Question
Compare DNA and RNA in terms of their sugar, bases, structure, and biological function. Name the three types of RNA and state their roles.
(NCERT Class 12 — direct CBSE board question, also asked in NEET and JEE Main)
Solution — Step by Step
- DNA contains deoxyribose sugar — the 2’ carbon has only a hydrogen (), no hydroxyl group
- RNA contains ribose sugar — the 2’ carbon carries a hydroxyl group ()
This single difference makes RNA more reactive and less stable than DNA. That’s why DNA is better suited for long-term genetic storage.
Both share three bases: adenine (A), guanine (G), and cytosine (C).
The difference:
- DNA has thymine (T) — a methylated base
- RNA has uracil (U) — no methyl group
Base pairing: In DNA, (2 hydrogen bonds) and (3 hydrogen bonds). In RNA, replaces .
- DNA is usually double-stranded — two antiparallel strands forming a double helix (B-form, right-handed)
- RNA is usually single-stranded — though it can fold back on itself to form secondary structures (like tRNA’s cloverleaf)
| Type | Full Name | Function |
|---|---|---|
| mRNA | Messenger RNA | Carries the genetic code from DNA to ribosomes for protein synthesis |
| tRNA | Transfer RNA | Carries specific amino acids to the ribosome; has an anticodon that pairs with mRNA codon |
| rRNA | Ribosomal RNA | Structural and catalytic component of ribosomes; forms the site where proteins are assembled |
rRNA is the most abundant RNA in the cell (about 80%), followed by tRNA (~15%), and mRNA (~5%).
Why This Works
The structural differences between DNA and RNA are directly linked to their functions.
DNA needs to be stable for long-term storage of genetic information — the double-stranded helix with deoxyribose sugar and thymine (which resists deamination better than uracil) provides that stability. The double strand also allows error correction during replication.
RNA is meant to be temporary — a working copy. The single strand with ribose sugar makes it more flexible and reactive. mRNA is made, used, and degraded. tRNA and rRNA are more stable because of their extensive secondary structures, but they’re still recycled faster than DNA.
Alternative Method — Quick Comparison Table
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Unique base | Thymine (T) | Uracil (U) |
| Strands | Double-stranded | Single-stranded |
| Location | Nucleus (mostly) | Cytoplasm + nucleus |
| Function | Stores genetic info | Protein synthesis |
| Stability | High | Low (easily hydrolysed) |
| Replication | Self-replicating | Transcribed from DNA |
For NEET: this comparison table is a guaranteed 1-mark question. Memorise the sugar difference (deoxy vs ribose) and the base difference (T vs U). Also remember that RNA can sometimes be double-stranded — in some viruses like reovirus. This is a favourite NEET trap.
Common Mistake
Students frequently write that “DNA is found only in the nucleus and RNA only in the cytoplasm.” This is wrong. DNA is also found in mitochondria and chloroplasts (organellar DNA). RNA is found in the nucleus (as hnRNA/pre-mRNA) as well as in the cytoplasm. The question asks where they are mostly found — but stating “only” will lose you marks.