Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 17.3 Eukaryotic Cells Modify RNA After Transcription
In eukaryotic cells, RNA is modified extensively after transcription and before translation. These modifications include adding protective caps and tails, as well as splicing out noncoding segments called introns.
Alteration of mRNA Ends
- 5′ Cap: a modified guanine nucleotide is added at the 5′ end of pre-mRNA
- Poly-A Tail: 50–250 adenine nucleotides are added at the 3′ end after a polyadenylation signal (AAUAAA)
- These modifications help:
- Facilitate export of mRNA from the nucleus
- Protect mRNA from hydrolytic enzymes
- Assist ribosome attachment during translation
- mRNA also has untranslated regions (5′ UTR and 3′ UTR) which aid ribosome binding but aren't translated into protein
RNA Splicing
- Eukaryotic genes contain coding (exons) and noncoding sequences (introns)
- Introns are removed and exons joined together, forming a continuous coding sequence
- Splicing is carried out by a complex called the spliceosome, consisting of proteins and small RNAs
- Small RNAs in spliceosomes recognize splice sites and catalyze intron removal
Ribozymes and RNA Catalysis
- Ribozymes: RNA molecules that function as enzymes
- Example: self-splicing RNA in certain organisms
- Properties allowing RNA catalytic function:
- Single-stranded, able to fold into complex shapes
- Contains functional groups for catalysis
- Can base-pair with RNA or DNA to add specificity
Functional and Evolutionary Importance of Introns
- Alternative RNA Splicing: one gene can code for multiple proteins by varying exon combinations
- Human genome (~20,000 genes) produces 75,000–100,000 proteins due to alternative splicing
- Introns may enhance evolution through exon shuffling, enabling new gene combinations and protein functions
Protein Domains and Exons
- Proteins often have distinct regions (domains) with specific functions
- Each exon can code for different protein domains
- Exon shuffling can rearrange domains to create proteins with novel functionalities
In a Nutshell
Eukaryotic RNA processing involves capping, tailing, and splicing. Introns are removed, exons reconnected, and the resulting mature mRNA is exported from the nucleus for translation. Alternative RNA splicing and exon shuffling increase protein diversity and evolutionary potential.