Learning Objectives

• Identify the key components of the tRNA cloverleaf structure (T-arm, D-arm, and Acceptor stem).
• Explain the role of Aminoacyl-tRNA synthetase in ensuring translational accuracy.
• Describe the charging process and the energy requirements involved.
• Understand the consequences of mischarged tRNA on the final protein product.

1. tRNA Structure: The Cloverleaf

tRNAs are small RNA molecules (75–90 nucleotides) that act as the physical bridge between the mRNA codon and the amino acid.

• 3′ Acceptor End: All tRNAs end in the sequence 5′-CCA-3′. The amino acid is covalently bound to the hydroxyl group of the 3′ adenosine.Mnemonic: CCA Can Carry Amino acids.
• T-arm: Contains the $T\psi C$ sequence.Mnemonic: T-arm Tethers the tRNA to the ribosome.
• D-arm: Contains Dihydrouridine residues.Mnemonic: D-arm allows the detection of the tRNA by aminoacyl-tRNA synthetase.
• Anticodon Loop: Found at the opposite end of the 3′ aminoacyl end; recognizes and base-pairs with the mRNA codon.

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2. tRNA Charging (Aminoacylation)

This process attaches the correct amino acid to its corresponding tRNA. It is the primary checkpoint for the accuracy of protein synthesis.

• Enzyme: Aminoacyl-tRNA synthetase. There is one unique enzyme for each amino acid.
• Energy: Requires ATP.
• Proofreading: The enzyme scrutinizes the amino acid both before and after binding. If it’s the wrong fit, the bond is hydrolyzed (broken).

3. Clinical High-Yield: Mischarging

What happens if the proofreading fails?

• If a mischarged tRNA (a tRNA carrying the wrong amino acid) reaches the ribosome, it will still read the usual codon but will insert the wrong amino acid into the polypeptide chain.
• The ribosome cannot verify the amino acid; it only verifies the tRNA-mRNA base pairing.

4. Summary of tRNA “Arms.”

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