DNA Structure Diagram IGCSE Biology 0610

DNA Structure Diagram: Double Helix and Base Pairs

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key diagram

DNA is a double helix made of two polynucleotide strands. Each nucleotide has a phosphate group, a deoxyribose sugar and a base. Complementary base pairing (adenine-thymine, cytosine-guanine) holds the two strands together by hydrogen bonds.

The DNA structure diagram is tested at Extended level and links to questions on protein synthesis, mutations and genetic engineering. Students must show the double helix arrangement and the complementary base pairing rule clearly.

Drawing the double helix

Draw two parallel lines that twist around each other to form a spiral ladder shape: this is the double helix. The sides of the ladder are the sugar-phosphate backbone, made of alternating deoxyribose sugar and phosphate groups. The rungs of the ladder are base pairs connected by hydrogen bonds.

If drawing a simplified "ladder" version (untwisted), draw two vertical lines for the backbone and horizontal lines between them for the base pairs.

Nucleotide structure

Each nucleotide consists of three parts: a phosphate group, a deoxyribose sugar and a nitrogenous base. The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). In a diagram, show one nucleotide with all three components clearly labelled.

Complementary base pairing

Adenine always pairs with thymine (A-T), and cytosine always pairs with guanine (C-G). The bases are held together by hydrogen bonds. This complementary base pairing means that if you know the sequence on one strand, you can determine the sequence on the other strand.

In exam answers, always state both pairs and mention hydrogen bonds. Writing "A pairs with T" without mentioning "C pairs with G" may lose marks if the question asks for the base pairing rule.

Genes and the genetic code

A gene is a section of DNA that codes for a specific protein. The sequence of bases along the gene provides the code: each triplet of bases codes for one amino acid. The sequence of amino acids determines the type of protein produced. This links the DNA diagram to protein synthesis and the concept of genetic information.

Label checklist

Check these labels are present and correctly positioned before finishing your drawing.

✓ Double helix
✓ Sugar-phosphate backbone
✓ Deoxyribose sugar
✓ Phosphate group
✓ Base pair
✓ Hydrogen bonds
✓ Adenine (A)
✓ Thymine (T)
✓ Cytosine (C)
✓ Guanine (G)
✓ Nucleotide

Try a diagram question (4 marks)

Describe the structure of a DNA molecule. [4]

Show the answer and marking guidance

DNA is a double helix made of two polynucleotide strands.
Each nucleotide consists of a phosphate group, a deoxyribose sugar and a base.
The two strands are held together by hydrogen bonds between complementary base pairs.
Adenine pairs with thymine and cytosine pairs with guanine.

Original Cambridge-style practice written for this site.

Common diagram mistakes

Check these against your own drawing before moving on.

Writing "adenine pairs with guanine": adenine pairs with thymine.
Forgetting to mention hydrogen bonds holding the base pairs together.
Drawing a single strand instead of a double helix.
Using "uracil" instead of "thymine": uracil is in RNA, not DNA.
Stating that the backbone is made of bases: it is sugar and phosphate.