DNA Structure — The Double Helix

DNA is the universal hereditary molecule of life. Its structure, solved by James Watson, Francis Crick, Rosalind Franklin and Maurice Wilkins in 1953, is one of the greatest discoveries in biology. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (c) — the structure of DNA — including the antiparallel nature of the two strands, complementary base pairing, hydrogen bonding and the double helix itself.

A detailed understanding of DNA structure is essential not only for this topic but also for genetics, mutations, gene expression and genetic technologies in Year 2.

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1. Historical Context (Non-Specification, Useful Background)

• In 1953, Watson and Crick proposed the double-helical model of DNA based in large part on X-ray diffraction images taken by Rosalind Franklin and Maurice Wilkins at King's College London.
• Erwin Chargaff had already shown that in any DNA sample, the amount of A equals the amount of T, and the amount of C equals the amount of G. This became known as Chargaff's rules.
• The model elegantly explained how DNA could store information and how it could be copied accurately — the two central functions of a hereditary molecule.

Exam Tip: You do not need the historical detail for marks, but mentioning Chargaff's rules (A=T, C=G) in an answer about base pairing can show depth of understanding.

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2. Overall Architecture

DNA is a double-stranded polynucleotide arranged as a right-handed double helix. Each strand is a polymer of nucleotides joined by phosphodiester bonds (see Lesson 1). The two strands wind around a common axis like a twisted ladder.

The main structural features are:

• Two polynucleotide strands running antiparallel to one another
• Sugar–phosphate backbone on the outside (polar, hydrophilic)
• Nitrogenous bases on the inside, paired by hydrogen bonding
• Complementary base pairing: A pairs with T, C pairs with G
• A right-handed double helix with approximately 10 base pairs per turn
• The helix contains a major groove and a minor groove (protein binding sites)

5' —P—S—P—S—P—S—P—S— 3'
        |   |   |   |
        A   T   G   C
        ||  ||  |||  |||
        T   A   C   G
        |   |   |   |
3' —S—P—S—P—S—P—S—P— 5'

Key: S = deoxyribose sugar, P = phosphate, || = 2 hydrogen bonds, ||| = 3 hydrogen bonds.

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3. The Antiparallel Strands

The two strands of DNA run in opposite directions: one runs 5' → 3' while the other runs 3' → 5'. This is what is meant by "antiparallel".

Strand 1:   5' — A — T — G — C — 3'
                 |   |   |   |
Strand 2:   3' — T — A — C — G — 5'

Why antiparallel?

• It allows the bases on each strand to align so they can form stable hydrogen bonds.
• It enables the enzymes of replication and transcription to recognise and move along the strands in the correct direction.
• DNA polymerase can only add nucleotides to the 3' end of a growing strand — so one strand is synthesised continuously and the other discontinuously during replication (see Lesson 4).

Exam Tip: When drawing DNA, always label the 5' and 3' ends and make sure they are on opposite ends of the two strands.

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4. Complementary Base Pairing

The bases project inwards from the sugar–phosphate backbones and pair via hydrogen bonds according to strict rules:

Pair	Bonding	Hydrogen bonds
Adenine – Thymine (A–T)	Purine–pyrimidine	2 hydrogen bonds
Cytosine – Guanine (C–G)	Purine–pyrimidine	3 hydrogen bonds

Key consequences of this pairing:

• A purine always pairs with a pyrimidine, keeping the width of the helix constant at approximately 2 nm. Two purines would be too wide; two pyrimidines would be too narrow.
• C–G pairs are more stable than A–T pairs because three hydrogen bonds are stronger than two. DNA that is rich in G and C has a higher melting temperature.
• Complementary base pairing means that if you know the sequence of one strand, you automatically know the sequence of the other. This is the basis of accurate DNA replication.

Key Definition — Complementary base pairing: The specific pairing of adenine with thymine (two hydrogen bonds) and cytosine with guanine (three hydrogen bonds) by hydrogen bonding between the bases in double-stranded DNA.

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