Part 6: Translation and Protein Synthesis
RNA → Protein
Translation is the process by which the genetic code in mRNA is decoded to produce a polypeptide chain. This occurs on ribosomes and requires tRNA, amino acids, and numerous protein factors.
The Genetic Code
Properties
- Triplet: 3 nucleotides = 1 codon
- Degenerate: 64 codons, 20 amino acids
- Non-overlapping: Read in frame
- Universal: Same in most organisms
- Unambiguous: Each codon = one AA
Special Codons
- AUG: Start codon (Methionine)
- UAA: Stop (Ochre)
- UAG: Stop (Amber)
- UGA: Stop (Opal)
Translation Steps
1. Initiation
- • Small ribosomal subunit binds mRNA
- • Initiator tRNA (Met-tRNA) binds AUG start codon
- • Large subunit joins, forming complete ribosome
- • Initiation factors: IF1, IF2, IF3 (prokaryotes); eIF1-6 (eukaryotes)
2. Elongation
A site
Aminoacyl-tRNA entry
P site
Peptidyl-tRNA
E site
Exit for empty tRNA
Elongation factors: EF-Tu, EF-G (prokaryotes)
3. Termination
- • Stop codon reaches A site
- • Release factors (RF1, RF2, RF3) recognize stop codons
- • Polypeptide released, ribosome dissociates
The Ribosome
Prokaryotic (70S)
- • Small subunit: 30S (16S rRNA)
- • Large subunit: 50S (23S + 5S rRNA)
- • Target for antibiotics
Eukaryotic (80S)
- • Small subunit: 40S (18S rRNA)
- • Large subunit: 60S (28S + 5.8S + 5S)
- • More complex regulation