Answers :
1️⃣ Question: Which base pair is correct in the Watson–Crick model?
🧬 Answer: A pairs with T.
🧠 Explanation: Adenine always bonds with thymine and guanine with cytosine. This complementary pattern helps DNA copy itself accurately during replication.
2️⃣ Question: The two DNA strands run antiparallel.
✅ Answer: True.
💭 Explanation: One strand runs 5′→3′, while the other goes 3′→5′. This opposite direction is what allows the bases to match up properly.
3️⃣ Question: What forms the backbone of the DNA double helix?
🧪 Answer: Sugar and phosphate.
📘 Explanation: The alternating deoxyribose sugar and phosphate groups make up the sturdy outer frame, while the nitrogen bases stay tucked inside.
4️⃣ Question: What is the diameter of B-DNA?
📏 Answer: About 2 nm.
💬 Explanation: Each purine pairs with a pyrimidine, so the helix keeps a steady width all along — like a perfectly twisted ladder.
5️⃣ Question: Distance between adjacent base pairs in B-DNA is—
📐 Answer: 0.34 nm.
🌀 Explanation: Ten such stacked pairs make one full helical turn of about 3.4 nm.
6️⃣ Question: How many base pairs occur per helical turn in B-DNA?
🔢 Answer: Around 10 base pairs.
📘 Explanation: In the common B-form, every full spiral (3.4 nm) fits roughly ten base pairs — neat and regular!
7️⃣ Question: How many hydrogen bonds connect G and C?
💧 Answer: Three.
🔬 Explanation: G≡C has three hydrogen bonds, while A=T has two. That’s why G–C regions are a bit more stable and harder to separate.
8️⃣ Question: The sugar–phosphate backbone lies inside the helix.
❌ Answer: False.
💭 Explanation: In the Watson–Crick model, the backbone actually stays on the outside, wrapping around the inner base pairs.
9️⃣ Question: Which evidence helped identify the double-helix structure?
📸 Answer: X-ray diffraction.
🧠 Explanation: The X-ray images — especially Rosalind Franklin’s — revealed a repeating helical pattern that led to the famous model.
🔟 Question: What best defines complementary base pairing?
🧬 Answer: One strand’s sequence decides the other.
💬 Explanation: A always pairs with T and G with C. This rule lets DNA copy itself precisely every time a cell divides.
11️⃣ Question: Which statement about B-DNA is correct?
📘 Answer: It’s a right-handed helix with a 3.4 nm pitch.
💭 Explanation: The B-form DNA turns gently to the right and makes one full spiral every 3.4 nanometres.
12️⃣ Question: Does B-DNA have major and minor grooves?
✔️ Answer: Yes, it does.
🔎 Explanation: Because the two backbones aren’t evenly spaced, the helix shows alternating grooves — spots where proteins can attach and read DNA information.
13️⃣ Question: If A = 30% and C = 20% in dsDNA, what are T and G?
📊 Answer: T = 30%, G = 20%.
📗 Explanation: Chargaff’s rule says adenine equals thymine, and guanine equals cytosine — always in matching proportions.
14️⃣ Question: What keeps the DNA helix at a uniform 2 nm width?
⚖️ Answer: Pairing of purine with pyrimidine.
🧠 Explanation: A larger purine base always bonds with a smaller pyrimidine, keeping the structure even and stable.
15️⃣ Question: What is the helical sense of B-DNA?
➡️ Answer: Right-handed.
💡 Explanation: The classic B-form described by Watson and Crick twists to the right — the one most commonly found in living organisms.
📚 Topics Covered
📚 What this roundup covers — quick peek
A compact Class 12 CBSE revision of the Watson–Crick DNA model: its shape, building blocks, base-pairing rules, a few neat memory tricks, and exam-friendly checks.
🧠 Core ideas — in plain language
DNA (deoxyribonucleic acid) is the molecule that carries genetic instructions — basically the cell’s instruction manual. In 1953 James Watson and Francis Crick proposed the double-helix as the structure for DNA, a model built on Rosalind Franklin’s X-ray diffraction images and Maurice Wilkins’ data.
Picture two long strands winding around each other like a spiral staircase — that’s the double helix. Each strand is a chain of nucleotides; every nucleotide contains a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C) or guanine (G).
Bases pair specifically: A with T, and G with C. That pairing follows Chargaff’s observations and is held together by hydrogen bonds — two between A–T and three between G–C — which both stabilise the molecule and let the strands unzip when needed (replication, transcription). The strands run in opposite directions (antiparallel): one from 5′ to 3′, the other from 3′ to 5′.
⚡ Quick facts worth memorizing
• One helical turn = ~3.4 nm and contains about 10 base pairs.
• Purine (A, G) always pairs with a pyrimidine (T, C) so the helix keeps a constant diameter.
• The sugar-phosphate backbone sits on the outside; the bases face inward like the rungs of a twisted ladder.
• Hydrogen bonds stabilise the helix but are weak enough to allow strand separation.
• Major and minor grooves form along the helix and are important sites for protein binding and regulation.
✅ Revision checklist & exam hacks
• Always give the discovery year: 1953 — and name Watson & Crick.
• In diagrams, label both strands and show directions (5′→3′ vs 3′→5′).
• Draw base pairs clearly and indicate hydrogen bonds (A–T: 2, G–C: 3). A tiny mnemonic: “AT = 2, GC = 3” — short and sticky.
• Mention Franklin’s X-ray diffraction as the key supporting evidence.
• Define and highlight “complementary base pairing” and “antiparallel strands.” These phrases often appear in questions.
• If asked to compare with RNA: note RNA is usually single-stranded, uses ribose (not deoxyribose), and has uracil (U) instead of thymine (T).
• For stability questions: cite hydrogen bonding and base stacking.
• Explain replication simply: because strands are complementary, each can serve as a template for a new partner strand.
• In MCQs, point out Chargaff’s rule (equal amounts of purines and pyrimidines) whenever base ratios come up.
🧩 Memory nudges (quick)
• Visualize the helix as a spiral staircase: sugar-phosphate pillars outside, base rungs inside.
• “AT two, GC three” — say it once, and it often sticks.
• To remember antiparallel: imagine two people walking side by side but facing opposite directions.
🗂️ Mini glossary
Double helix — two strands twisted into a uniform spiral.
Antiparallel — strands run in opposite orientations (5′→3′ and 3′→5′).
Base pairing — specific hydrogen bonding: A–T and G–C.
Chargaff’s rule — purine and pyrimidine amounts are equal (A ≈ T, G ≈ C).
Major groove — the wider groove of DNA where many proteins bind.
