The trap that costs marks: writing A=U for DNA or A=T for RNA. This single swap — confusing which nucleic acid uses thymine and which uses uracil — is a common confusion in NEET biomolecule questions, and it hands away easy marks on what should be straightforward recall.
DNA vs RNA — the chemical differences that matter.
Both DNA and RNA are polynucleotides built from nucleotide monomers. Each nucleotide has three parts: a nitrogenous base, a pentose sugar, and a phosphate group (NCERT Class 12 Chemistry, Chapter 14, page 26).
The critical distinctions:
Sugar: DNA contains 2′-deoxyribose (missing the –OH at carbon 2′). RNA contains ribose (–OH present at carbon 2′). This single oxygen difference makes RNA more susceptible to alkaline hydrolysis.
Bases: Four bases in each, but one differs.
Thymine is 5-methyluracil — it carries an extra methyl group that uracil lacks. NEET questions exploit the T↔U swap precisely because students memorize "four bases" without anchoring which base belongs to which nucleic acid.
Base pairing (Watson–Crick model):
The G≡C pair (3 H-bonds) is stronger than A=T or A=U (2 H-bonds). Higher GC content means a more thermally stable duplex.
Structure: DNA is typically double-stranded (antiparallel, right-handed double helix). RNA is typically single-stranded, though it can fold into secondary structures through intramolecular base pairing.
Watch out: When a question mentions "complementary base pairing in mRNA," the answer uses A-U, not A-T. The moment you see "RNA" in any form — mRNA, tRNA, rRNA — thymine is absent.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which nitrogenous base is present in RNA but absent in DNA?
The sugar present in the backbone of DNA is:
How many hydrogen bonds form between adenine and thymine in DNA?
In an mRNA molecule, adenine would pair with which base during translation-related base pairing?
A DNA segment has 200 base pairs. If 40% of the bases are guanine, how many hydrogen bonds are present in this segment?
Thymine is chemically related to uracil. What is the structural difference between thymine and uracil?
A researcher finds that a nucleic acid sample contains equal proportions of adenine and uracil but no thymine. Which of the following is the most likely identity of this molecule?
A double-stranded DNA molecule contains 1000 base pairs. If the molecule has 600 adenine bases, calculate the total number of hydrogen bonds in this DNA molecule.
Given
A double-stranded DNA molecule has 500 base pairs. The GC content is 36%.
Required
Total number of hydrogen bonds in the DNA molecule.
Concept
In Watson–Crick base pairing, A pairs with T via 2 hydrogen bonds and G pairs with C via 3 hydrogen bonds. The total H-bond count depends on the proportion of each pair type.
Formula
Total H-bonds = (number of A-T pairs × 2) + (number of G-C pairs × 3)
Substitution
- GC pairs = 36% of 500 = 180 - AT pairs = 500 − 180 = 320 - H-bonds = (320 × 2) + (180 × 3)
Calculation
- AT contribution: 320 × 2 = 640 - GC contribution: 180 × 3 = 540 - Total = 640 + 540 = **1180** Note: The integers 2, 3, 500, and the percentage 36% are either counting numbers or problem-defined exact values. They do not limit significant figures.
Final answer
**1180 hydrogen bonds**
Common trap
Using 2 H-bonds for GC pairs (confusing the H-bond counts between AT and GC). This would give 320 × 2 + 180 × 2 = 1000 — a wrong answer that NTA can place as a distractor. Another trap: misidentifying GC content as referring to individual bases rather than pairs. In a duplex, "36% GC content" means 36% of the base pairs are G-C.
Similar NEET-style question
"A DNA fragment has 800 base pairs with 240 guanine bases. How many hydrogen bonds does this fragment contain?" (Answer: G-C pairs = 240, A-T pairs = 560; H-bonds = 560 × 2 + 240 × 3 = 1120 + 720 = 1840.) ---
DNA: deoxyribose sugar, A-T-G-C bases, double helix. RNA: ribose, A-U-G-C bases, mostly single-strand. Phosphate-sugar backbone. Bases: purines (A, G) and pyrimidines (T/U, C). H-bond pairs A=T (DNA), A=U (RNA), G≡C.
-- NCERT, p. 26Used to compute total H-bonds in a duplex of given GC%/AT% composition.
| Symbol | Quantity | SI Unit |
|---|---|---|
| %GC | GC content | - |
Empirical formula of simple monosaccharides; glucose/fructose are C6H12O6.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | carbon count | - |
Number of amide (peptide) bonds in a linear polypeptide of N amino acids.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N | residue count | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student writes A=U for DNA or A=T for RNA. DNA: A=T, G≡C. RNA: A=U (no T), G≡C.
Question on base pairing or sugar identity.
DNA: deoxyribose, A-T-G-C bases. RNA: ribose, A-U-G-C bases (uracil instead of thymine). H-bond pairs: A=T (DNA) or A=U (RNA), G≡C (3 H-bonds, both).
Category: Similar Terms
Student claims denaturation breaks peptide bonds. Denaturation only breaks H-bonds, ionic, hydrophobic interactions; primary structure (peptide bonds) intact.
Question about protein denaturation effects.
Denaturation: heat/pH/organic solvents disrupt secondary, tertiary, quaternary structure. Primary structure (covalent peptide bonds) requires hydrolysis to break.
Root cause: concept gap
Denaturation breaks H-bonds, ionic, hydrophobic; preserves peptide bonds. Hydrolysis (acid/base/enzyme) breaks primary structure.
Root cause: concept gap
DNA: deoxyribose, A=T, G≡C. RNA: ribose, A=U (no T), G≡C.
6 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
The incorrect statement regarding enzymes is
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
swapped classes
Tempts surface-level recall.
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