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🧪 Introduction

Translation—the process of synthesizing proteins from mRNA—is a highly specific biological mechanism. Transfer RNAs (tRNAs) play a critical role, serving as adaptors that recognize mRNA codons and deliver the correct amino acids. But what happens if the amino acid on a tRNA is chemically modified after charging?

In this article, we’ll explore a fascinating scenario where a tRNA with a leucine anticodon is charged with leucine, and then the leucine is chemically converted to arginine. What amino acid gets incorporated into the protein—and why?

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🔍 The Scenario Explained

Let’s break down the key components of the question:

1. The tRNA has an anticodon for leucine
   → This means it will pair with a leucine codon on the mRNA.

2. The tRNA is originally charged with leucine
   → Aminoacyl-tRNA synthetase attaches the correct amino acid.

3. The leucine is chemically modified to arginine
   → The physical amino acid on the tRNA is now arginine, but its identity hasn’t changed at the codon level.

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🧠 Core Concept: What Determines Amino Acid Incorporation?

The anticodon on the tRNA determines which codon it pairs with on the mRNA. The ribosome does not “read” the amino acid attached; it only aligns the anticodon with the mRNA codon. Therefore:

🧾 The amino acid incorporated during translation is the one that is physically attached to the tRNA, regardless of whether it matches the codon-anticodon pair.

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✅ Answer Explained

The tRNA has an anticodon for leucine, so it binds to a leucine codon on the mRNA.
However, due to chemical modification, it carries arginine, not leucine.

👉 So the ribosome will insert arginine at a position where the codon calls for leucine.

✅ Final Answer:

(3) Arginine against codon of leucine in mRNA

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🧬 Why This Is Biologically Important

This concept underlies classic experiments in molecular biology—most notably, those by Zamecnik and Hoagland, which helped establish the adaptor hypothesis. Their work showed that:

• tRNA’s anticodon dictates where it goes.

• The ribosome doesn’t verify the amino acid; it trusts the tRNA to be correctly charged.

This has major implications for:

• Synthetic biology (e.g., incorporating unnatural amino acids)

• Protein engineering

• Understanding translation fidelity

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🏷 Tags & Keywords:

• tRNA anticodon and amino acid incorporation

• protein translation mechanism

• ribosome and tRNA interaction

• modified amino acid on tRNA

• leucine codon vs amino acid identity

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📌 Conclusion

The example clearly shows that anticodon recognition by the mRNA codon guides positioning, while the amino acid on the tRNA—regardless of whether it’s modified—gets incorporated into the growing polypeptide chain. This key insight into molecular biology emphasizes the importance of understanding codon-anticodon specificity over amino acid identity.

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📍 Correct Answer: (3) Arginine against codon of leucine in mRNA ✅