1. Introns and Exons Introns--Untranslated intervening sequences in mRNA Introns--Untranslated intervening sequences in mRNA Exons– Translated sequences Exons– Translated sequences Process-RNA splicing Process-RNA splicing Heterogeneous nuclear RNA (hnRNA)-Transcript before splicing is complete Heterogeneous nuclear RNA (hnRNA)-Transcript before splicing is complete

2. Splicing Overview Occurs in the nucleus Occurs in the nucleus hnRNAs complexed with specific proteins, form a ribonucleoprotein particle (RNP) hnRNAs complexed with specific proteins, form a ribonucleoprotein particle (RNP) Primary transcripts assembled into hnRNP Primary transcripts assembled into hnRNP Splicing occurs on spliceosomes consist of Splicing occurs on spliceosomes consist of Small nuclear ribonucleoproteins (SnRNPs) Small nuclear ribonucleoproteins (SnRNPs) components of spliceosomes components of spliceosomes Contain small nuclear RNA (snRNA) Contain small nuclear RNA (snRNA) Many types of snRNA with different functions in the splicing process Many types of snRNA with different functions in the splicing process

3. Spliceosome

4. Splice Site Recognition Introns contain invariant 5’-GU and 3’-AG sequences at their borders (GU-AG Rule) Introns contain invariant 5’-GU and 3’-AG sequences at their borders (GU-AG Rule) Recognized by Recognized by small nuclear ribonucleoprotein particles (snRNPs) that catalyze the cutting and splicing reactions. Internal intron sequences are highly variable even between closely related homologous genes. Internal intron sequences are highly variable even between closely related homologous genes. Alternative splicing allows different proteins from a single original transcript Alternative splicing allows different proteins from a single original transcript

5. Simplified Splicing Mechanism

6. Close-up of Internal A

7. Alternative Splicing I Exon removed with intron Exon removed with intron

8. Alternative Splicing II Multiple 3’ cleavage sites Multiple 3’ cleavage sites EX. AG found at 5’ end of exon 2 and inside exon 2 EX. AG found at 5’ end of exon 2 and inside exon 2