Eukaryotic transcripts possess extra segments called introns or intervening sequences or non- coding sequences. RNA contains both the exons and introns. The functional coding sequences are called exons. Splicing is removal of introns and fusion of exons to form functional hnRNAs.

Each intron starts with dinucleotide GU and ends with dinucleotide AG (GU-AG rule). They are recognised by components of splicing apparatus of Sn-RNPs or small nuclear ribonucleoproteins (viz, U1, U2, U4, U5, U6). A complex called spliceosome is formed between 5ï‚¢ end (GU) and 3ï‚¢ end (AG) of intron. Energy is obtained from ATP. It removes the intron. The adjacent exons are brought together. The ends are then sealed by RNA ligase