Transfer RNA (tRNA) architecture: how many stem–loop arms are present in the canonical cloverleaf secondary structure? Choose the best answer.

Introduction / Context:
tRNAs adopt a conserved secondary structure often depicted as a cloverleaf. Understanding the standard arms and loops is essential for decoding, aminoacylation, and ribosome interactions.

Given Data / Assumptions:

• Common arms: acceptor stem, D (dihydrouridine) arm, anticodon arm, TψC arm.
• Many tRNAs also have a variable arm of differing length.

Concept / Approach:
In the cloverleaf, there are three principal stem–loop structures: the D-loop, the anticodon loop, and the TψC loop. The acceptor arm is a stem without a loop at the top. A variable arm may be present but is not universal as a distinct stem–loop.

Step-by-Step Solution:
Identify loops: D-loop, anticodon loop, TψC loop → total three.Recognize acceptor stem is not a loop; it terminates in the 3′ CCA tail for amino acid attachment.Account for variable arm as optional and not always forming a canonical loop counted among the three core loops.

Verification / Alternative check:
Structural studies show L-shaped tertiary structure arises from coaxial stacking of these elements, consistent with three core loops.

Why Other Options Are Wrong:
Two loops undercounts the standard architecture; four or five are not consistently present across tRNAs. 'Either (a) or (b)' is imprecise.

Common Pitfalls:
Counting the acceptor stem as a loop or treating the variable arm as universally looped.

Final Answer:
Three stem loops.