Key Fact
DNA structure
Watson-Crick double helix: antiparallel strands; A=T (2 H-bonds), G≡C (3 H-bonds); 10 bp/turn; pitch 3.4 nm; diameter 2 nm. Chargaff's rule: A=T, G=C. Z-DNA, A-DNA, B-DNA forms.
-- NCERT Class 12 Biology, Ch. 5, p. 81DNA RNA Structure
8 MCQs9-step worked example
Source: NCERT Genetics and EvolutionPYQ coverage: NEET 2021Official key: NTA-verifiedLast updated: 26 Sep 2026
DNA RNA Structure, explained for NEET
NEET regularly tests whether you can distinguish the structural features of DNA from RNA and recall the details of the Watson-Crick model. The high-frequency trap here is confusing the directionality and bonding specifics — getting the strand polarity, base-pairing rules, or groove nomenclature wrong costs marks under negative marking.
The Watson-Crick model (1953) describes DNA as a right-handed double helix with two antiparallel polynucleotide strands. "Antiparallel" means one strand runs 5'→3' while its complement runs 3'→5'. The strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine via two H-bonds (A=T), and guanine pairs with cytosine via three H-bonds (G≡C). This is Chargaff's rule in structural form — the molar ratio of A/T and G/C each equals 1 (NCERT Class 12 Biology Chapter 5, page 81).
The backbone consists of alternating deoxyribose sugar and phosphate groups linked by 3'–5' phosphodiester bonds. The bases project inward, stacked 0.34 nm apart; one full turn spans 3.4 nm and contains approximately 10 base pairs. The helix has a major groove and a minor groove — proteins that read DNA (transcription factors, regulatory proteins) typically interact via the major groove, where more of the base-pair's chemical identity is exposed.
DNA vs RNA — the four structural differences NEET expects:
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose (–H at 2' carbon) | Ribose (–OH at 2' carbon) |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double-stranded (usually) | Single-stranded (usually) |
| Stability | More stable (no 2'–OH) | Less stable, more reactive |
Thymine in DNA is replaced by uracil in RNA — both pair with adenine, but uracil lacks the methyl group at the 5-position.
Watch-out for NEET: when a question says "which base is absent in RNA," the answer is thymine, not uracil. Reversing this is a common negative-marking loss.
Can you answer these DNA RNA Structure MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The distance between two adjacent base pairs in the B-form DNA double helix is:
Show answer and why every option is right or wrong
Answer: C. The rise per base pair in B-DNA is 0.34 nm (3.4 Å). One full helical turn of 3.4 nm contains approximately 10 base pairs (NCERT Class 12 Biology Chapter 5, page 81).
Why A is wrong: A is wrong because 0.034 nm is ten times too small — this confuses the rise per base pair (0.34 nm) with a decimal-place error.
Why B is wrong: B is wrong because 3.4 nm is the pitch (one full helical turn), not the distance between adjacent base pairs.
Why D is wrong: D is wrong because 34 nm represents ten full helical turns, not a single base-pair spacing.
MCQ 2Easy RecallPractice
The number of hydrogen bonds between adenine and thymine in a DNA double helix is:
Show answer and why every option is right or wrong
Answer: D. Adenine and thymine are held together by two hydrogen bonds (A=T), while guanine and cytosine form three hydrogen bonds (G≡C) (NCERT Class 12 Biology Chapter 5, page 81).
Why A is wrong: A is wrong because one hydrogen bond is too few — even the weaker A-T pair uses two H-bonds for stable base-pairing.
Why B is wrong: B is wrong because no standard Watson-Crick base pair uses four hydrogen bonds.
Why C is wrong: C is wrong because three hydrogen bonds are formed between G and C, not between A and T. Confusing the two pairs is a common trap.
MCQ 3Easy RecallPractice
Which of the following bases is present in RNA but absent in DNA?
Show answer and why every option is right or wrong
Answer: D. Uracil replaces thymine in RNA. DNA contains A, T, G, C; RNA contains A, U, G, C. Uracil lacks the methyl group at the 5-position that thymine carries (NCERT Class 12 Biology Chapter 5, page 81).
Why A is wrong: A is wrong because adenine is present in both DNA and RNA — it is a common purine base.
Why B is wrong: B is wrong because guanine is present in both DNA and RNA — it is a common purine base.
Why C is wrong: C is wrong because cytosine is present in both DNA and RNA — it is a common pyrimidine base.
MCQ 4Direct ApplicationPractice
A DNA molecule has 30% adenine. What is the percentage of guanine in the same molecule?
Show answer and why every option is right or wrong
Answer: C. By Chargaff's rule, A = T = 30%. So A + T = 60%. Since A + T + G + C = 100%, G + C = 40%. Since G = C, each is 20% (NCERT Class 12 Biology Chapter 5, page 81).
Why A is wrong: A is wrong because 30% would mean A = T = G = C = 30%, which sums to 120%. This error arises from assuming all bases are equimolar.
Why B is wrong: B is wrong because 40% is the combined G + C content, not the percentage of guanine alone. You need to divide by 2 since G = C.
Why D is wrong: D is wrong because 10% would give G + C = 20% and A + T = 80%, which does not follow from 30% adenine.
MCQ 5Direct ApplicationPractice
A DNA molecule contains 600 base pairs. How many complete helical turns does it have in B-form?
Show answer and why every option is right or wrong
Answer: B. B-DNA has approximately 10 base pairs per helical turn. 600 bp ÷ 10 bp/turn = 60 turns (NCERT Class 12 Biology Chapter 5, page 81).
Why A is wrong: A is wrong because 20 turns would require only 200 bp. This likely comes from dividing 600 by 30 or another incorrect divisor.
Why C is wrong: C is wrong: 180 turns at 10 bp per turn would need 1,800 bp, three times the 600 bp given. (Dividing by the 3.4 nm pitch, a length rather than a count, gives about 176, not 180.)
Why D is wrong: D is wrong because 6000 comes from multiplying 600 by 10 instead of dividing. The question asks for turns, not total rise distance.
MCQ 6Direct ApplicationPractice
The two strands of a DNA double helix are described as antiparallel because:
Show answer and why every option is right or wrong
Answer: A. "Antiparallel" refers to the opposite chemical polarity of the two strands — one runs 5'→3' and the complementary strand runs 3'→5'. This is a fundamental feature of the Watson-Crick model (NCERT Class 12 Biology Chapter 5, page 81).
Why B is wrong: B is wrong because by Chargaff's rule, both strands are complementary in base composition (A pairs with T, G pairs with C) — antiparallel refers to directionality, not composition.
Why C is wrong: C is wrong because the major and minor grooves are features of the helical geometry, not the reason the strands are called antiparallel.
Why D is wrong: D is wrong because both strands contain both purines and pyrimidines — each base pair has one purine and one pyrimidine. This is complementarity, not antiparallelism.
MCQ 7Concept TrapPractice
DNA is more stable than RNA under physiological conditions. Which structural feature of DNA primarily accounts for this greater stability?
Show answer and why every option is right or wrong
Answer: B. The 2'–OH group on ribose in RNA makes it susceptible to alkaline hydrolysis — the hydroxyl can attack the adjacent phosphodiester bond. DNA's deoxyribose lacks this 2'–OH, making the backbone more resistant to cleavage (NCERT Class 12 Biology Chapter 5, page 87).
Why A is wrong: A is wrong because while thymine does have a methyl group that uracil lacks, this difference relates to base recognition and repair mechanisms, not to the chemical stability of the sugar-phosphate backbone.
Why C is wrong: C is wrong because molecular weight does not determine chemical stability. A higher molecular weight simply means a longer polymer, not a more hydrolysis-resistant one.
Why D is wrong: D is wrong because both DNA and RNA have 3'–5' phosphodiester bonds in their backbone. This shared feature does not explain why DNA is more stable than RNA.
MCQ 8Concept TrapPractice
Regulatory proteins that need to read the DNA sequence without unwinding the helix typically bind at:
Show answer and why every option is right or wrong
Answer: A. The major groove exposes more of the edges of the base pairs, allowing sequence-specific recognition without disrupting base-pairing. Transcription factors and other regulatory proteins commonly interact through the major groove (beyond NCERT: NCERT Class 12 Biology Chapter 5, page 81 describes the double helix but not its grooves).
Why B is wrong: B is wrong because while the minor groove can be used by some proteins, it provides less chemical information about base identity. NEET-level texts emphasise the major groove as the primary site for sequence-specific recognition.
Why C is wrong: C is wrong because the sugar-phosphate backbone is chemically uniform regardless of sequence — binding here gives no sequence-specific information.
Why D is wrong: D is wrong because the 3' end is a single point on the strand, not a groove that runs along the length of the helix. Regulatory proteins bind extended DNA stretches, not a terminal position.
Free NEET study resources
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
How do you solve a DNA RNA Structure question? A worked example
- 1
Given
A double-stranded DNA molecule has 1,000 base pairs. Analysis of one strand shows: A = 200, T = 150, G = 350, C = 300.
- 2
Required
Find the base composition (number of each base) of the complementary strand and the complete molecule.
- 3
Concept
In double-stranded DNA, adenine on one strand pairs with thymine on the complementary strand, and guanine pairs with cytosine (Watson-Crick base-pairing). Chargaff's rule ensures that for the whole molecule, A = T and G = C.
- 4
Formula/Rule
Complementary strand base = partner of the given strand base:• A in given strand → T in complementary strand• T in given strand → A in complementary strand• G in given strand → C in complementary strand• C in given strand → G in complementary strand
- 5
Substitution
Complementary strand:• T (paired with A = 200) = 200• A (paired with T = 150) = 150• C (paired with G = 350) = 350• G (paired with C = 300) = 300
- 6
Calculation
Whole molecule totals:• Total A = 200 (strand 1) + 150 (strand 2) = 350• Total T = 150 (strand 1) + 200 (strand 2) = 350 ✓ (A = T)• Total G = 350 (strand 1) + 300 (strand 2) = 650• Total C = 300 (strand 1) + 350 (strand 2) = 650 ✓ (G = C)• Grand total = 350 + 350 + 650 + 650 = 2,000 bases = 1,000 base pairs ✓
Note: the counts 200, 150, 350, 300, and 1,000 are exact counting integers — they do not involve significant-figure considerations. - 7
Final answer
Complementary strand: A = 150, T = 200, G = 300, C = 350.
Whole molecule: A = T = 350; G = C = 650. Total = 2,000 bases. - 8
Common trap
Students sometimes apply Chargaff's rule to a single strand (expecting A = T within that strand). Chargaff's rule applies to the double-stranded molecule as a whole. In a single strand, A ≠ T and G ≠ C is perfectly normal.
- 9
Similar NEET-style question
"If one strand of DNA has the base composition A : T : G : C = 20% : 30% : 15% : 35%, what is the base composition of the complete double-stranded DNA molecule?" (Apply the same complementary-pairing logic, then average across both strands.)
---
What to remember before solving DNA RNA Structure questions
More in Genetics and Evolution: 21 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.
DNA RNA Structure questions from past NEET papers
1 question from NEET 2021. Answers verified against NTA official keys.
NEET 2021
1T : 20 ; G : 25 ; C : 25
2T : 20 ; G : 30 ; C : 20
3T : 20 ; G : 20 ; C : 30
4T : 30 ; G : 20 ; C : 20
NTA Answer: Option 4(final)
Sources
NCERT refs: Class 12 Biology Chapter 5, p.81
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.
Test yourself on this topic with real past-paper questions:
Practice this topic →