Question
Compare the three types of RNA — mRNA, tRNA, and rRNA — in terms of their structure, function, and percentage composition in the cell.
(NEET and CBSE 12 — regularly asked as a comparison question)
Solution — Step by Step
Messenger RNA carries the genetic code from DNA to the ribosome. It is a linear, single-stranded molecule with a 5’ cap and 3’ poly-A tail in eukaryotes. Each set of three nucleotides (codon) specifies one amino acid.
mRNA is the shortest-lived of the three — it is degraded after translation. It makes up only about 3-5% of total cellular RNA.
Transfer RNA brings amino acids to the ribosome during translation. It has a characteristic cloverleaf secondary structure (L-shaped in 3D) with an anticodon loop and an amino acid acceptor arm (3’-CCA end).
tRNA is small (~75-90 nucleotides) and makes up about 15% of total RNA. There is at least one tRNA for each of the 20 amino acids.
Ribosomal RNA is the most abundant RNA, making up about 80% of total cellular RNA. It forms the structural and catalytic core of ribosomes.
In prokaryotes: 23S, 16S, and 5S rRNA. In eukaryotes: 28S, 18S, 5.8S, and 5S rRNA. The 23S rRNA (prokaryotes) acts as a ribozyme — it catalyses peptide bond formation.
graph TD
A[RNA Types] --> B[mRNA]
A --> C[tRNA]
A --> D[rRNA]
B --> B1["Linear, single-stranded"]
B --> B2["3-5% of total RNA"]
B --> B3["Carries genetic code"]
C --> C1["Cloverleaf shape"]
C --> C2["~15% of total RNA"]
C --> C3["Carries amino acids"]
D --> D1["Complex folded structure"]
D --> D2["~80% of total RNA"]
D --> D3["Forms ribosome core"]Why This Works
The three RNA types work as a team during translation. DNA is transcribed into mRNA (the message), tRNA reads the message and fetches the correct amino acids (the adaptor), and rRNA provides the factory floor where peptide bonds are actually formed (the ribosome).
| Feature | mRNA | tRNA | rRNA |
|---|---|---|---|
| Structure | Linear | Cloverleaf / L-shaped | Complex folded |
| Size | Variable | 75-90 nt | Large (varies by subunit) |
| % of total RNA | 3-5% | ~15% | ~80% |
| Function | Template for translation | Amino acid carrier | Ribosome structure + catalysis |
| Lifespan | Short | Moderate | Long |
Alternative Method
For NEET, remember the percentage trick: rRNA is the most abundant (80%), tRNA is second (15%), and mRNA is least abundant (3-5%). If a question asks “which RNA is most abundant in the cell?” — the answer is always rRNA, never mRNA despite mRNA being the most talked-about.
Common Mistake
Students frequently confuse the anticodon (on tRNA) with the codon (on mRNA). The codon is on the mRNA and is read 5’ to 3’. The anticodon on tRNA is complementary and antiparallel to the codon. If the codon is 5’-AUG-3’, the anticodon is 3’-UAC-5’.
Also, many students think mRNA is the most abundant RNA because it gets the most attention in textbooks. In reality, rRNA dominates because ribosomes are massive complexes present in thousands per cell, each containing multiple rRNA molecules.