RNA chemistry — Which RNA class characteristically contains unusual (modified) purines and pyrimidines as part of its normal structure?

Introduction:
Many RNAs undergo post-transcriptional modification. This question asks which RNA species is especially rich in unusual bases—modified purines and pyrimidines that fine-tune structure and function.

Given Data / Assumptions:

• Common RNA classes: mRNA, rRNA, tRNA, small RNAs.
• Modified bases include methylations, thiolations, and complex derivatives.
• Function of modifications is to stabilize structure and support decoding.

Concept / Approach:
Transfer RNAs (tRNAs) are heavily modified, often containing 10–20 modified nucleotides per molecule. Examples include pseudouridine, dihydrouridine, inosine, 1-methylguanosine, and queuosine. These modifications stabilize the L-shaped tertiary structure and modulate anticodon–codon pairing fidelity during translation.

Step-by-Step Solution:

Verification / Alternative check:
tRNA sequencing maps consistently reveal numerous modified nucleotides, particularly near the anticodon loop and D/T arms, underscoring their functional importance in translation accuracy.

Why Other Options Are Wrong:

• mRNA: relatively few internal base modifications in canonical eukaryotic mRNA beyond the 5′ cap and occasional internal marks.
• rRNA: contains modifications but less densely than tRNA.
• nRNA/pre-mRNA: primarily processed by splicing and capping; not heavily modified internally.
• siRNA: short regulatory RNAs without extensive base modification patterns.

Common Pitfalls:
Conflating the mRNA 5′ cap modification with high internal modification density; tRNA is the standout for unusual bases throughout the molecule.

Final Answer:
tRNA.