17. Which one of the following bases is NOT found in the T-arm of an aminoacyl t-RNA?
(A) Dihydrouridine
(B) Pseudouridine
(C) Uracil
(D) Guanine

The correct answer is (D) Guanine. Dihydrouridine, pseudouridine, and uracil appear in the T-arm, while guanine does not. This multiple-choice question tests tRNA structure knowledge relevant to molecular biology exams.​

T-arm Structure Overview

The T-arm, or TψC arm, forms part of transfer RNA (tRNA) cloverleaf secondary structure. It features a stem-loop with the conserved TψC sequence (ribothymidine, pseudouridine, cytidine). Common modified bases stabilize this region for ribosome interaction during translation.​

Option Analysis

• (A) Dihydrouridine: Found mainly in the D-arm (dihydrouridine loop), not the T-arm. This reduced uridine derivative aids tRNA flexibility but marks the opposite arm.​

• (B) Pseudouridine: Present in the T-arm as ψ in TψC loop. This isomerized uridine enhances base stacking and tRNA stability.​

• (C) Uracil: Occurs as ribothymidine (derived from uracil) in the TψC sequence. Unmodified uracil may also appear in loop variants.​

• (D) Guanine: Absent from the T-arm. Standard bases like guanine reside elsewhere, such as anticodon or acceptor stem, but T-arm favors uridine derivatives.​

Why Guanine Stands Out

tRNA arms contain modified bases for function: D-arm has dihydrouridine, T-arm prioritizes pseudouridine and thymidine. Guanine, a purine base, lacks specific T-arm association, making it the exclusion.​