BiologyICSE Class 10 · Structure of Chromosome

DNA Double Helix and Base Pairing: Watson and Crick Model (ICSE Class 10 Biology)

Updated 15 October 2026 · Fact-checked

DNA is a double helix proposed by Watson and Crick in 1953: two strands twisted round each other like a spiral ladder. The sides are sugar–phosphate backbones and the rungs are pairs of nitrogenous bases joined by hydrogen bonds. Adenine always pairs with thymine, and guanine with cytosine, so the two strands are complementary.

Understand DNA Double Helix and Base Pairing

DNA: the hereditary material. Chromosomes are made of DNA (deoxyribonucleic acid) wound round proteins. Genes are segments of this DNA, so the structure of DNA explains how information is stored and copied.

Building block: the nucleotide. DNA is a very long chain (polymer) of units called nucleotides. Each nucleotide has three parts:

a sugar: deoxyribose (a five-carbon sugar);
a phosphate group;
a nitrogenous base: one of adenine (A), thymine (T), guanine (G) or cytosine (C).

Sugar and phosphate of neighbouring nucleotides join to form a long sugar–phosphate backbone; the bases stick out from it.

The Watson and Crick model (1953). James Watson and Francis Crick proposed the double helix, using X-ray pictures of DNA made by Maurice Wilkins and Rosalind Franklin and Erwin Chargaff's finding that A equals T and G equals C in DNA. Its main features:
1. DNA has two strands (two polynucleotide chains).
2. The strands are twisted round each other into a right-handed helix, like a spiral staircase or a twisted ladder.
3. The sugar–phosphate backbones form the two sides (rails) of the ladder, on the outside.
4. The bases point inwards and pair with bases of the other strand to form the rungs (steps).
5. Paired bases are held together by weak hydrogen bonds: A–T by two hydrogen bonds and G–C by three.
6. The two strands run in opposite directions (they are anti-parallel).
7. One complete turn of the helix contains about 10 base pairs.

Complementary base pairing. A pairs only with T, and G pairs only with C. So if one strand reads 5′ A T G C C A …, the partner reads T A C G G T. The strands are complementary, not identical. Consequences:

In any double-stranded DNA, amount of A equals amount of T, and amount of G equals amount of C.
Hence A + G equals T + C (purines equal pyrimidines).
Knowing one strand lets you write the other, which is how DNA is copied exactly before cell division: the strands separate and each acts as a template for a new partner.

Percentage rule (useful for numericals). If A is x per cent, then T is also x per cent, and G and C are each (100 − 2x) ÷ 2 per cent.

From DNA to chromosome (elementary). The very long DNA double helix is wound round proteins to form chromatin; during division chromatin coils tightly into a chromosome. A gene is a particular segment of the DNA, a particular sequence of bases, that carries information for one character.

Flow chart: levels of organisation. Nucleotide (sugar + phosphate + base) → polynucleotide strand → double helix (two complementary strands) → DNA wound on proteins (chromatin) → condensed chromosome; a gene is a stretch of the double helix.

Draw-and-label checklist: DNA double helix. Two ribbons twisting round each other several times; horizontal rungs joining them; a few rungs labelled A–T and G–C (with A opposite T and G opposite C); show one sugar and one phosphate on a ribbon. Labels: sugar–phosphate backbone, base pair, hydrogen bond, nitrogenous base, one complete turn. Title it DNA double helix (Watson and Crick model).

DNA Double Helix and Base Pairing: key facts

Base-pairing rule: Adenine (A) pairs with thymine (T), two hydrogen bonds
Guanine (G) pairs with cytosine (C), three hydrogen bonds
A with C or G with T never; Spell thymine and adenine correctly.
Parts of a nucleotide: Sugar deoxyribose
Phosphate phosphate group
Base adenine, thymine, guanine or cytosine; Sugar and phosphate make the backbone; bases make the rungs.
Percentage rule in double-stranded DNA: $% A = % T, % G = % C = \frac{100 - 2 ( % A )}{2}$; Chargaff's observation, used by Watson and Crick.
Model facts: Proposed by Watson and Crick, 1953
Shape right-handed double helix
Strands two, anti-parallel, complementary
Turn about 10 base pairs; Write 'double helix', not just 'spiral'.

Adenine (A)	pairs with thymine (T), two hydrogen bonds
Guanine (G)	pairs with cytosine (C), three hydrogen bonds
A with C or G with T	never

Sugar	deoxyribose
Phosphate	phosphate group
Base	adenine, thymine, guanine or cytosine

Proposed by	Watson and Crick, 1953
Shape	right-handed double helix
Strands	two, anti-parallel, complementary
Turn	about 10 base pairs

How to answer DNA Double Helix and Base Pairing questions

Expect a 1-mark question on who proposed the model or what its shape is, 2 marks on the base pairs, a diagram, and short numericals on complementary strands or base percentages.

1For 'structure of DNA as proposed by Watson and Crick', write the term double helix, then add: two strands, sugar–phosphate backbone outside, bases inside joined by hydrogen bonds.
2For base pairing, write both pairs in full: adenine–thymine and guanine–cytosine (with H-bonds if 2 marks).
3For a complementary strand, replace every base letter by its partner, keeping the same order.
4For percentages, set %T equal to %A, then share what is left equally between G and C. Check that the four add to 100.
5In a diagram, label at least one A–T and one G–C pair and the sugar–phosphate backbone.

DNA Double Helix and Base Pairing in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Complementary strands as a backup copy

Suppose a librarian keeps every book in two copies written in a code where each letter is replaced by its 'partner' letter (A↔T, G↔C). One copy of a page gets torn.

Explain how the torn page can be restored exactly, and relate this to DNA copying.

Try it first, then show the solutionSolution

Each letter on the surviving page fixes exactly one partner letter.
So the missing page can be rebuilt letter by letter from the surviving one.
DNA works the same way: its two strands are complementary, A opposite T and G opposite C.
Before division the strands separate and each builds a new partner by the pairing rule, giving two identical double helices.

Answer: Because each base has one fixed partner, one strand fully specifies the other; DNA uses this to make exact copies before cell division.

Memory hook: One strand is the mould for the other.

Chargaff's rule in a test

Suppose a forensic laboratory analyses a DNA sample and reports that 22 per cent of its bases are guanine.

Work out the percentages of the other three bases.

Try it first, then show the solutionSolution

G pairs with C, so cytosine is also 22 per cent.
G + C together make 44 per cent.
A + T make 100 − 44 = 56 per cent.
A equals T, so each is 56 ÷ 2 = 28 per cent.

Answer: C 22%, A 28%, T 28%.

Memory hook: Partners are equal; split the rest.

Tricks to remember DNA Double Helix and Base Pairing

Short memory aids. Say each one aloud once, then use it on a question the same day.

Apple in a Tree, Car in a Garage
A goes with T (Apple–Tree); C goes with G (Car–Garage).
Use it when: Never mixing up the base pairs.
AT two, GC three
'AT' has two letters that look like two bonds; 'GC' is the stronger pair with three bonds.
Use it when: Number of hydrogen bonds.
Rails and rungs
Rails are sugar–phosphate; rungs are bases.
Use it when: Labelling the double helix.

DNA Double Helix and Base Pairing: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: The two base pairs · 2 marks · Short answerBoard level

A model kit for DNA has four colours of rungs, one for each base. Which bases must the student join together across the two strands? Name both pairs in full.

Try it first, then show the solutionSolution

Adenine pairs with thymine (1 mark).
Guanine pairs with cytosine (1 mark).

Answer: Adenine–thymine and guanine–cytosine.

Example 2: Write the partner strand · 2 marks · Short answerBoard level

One strand of a DNA segment has the sequence G G A T C C T A. Write the sequence of its complementary strand and state the rule you used.

Try it first, then show the solutionSolution

Rule: A pairs with T and G pairs with C (complementary base pairing) (1 mark).
Complementary strand: C C T A G G A T (1 mark).

Answer: C C T A G G A T.

Example 3: Percentages of bases · 2 marks · Short answerHarder board level

A double-stranded DNA sample contains 30 per cent adenine. Calculate the percentages of thymine, guanine and cytosine. Give a reason.

Try it first, then show the solutionSolution

A pairs with T, so T = 30% (½ mark).
A + T = 60%, leaving 40% for G + C (½ mark).
G pairs with C, so G = C = 20% each (1 mark).

Answer: T 30%, G 20%, C 20%.

Example 4: Features of the model · 3 marks · Short answerHarder board level

Describe the Watson and Crick model of a DNA molecule under four headings: shape, backbone, rungs, bonds.

Try it first, then show the solutionSolution

Shape: two strands twisted into a double helix (½ + ½ mark).
Backbone: alternating deoxyribose sugar and phosphate groups form the two outer sides (1 mark).
Rungs and bonds: nitrogenous bases pair inside, A with T and G with C, joined by hydrogen bonds (1 mark).

Answer: A double helix of two complementary strands with sugar–phosphate backbones outside and A–T, G–C base pairs inside held by hydrogen bonds.

Example 5: Counting bonds · 3 marks · Short answerHOTS / competency

A DNA segment has 100 base pairs, of which 60 are A–T pairs. (a) How many G–C pairs does it have? (b) How many hydrogen bonds hold its two strands together? (c) Which segment would be harder to separate by heating: this one or one of 100 base pairs with 30 A–T pairs? Explain.

Try it first, then show the solutionSolution

(a) 100 − 60 = 40 G–C pairs (1 mark).
(b) A–T has 2 hydrogen bonds and G–C has 3: 60 × 2 + 40 × 3 = 120 + 120 = 240 (1 mark).
(c) The second segment has 70 G–C pairs: 30 × 2 + 70 × 3 = 270 bonds, more than 240, so it is harder to separate (1 mark).

Answer: (a) 40 (b) 240 (c) The one with 30 A–T pairs (270 hydrogen bonds).

Example 6: Diagram of the double helix · 3 marks · Long answerHarder board level

Draw a neat, labelled diagram of the DNA double helix. Label: sugar–phosphate backbone, base pair, hydrogen bond and one A–T pair.

Try it first, then show the solutionSolution

Two ribbons twisting round each other with at least one and a half turns (1 mark).
Horizontal rungs between them, a few marked A–T and G–C with dotted lines for hydrogen bonds (1 mark).
Four labels with straight label lines (1 mark).

Answer: Two twisted sugar–phosphate ribbons with base-pair rungs (A–T, G–C) joined by hydrogen bonds, all labelled.

Where students lose marks in DNA Double Helix and Base Pairing

The slips examiners see most often, and what to write instead.

Mistake: Wrong pairs and misspelt bases
CISCE examiners' comments (2025): candidates gave incorrect base pairing and misspelt thymine as 'thiamine' and adenine as 'adenosine'.
Wrong working
Adenosine–thiamine; guanine–adenine.
Right working
Adenine–thymine; guanine–cytosine.
Do this: Learn the spellings: adenine, thymine, guanine, cytosine.
Usually costs: 1-2 marks.
Mistake: 'Spiral' or 'ladder' without 'double helix'
CISCE examiners' comments (2025): for the structure proposed by Watson and Crick, candidates wrote spiral, ladder or twisted ladder without naming the double helix.
Do this: Write the key term double helix first; the ladder can be an extra explanation.
Usually costs: 1 mark.
Mistake: Forgetting that percentages cover both strands
Students halve the remainder wrongly or give G and C different values.
Wrong working
A = 30%, so G = 70%.
Right working
A = T = 30%; G = C = (100 − 60) ÷ 2 = 20%.
Do this: Write all four percentages and check they add to 100.
Usually costs: 1 mark.

Exam tips

Use 'double helix' and 'complementary base pairing' as key terms.
When asked for bases, write full names, not just letters.
In a diagram, the backbone is on the outside and bases on the inside.
Mention hydrogen bonds whenever you describe the rungs.

Practice questions on DNA Double Helix and Base Pairing

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In DNA, guanine pairs with:
(a) Adenine (b) Thymine (c) Cytosine (d) Uracil
Hint
Car in a Garage.
Answer
(c) Cytosine.
Q2 · 1 mark · Very short answerBoard level
Who proposed the double helix model of DNA? (Asked in 2025 in the form 'structure of DNA as proposed by Watson and Crick'.)
Hint
Two scientists, 1953.
Answer
James Watson and Francis Crick (1953).
Q3 · 1 mark · Very short answerHarder board level
A DNA sample has 18 per cent cytosine. Find the percentage of adenine.
Hint
C = G, then split the rest.
Answer
G = 18%, so G + C = 36%; A + T = 64%; A = 32%.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): In double-stranded DNA the amount of adenine equals the amount of thymine.
Reason (R): Adenine always pairs with thymine.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Every A has a T opposite it.
Answer
(a).
Q5 · 2 marks · Short answerHarder board level
Name the three components of a nucleotide. Which two of them form the backbone of a DNA strand?
Hint
Sugar, phosphate, base.
Answer
Deoxyribose sugar, phosphate group and a nitrogenous base; sugar and phosphate form the backbone.
Q6 · 2 marks · Short answerHarder board level
Why are the two strands of DNA described as complementary rather than identical?
Hint
Compare a strand with its partner.
Answer
The two strands do not have the same sequence; each base on one strand is matched by its pairing partner (A with T, G with C) on the other, so one strand fits the other like a mould, and either can be worked out from the other.
Q7 · 2 marks · Short answerHOTS / competency
Suppose a student's model of DNA shows adenine paired with guanine and thymine paired with cytosine. Point out the error and explain why the width of the real helix stays uniform.
Hint
A and G are both large (purines).
Answer
The pairing must be A–T and G–C. In each correct pair a larger base (A or G) pairs with a smaller base (T or C), so every rung is the same length and the two strands stay the same distance apart; two large bases together would bulge the helix.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose one strand of a short DNA segment reads T A C G G A T C.
(a) Write the complementary strand. (1)
(b) How many hydrogen bonds join the two strands? (1)
(c) Give the percentage of G in this double-stranded segment. (1)
Hint
Count A–T and G–C pairs.
Answer
(a) A T G C C T A G. (b) Pairs: T–A, A–T, C–G, G–C, G–C, A–T, T–A, C–G = 4 A–T and 4 G–C pairs: 4 × 2 + 4 × 3 = 20. (c) Strand 1 has 2 G and strand 2 has 2 G: 4 G out of 16 bases = 25%.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which sugar is present in DNA?Show answer
Deoxyribose.
2.How many hydrogen bonds join guanine and cytosine?Show answer
Three.
3.In which year was the double helix model proposed?Show answer
1953.
4.If a strand reads A T G, what does its partner read?Show answer
T A C.
5.A DNA sample has 40% guanine. What percentage is thymine?Show answer
10%.
6.What is a nucleotide?Show answer
The unit of DNA: a deoxyribose sugar, a phosphate group and a nitrogenous base.

DNA Double Helix and Base Pairing: frequently asked questions

What is the structure of DNA according to Watson and Crick?

A double helix of two complementary strands, with sugar–phosphate backbones on the outside and paired bases (A–T, G–C) inside, joined by hydrogen bonds.

Which bases pair in DNA?

Adenine pairs with thymine, and guanine pairs with cytosine.

Why is base pairing important?

It makes the strands complementary, so each strand can act as a template to make an exact copy of DNA before cell division.

Is the detailed chemistry of DNA in the ICSE Class 10 syllabus?

No. The syllabus asks for an elementary understanding of DNA structure; the double helix, nucleotides and base pairing are enough.