Overview
This lecture explains the process of translation (protein synthesis), detailing the roles of RNA types, the genetic code, ribosomes, and the steps of translation, as well as protein targeting and modification.
What is Translation?
- Translation is the process of making proteins from mRNA using ribosomes.
- mRNA is produced from DNA via transcription.
- Three main RNAs involved: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).
The Genetic Code
- The genetic code is based on codons—triplets of nucleotides in mRNA.
- RNA bases: Adenine (A), Guanine (G), Cytosine (C), Uracil (U).
- 64 possible codons; 61 code for amino acids, 3 code for stop signals.
- AUG is the universal start codon and codes for methionine.
- Codons code for amino acids via complementary tRNA anticodons.
- The code is continuous (comma-less), non-overlapping, and redundant (degenerate)—most amino acids have multiple codons.
- Exceptions: Methionine (AUG) and tryptophan (UGG) have only one codon.
- Wobble effect: Flexibility at the third codon position reduces mutation risk.
tRNA Structure and Charging
- tRNA has three loops: anticodon loop (binds codon), D-arm (enzyme recognition), T-arm (ribosome binding).
- Amino acid attaches at the 3’ CCA end.
- Charging: Amino acid + ATP attaches to tRNA by aminoacyl-tRNA synthetase enzyme, forming a "charged" tRNA.
Ribosomes: Structure and Function
- Ribosomes have large and small subunits, made of rRNA and proteins.
- Eukaryotic ribosomes: 60S + 40S = 80S.
- Prokaryotic ribosomes: 50S + 30S = 70S.
- Ribosome differences allow antibiotics (e.g., aminoglycosides, tetracyclines, macrolides) to target bacterial protein synthesis.
Phases of Translation
Initiation
- Prokaryotes: Ribosome binds Shine-Dalgarno sequence, start codon (AUG) codes for formyl-methionine.
- Eukaryotes: Initiation factors bind 5’ cap, start codon (AUG) codes for methionine.
- Small subunit binds mRNA, initiator tRNA binds start codon, large subunit joins to form the initiation complex.
- Initiator tRNA is positioned in the P site.
Elongation
- tRNA enters A site, matching the next codon.
- Peptidyl transferase forms peptide bond, transferring peptide to tRNA in A site.
- Translocation shifts ribosome: A → P → E site, empty tRNA exits E site.
- Process repeats, elongating the polypeptide.
Termination
- When a stop codon is reached, release factors bind, stopping translation.
- Peptide is cleaved from tRNA, ribosome subunits and mRNA dissociate.
Protein Targeting: Free vs. Bound Ribosomes
- Free ribosomes: Synthesize proteins destined for cytosol, nucleus, mitochondria, peroxisomes.
- Ribosomes on rough ER: Synthesize proteins for secretion, membranes, or lysosomes.
- Signal sequence on growing peptide is recognized by signal recognition particle (SRP), directing the complex to the rough ER.
- Translation resumes, peptide enters ER lumen, signal peptide is cleaved.
Protein Modification
- Proteins can be modified after translation:
- Glycosylation: Add sugars (e.g., blood antigens)
- Lipidation: Add lipids (membrane association)
- Phosphorylation: Add phosphate (regulation)
- Hydroxylation: Add hydroxyl (collagen synthesis)
- Methylation/Acetylation: Add methyl/acetyl (gene expression)
- Trimming: Cutting parts off (protein activation)
Key Terms & Definitions
- Translation — synthesis of proteins from mRNA by ribosomes.
- Codon — three-nucleotide sequence on mRNA coding for an amino acid.
- Anticodon — complementary three-nucleotide sequence on tRNA.
- Wobble effect — flexibility in base pairing at the third codon position.
- Aminoacyl-tRNA synthetase — enzyme that charges tRNA with amino acid.
- Peptidyl transferase — ribosomal enzyme forming peptide bonds.
- Initiation/Elongation/Termination — phases of translation.
- Signal sequence — peptide segment directing ribosome to ER.
- Post-translational modification — chemical changes to proteins after synthesis.
Action Items / Next Steps
- Review genetic code chart in textbook appendix.
- Practice tracing the steps of translation for a given mRNA sequence.
- Read about antibiotic mechanisms targeting ribosomal subunits.