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 (pre-2023 edition), Chapter 14, page 427).
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?
Answer: C. Uracil replaces thymine in RNA. DNA contains A, T, G, C; RNA contains A, U, G, C (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A is wrong — thymine is present in DNA but absent in RNA, which is the reverse of what the question asks.
Why B is wrong: B is wrong — cytosine is present in both DNA and RNA.
Why D is wrong: D is wrong — guanine is present in both DNA and RNA.
The sugar present in the backbone of DNA is:
Answer: B. DNA contains 2′-deoxyribose, which lacks the hydroxyl group at the 2′ carbon of the sugar ring (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A is wrong — ribose is the sugar found in RNA, not DNA. The 2′-OH is present in ribose. (trap: DNA–RNA sugar swap)
Why C is wrong: C is wrong — fructose is a monosaccharide (ketohexose) found in fruits, not a component of nucleic acid backbones.
Why D is wrong: D is wrong — ribulose is a ketopentose involved in the Calvin cycle, not a nucleic acid component.
How many hydrogen bonds form between adenine and thymine in DNA?
Answer: D. The A=T base pair in DNA is held together by 2 hydrogen bonds, while G≡C is held by 3 hydrogen bonds (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A is wrong — a single hydrogen bond would be too weak to stabilize the double helix; A-T forms 2 H-bonds.
Why B is wrong: B is wrong — no standard Watson–Crick base pair in DNA forms 4 hydrogen bonds.
Why C is wrong: C is wrong — 3 hydrogen bonds is the count for the G≡C pair, not A=T. Confusing the H-bond count between the two pairs is a common error.
In an mRNA molecule, adenine would pair with which base during translation-related base pairing?
Answer: B. In RNA, adenine pairs with uracil (A=U, 2 H-bonds). Thymine is absent from RNA. This applies to any RNA-based pairing event — mRNA–tRNA codon-anticodon interaction, or any intramolecular RNA folding (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A is wrong — thymine is a DNA-specific base. The moment the question mentions mRNA, thymine is ruled out. Writing A=T for RNA is the classic base-pairing swap trap.
Why C is wrong: C is wrong — cytosine pairs with guanine (C≡G), not with adenine, in both DNA and RNA.
Why D is wrong: D is wrong — guanine pairs with cytosine, not adenine. A pairs with T in DNA and U in RNA.
A DNA segment has 200 base pairs. If 40% of the bases are guanine, how many hydrogen bonds are present in this segment?
Answer: D. D is correct, and the wording of the stem is what decides it. 200 base pairs means 400 bases. "40% of the bases are guanine" gives G = 0.40 × 400 = 160, so by Chargaff C = 160 as well — that is 160 G–C pairs. The remaining 200 − 160 = 40 pairs are A–T. H-bonds = (160 × 3) + (40 × 2) = 480 + 80 = 560. Read the percentage carefully: it is stated per BASE and for guanine alone, so the G–C share of the pairs is 80%, not 40% (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A (480) is the answer to a different question — it treats '40% guanine' as '40% of the PAIRS are G–C', giving 80 GC pairs and 120 AT pairs: 80 × 3 + 120 × 2 = 480. But 40% of the bases being guanine means C is another 40%, so G–C accounts for 80% of the bases and 160 of the 200 pairs. This is the single most common misreading in composition questions.
Why B is wrong: B (520) is wrong — it falls between the two readings above and follows from neither. With 160 GC and 40 AT pairs the total is 560; with 80 GC and 120 AT it is 480.
Why C is wrong: C (440) is wrong — it arises from using 2 H-bonds for G–C pairs somewhere in the count. G≡C forms 3 hydrogen bonds, A=T forms 2.
Thymine is chemically related to uracil. What is the structural difference between thymine and uracil?
Answer: D. Thymine is 5-methyluracil — it differs from uracil only by the presence of a methyl group (–CH₃) at the C-5 position of the pyrimidine ring (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A is wrong — the difference is a methyl group, not an amino group. Both thymine and uracil are pyrimidine bases with keto groups at C-2 and C-4.
Why B is wrong: B is wrong — both thymine and uracil have keto groups at C-2 and C-4. The difference is the methyl group on thymine, not a keto group.
Why C is wrong: C is wrong — uracil does not have an extra hydroxyl group compared to thymine. The only structural difference is the methyl at C-5 present in thymine.
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?
Answer: C. The presence of uracil and absence of thymine confirms this is RNA. Equal proportions of A and U indicate Watson–Crick base pairing (A=U), which occurs in double-stranded RNA. Single-stranded RNA would not necessarily show A=U equivalence (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A is wrong — DNA contains thymine, not uracil. Any sample with uracil and no thymine cannot be DNA.
Why B is wrong: B is wrong — double-stranded DNA would contain thymine (paired with adenine) and no uracil.
Why D is wrong: D is wrong — while single-stranded RNA does contain uracil and no thymine, the equal A and U proportions suggest complementary base pairing (A=U), which is characteristic of a double-stranded RNA structure. Single-stranded RNA has no requirement for A=U equality.
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.
Answer: B. B is correct. If A = 600 then T = 600 (Chargaff's rule), so there are 600 A–T pairs. The remaining pairs are 1000 − 600 = 400 G–C pairs. H-bonds: (600 × 2) + (400 × 3) = 1200 + 1200 = 2400. The two contributions happen to be equal here, which is a useful check — 400 × 3 is 1200, not 1600 (NCERT Class 12 Chemistry, Chapter 14).
Why A is wrong: A (2800) is wrong — it comes from evaluating the G–C contribution as 400 × 3 = 1600. That product is 1200, so the total is 1200 + 1200 = 2400. The AT and GC contributions are equal in this particular molecule.
Why C is wrong: C (3200) is impossible: even if all 1000 pairs were G–C, there would be only 3 × 1000 = 3000 hydrogen bonds.
Why D is wrong: D (2600) would need 400 A–T and 600 G–C pairs (800 + 1800); with 600 adenines there are 600 A–T pairs and 400 G–C pairs.
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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.)
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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 Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 428These 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).
Root cause: concept gap
DNA: deoxyribose, A=T, G≡C. RNA: ribose, A=U (no T), G≡C.
More in Biomolecules: 2 exam traps and mistakes · 3 formulas from its other lessons.
1 question from NEET 2023. Answers verified against NTA official keys.
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
swapped classes
Tempts surface-level recall.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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