Watson–Crick Model of the DNA Double Helix

Introduction

James Watson and Francis Crick proposed the double-helix model of DNA in 1953, using important experimental evidence from researchers including Rosalind Franklin, Maurice Wilkins, and Erwin Chargaff. According to the model, DNA is made of two long polynucleotide strands twisted around the same axis to form a right-handed double helix.

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1. Labelled Schematic Diagram

How to read the diagram: The sugar–phosphate chains form the two outer backbones. The paired nitrogenous bases form the inner “steps” of the ladder. One strand runs 5′ → 3′, while the other runs 3′ → 5′.

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2. Flowchart of the Model

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3. Infographic: Important Features

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4. Mind Map

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5. Complementary Base-Pairing Diagram

According to the base-pairing rule:

• A = T • G = C • A purine always pairs with a pyrimidine, maintaining the uniform diameter of the DNA molecule.

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6. Dimensions of DNA

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7. Exam-Ready Explanation

The Watson–Crick model states that DNA consists of two polynucleotide strands coiled around a common axis to form a right-handed double helix. Each strand has an outer backbone made of alternating deoxyribose sugar and phosphate groups, while the nitrogenous bases project towards the centre.

The two strands are antiparallel: one runs in the 5′ → 3′ direction, whereas the other runs in the 3′ → 5′ direction. The strands are joined by complementary base pairing. Adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. Therefore, a purine always pairs with a pyrimidine, maintaining a uniform DNA diameter of 2 nm.

Adjacent base pairs are separated by 0.34 nm. One complete turn of the helix measures 3.4 nm and contains approximately 10 base pairs. Complementary base pairing explains how DNA can store genetic information and produce accurate copies of itself during replication.

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Conclusion

The Watson–Crick model describes DNA as a twisted molecular ladder. The sugar–phosphate backbones form its sides, complementary base pairs form its steps, and the antiparallel strands twist into a stable double helix. This structure explains both the stability of DNA and its ability to replicate accurately.