7. RNA processing.

Introduction

• Prokaryotic RNA Processing:
• Primary RNA converted to mature RNA.
• Polycistronic mRNA for translation.
• 3 rRNAs (16S, 23S, 5S) transcribed as one primary transcript, then cut into three chains, needing further shortening and methylation.
• Similar processing for tRNA.
• Redundant parts removed by RNases, using short RNA as cofactor.
• Eukaryotic rRNA Maturation:
• Eukaryotic rRNAs (28S, 18S, 5.8S, 5S) synthesized as 45S chain, with nucleotide methylation and ribosomal proteins binding.
• Non-coding parts removed, and other proteins plus 5S rRNA join complex, assembling ribosomal subunits.
• Eukaryotic tRNA Maturation:
• Similar to prokaryotic tRNA processing.
• Missing CCA(cytosine cytosine adenine) added by enzyme post-transcription (tRNA nucleotidyl transferase).
• Some tRNAs have introns in anticodon arm.
• Maturation of Eukaryotic mRNA 5'-cap
• Addition of G to terminal phosphate creates a 5'-5' triphosphate link by guanylyl transferase.
• Methyl transferase adds methyl groups to G and the next two riboses.
• Resulting structure is called a 5' cap, ensuring mRNA stability and signaling translation start.
• Maturation of the 3'-polyA Tail
• mRNA gene's AATAAA signals transcription end.
• Enzyme recognizes AAUAAA, cutting chain and adding 100-200A to create a 3' polyA tail.
• PolyA tail protects mRNA from degradation and acts as a signaling structure.
• Communication between 5' cap and 3' polyA tail enhances translation.
• Splicing
• Introns must be removed.
• Spliceosome, comprising proteins and snRNA, cuts introns.
• Specific nucleotide sequences between exons and introns are recognized.
• Spliceosome joins mRNA during synthesis, and splice sites are identified by snRNA.