RNA size classes — Which of the following RNA types is generally the smallest by length in cells?

Introduction:
Cellular RNAs come in distinct size classes. Recognizing which are smallest helps in experimental design (e.g., gel percentage, library preparation) and in understanding RNA biology broadly.

Given Data / Assumptions:

• tRNAs are ~73–93 nt.
• mRNAs vary widely, often hundreds to thousands of nucleotides.
• rRNAs are among the largest and most abundant RNAs (e.g., 5S ~120 nt, 16S/18S and 23S/28S are much larger).

Concept / Approach:
Compare typical lengths: although 5S rRNA is small (~120 nt), most rRNA species and typical mRNA molecules are larger than tRNA. Therefore, in general cellular context, tRNA is the smallest of the listed major classes.

Step-by-Step Solution:

Verification / Alternative check:
Denaturing PAGE of total RNA shows a distinct small tRNA band below 5S rRNA, confirming its smaller average size.

Why Other Options Are Wrong:

Common Pitfalls:
Focusing on 5S rRNA size and forgetting that the question asks in general among the listed classes.

Final Answer:
Transfer RNA (tRNA).