Transfer RNA (tRNA) is the adaptor molecule that translates the genetic code from mRNA into a specific amino acid sequence during protein synthesis. It links each codon on the mRNA to its corresponding amino acid via complementary base pairing and enzymatic “charging.”
Structure of tRNA
| Feature | Description | Mnemonic / Function |
|---|---|---|
| Length | 75–90 nucleotides | — |
| Shape | Cloverleaf secondary structure | Stabilized by hydrogen bonding |
| 3′ End | Always ends with CCA, the site where the amino acid attaches covalently | “CCA – Can Carry Amino acids” |
| Anticodon Arm | Base-pairs with a complementary codon on mRNA | Determines amino acid sequence |
| T-arm | Contains TΨC (ribothymidine, pseudouridine, cytidine) | Tethers tRNA to the ribosome |
| D-arm | Contains dihydrouridine residues | Detects correct aminoacyl-tRNA synthetase |
| Amino Acid Attachment Site | Located at 3′–ACC–5′ end | ACCeptor site |
Charging of tRNA
tRNA charging is catalyzed by aminoacyl-tRNA synthetase, a highly specific enzyme that ensures each tRNA carries the correct amino acid.
- Reaction: Amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi
- Proofreading: The enzyme hydrolyzes mischarged amino acids to maintain translational fidelity.
- Clinical correlation:
- A mischarged tRNA will insert the wrong amino acid but still read the correct codon, → can lead to dysfunctional proteins.
| Step | Enzyme | Energy Source | Purpose |
|---|---|---|---|
| 1. Activation | Aminoacyl-tRNA synthetase | ATP | Binds an amino acid to an enzyme |
| 2. Transfer | Same enzyme | — | Transfers amino acid to tRNA |
| 3. Proofreading | Synthetase | — | Ensures accuracy; hydrolyzes mischarged tRNA |
Key Points to Remember
- All tRNAs end with CCA at the 3′ end → amino acid attachment site.
- T-arm → Tethers tRNA to ribosome.
- D-arm → Detects correct aminoacyl-tRNA synthetase.
- Charging uses ATP.
- Accuracy depends on aminoacyl-tRNA synthetase and codon–anticodon pairing.
Learning Objective
Explain the structure and function of tRNA, identify key structural arms, and describe the role of aminoacyl-tRNA synthetase in ensuring translational fidelity — with attention to how errors can lead to clinical consequences.
