RNA Processing

Overview

• Only in eukaryotes
  • pre-processed transcript = hnRNA
• Process 
  • capping
    • 5′ end
      • 7-methylguanosine cap
      • occurs before transcription has finished
      • functions to help in ribosomal binding
        • also protects against degradation
    • 3′ end
      • polyadenylated (poly-A) tail
      • added by poly-A-polymerase
      • functions to protect from degradation 
    • some viruses can steal host cell caps so that the viral mRNA gets translated
  • splicing 
    • process of intron removal
    • mediated by the spliceosome composed of small nuclear ribonucleoproteins (snRNPs) 
      • lariat is formed
      • adjacent exons are attached
  • alternative splicing  
    • one hnRNA can be spliced differently to produce different protein products
    • this requires conserved sequences including
      • GU at the beginning of the intron
      • AG at the end of the intron
    • mutations that create new splice sites (AG) in the middle of the intron can lead to
      • abberant splicing that creates new nonfunctional proteins
      • longer proteins than the wildtype variant
• Location
  • occurs in nucleus after transcription
• only processed RNA is transported out of the nucleus