Final answer:
Cells remove introns from pre-mRNA through splicing conducted by spliceosomes, which recognize specific sequences at intron ends, guaranteeing accurate exon joining.
Step-by-step explanation:
A cell knows which sequences to remove from the pre-mRNA transcript as introns through a process called splicing. This precise action is conducted by complexes known as spliceosomes, which are composed of proteins and RNA molecules, including small nuclear RNAs (snRNAs). Spliceosomes recognize specific nucleotide sequences at the 5' and 3' ends of introns—GU at the 5' end and AG at the 3' end—to ensure correct removal of introns. If this process is inaccurate by even one nucleotide, the protein produced could be dysfunctional due to the resulting shift in the reading frame of the exons. Precision is mandatory, as more than 70 introns may need to be spliced out to create a single functional mRNA molecule.