Overview
This section explains the essential steps and significance of RNA processing in eukaryotes, focusing on how pre-mRNA, tRNA, and rRNA are modified before functioning in protein synthesis.
Steps of mRNA Processing in Eukaryotes
- Eukaryotic pre-mRNA requires processing following transcription before it can be translated into protein.
- Protein-coding sequences (exons) in eukaryotic genes are interrupted by noncoding introns, which must be removed.
- Pre-mRNAs are coated with RNA-stabilizing proteins to prevent degradation during processing and export from the nucleus.
- Processing includes addition of a 5' cap, 3' poly-A tail, and precise removal of introns (splicing).
- The 5' cap is a 7-methylguanosine added during transcription to protect mRNA and assist translation initiation.
- A 3' poly-A tail (about 200 adenines) is added after cleavage at the AAUAAA sequence for stability and export.
- Introns are removed and exons joined via spliceosomes, complexes of proteins and small nuclear RNAs (snRNAs).
- Errors in splicing can cause diseases, including cancers, by disrupting the reading frame.
Exons, Introns, and Splicing
- Exons are expressed coding sequences that remain in mature mRNA.
- Introns are intervening noncoding regions, removed during splicing.
- Splicing requires specific sequence recognition (5' GU and 3' AG) to ensure accuracy.
- Multiple introns may be present; all must be correctly removed for a functional mRNA.
RNA Editing
- Some eukaryotes, like trypanosomes, use guide RNAs to add or modify nucleotides in pre-mRNA (RNA editing).
- Guide RNAs bind pre-mRNA, directing the insertion of specific nucleotides (e.g., uracil) for functional protein coding.
Processing of tRNAs and rRNAs
- tRNAs and rRNAs are transcribed as long precursor RNAs and then cleaved into functional units.
- Some bases in pre-rRNAs and pre-tRNAs are methylated for stability.
- Mature rRNAs combine with proteins to form ribosomes; some act catalytically.
- Mature tRNAs fold into a specific shape with an amino acid binding site at one end and anticodon at the other for translation.
Key Terms & Definitions
- pre-mRNA — Initial RNA transcript containing both exons and introns.
- Exon — Protein-coding sequence retained in mature mRNA.
- Intron — Noncoding sequence removed during RNA splicing.
- Spliceosome — Complex of proteins and snRNAs that catalyzes intron removal.
- 5' cap — Modified guanine nucleotide added to mRNA's 5' end for stability and translation.
- Poly-A tail — String of adenine nucleotides added to the 3' end of mRNA.
- RNA editing — Post-transcriptional alteration of RNA nucleotides guided by other RNA molecules.
- tRNA — Transfer RNA, carries amino acids to the ribosome during translation.
- rRNA — Ribosomal RNA, forms the core of ribosome structure and function.
- Anticodon — Three-nucleotide sequence in tRNA that pairs with mRNA codon.
Action Items / Next Steps
- Review the mechanisms of mRNA capping, polyadenylation, and splicing.
- Study examples of splicing errors and their implications.
- Explore animation or interactive resources on RNA splicing for visualization.