remove them from the primary RNA transcript through a process called splicing.