3. The different arms in the tRNA structure are shown in Column A. The specific signatures associated with the different arms are shown in Column B.
COLUMN A                                                            COLUMN B
A. Acceptor Arm                                                           (i) Dihydrouridine
B. Anticodon Arm                                                        (ii) 8 bp stem and CCA sequence
C. TѰC Arm                                                                  (iii) 5 bp stem
D. D-Arm                                                                      (iv) Pseudo-uridine

Choose the correct matches from the following:

(1) A – (ii); B – (iv); C – (i); D – (iii)
(2) A – (i); B – (iii); C – (iv); D- (ii)
(3) A – (iv); B – (i); C – (ii); D – (iii)
(4) A – (ii); B – (iii); C – (iv); D – (i)

Introduction

Transfer RNA (tRNA) is a critical molecule in protein synthesis, acting as an adaptor between mRNA codons and amino acids. Its cloverleaf structure contains four key arms, each with distinct features and roles. This article breaks down the structural signatures of tRNA arms, explaining how their unique characteristics enable precise translation.

The Four Arms of tRNA

1. Acceptor Arm

Signature: 8 bp stem and CCA sequence

• Located at the 3′ end of tRNA.

• Composed of a 7–9 base-pair stem (typically 8 bp in many tRNAs) formed by pairing the 5′ and 3′ ends of the tRNA.

• Terminates in a CCA sequence, where the amino acid attaches via its 3′ hydroxyl group.

• Function: Binds amino acids during aminoacylation, the first step of translation.

2. Anticodon Arm

Signature: 5 bp stem

• Contains a 5 base-pair stem and a loop with three unpaired nucleotides forming the anticodon.

• The anticodon pairs with the complementary mRNA codon during translation.

• Function: Ensures the correct amino acid is added to the growing polypeptide chain.

3. TΨC Arm

Signature: Pseudouridine (Ψ)

• Named for its conserved TΨC motif (thymine, pseudouridine, cytosine).

• Features a 5 base-pair stem and a loop stabilized by pseudouridine, a modified nucleotide.

• Function: Binds to the ribosome during translation initiation and elongation.

4. D-Arm

Signature: Dihydrouridine

• Contains a 4–5 base-pair stem and a loop with dihydrouridine (D), a modified pyrimidine.

• Function: Recognized by aminoacyl-tRNA synthetases, ensuring tRNA is charged with the correct amino acid.

Structural Comparison of tRNA Arms

Detailed Breakdown of tRNA Arm Functions

Acceptor Arm

• The CCA tail is universally conserved and acts as the amino acid docking site.

• Aminoacyl-tRNA synthetases catalyze the attachment of amino acids to the 3′ hydroxyl group of the terminal adenine in the CCA sequence.

Anticodon Arm

• The anticodon’s three nucleotides pair with mRNA codons via Watson-Crick base pairing.

• Modified bases like inosine in the anticodon loop enhance wobble pairing, allowing one tRNA to recognize multiple codons.

TΨC Arm

• Pseudouridine (Ψ) strengthens the tRNA’s tertiary structure and facilitates ribosome binding.

• The TΨC loop interacts with elongation factors (e.g., EF-Tu) during translation .

D-Arm

• Dihydrouridine introduces structural flexibility, aiding tRNA’s L-shaped folding.

• Critical for aminoacyl-tRNA synthetase recognition, ensuring tRNA is charged with the correct amino acid.

FAQs

Q: Why is the CCA sequence important in the acceptor arm?

A: The CCA sequence provides the site for amino acid attachment, enabling tRNA to deliver amino acids to the ribosome .

Q: How does dihydrouridine affect tRNA function?

A: Dihydrouridine increases flexibility in the D-arm, allowing proper folding and enzyme recognition.

Q: What happens if the TΨC arm is missing?

A: tRNAs lacking the TΨC arm fail to bind ribosomes efficiently, disrupting translation.

Summary: Matching tRNA Arms to Their Signatures

Conclusion

tRNA’s cloverleaf structure is elegantly designed to ensure accuracy in protein synthesis. Each arm—acceptor, anticodon, TΨC, and D-arm—has a unique signature critical for its role in translation. Understanding these structural features provides insight into the molecular machinery driving life’s central dogma.

Key Takeaways:

• The acceptor arm delivers amino acids via its CCA tail.

• The anticodon arm deciphers mRNA codons.

• The TΨC arm stabilizes ribosome interactions.

• The D-arm ensures proper tRNA charging.