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Introns and Exons

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

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Introns and Exons

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

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

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

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