GeneticsStep-through15 minLesson 1 of 2

DNA Structure

Every trait you inherit is written in a molecule just 2 nanometers wide. Unwind the double helix, pair up its bases, and zoom into a single nucleotide to see how DNA's shape makes it both stable enough to store information and easy enough to copy.
2 nm3.4 nm · one full turn ≈ 10 base pairsRight-handed double helixTwo sugar-phosphate backbones wind around a shared axisDNA · deoxyribonucleic acid
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Double Helix
Nucleotide

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Stage 1 of 5

Double Helix

Overall shape · ~2 nm wide

DNA is two long strands wound around each other into a right-handed double helix. The shape is remarkably uniform: about 2 nm wide, completing one full turn every 3.4 nm — roughly 10 base pairs. In real DNA the two backbones sit slightly off-center from each other, giving the helix a wide major groove and a narrow minor groove (drawn evenly here for clarity).

  • Two strands twisted into a right-handed helix
  • Constant width of ~2 nm along the entire molecule
  • One full turn every 3.4 nm ≈ 10 base pairs (0.34 nm per pair)
  • Major and minor grooves let proteins read the bases without unzipping the helix
  • AP exam: Rosalind Franklin's X-ray diffraction image (Photo 51) revealed the helical shape
Predict before you continue

If you untwisted the helix into a ladder, what would its two rails be made of?

Build a strand

Edit the top strand

Tap a base to cycle A → T → G → C. The partner strand and the helix diagram update to follow base-pairing rules.

5′3′
3′5′

Top strand 5′ A T G C G T A C C T A G 3′. Partner strand 3′ T A C G C A T G G A T C 5′.

Base composition (both strands)
Adenine
25%
Thymine
25%
Guanine
25%
Cytosine
25%

Chargaff's rule holds no matter what you type: %A = %T and %G = %C.

GC content
50%
Hydrogen bonds
30
More = harder to separate

Three things to keep straight

  • Same width everywhere

    A two-ring purine always pairs with a one-ring pyrimidine, so every rung is the same length. That's why the helix is a uniform ~2 nm wide — and why A–G or C–T pairs don't fit.

  • Strong rails, weak rungs

    Covalent phosphodiester bonds hold each backbone together. Only hydrogen bonds hold the two strands to each other, so DNA can be unzipped for copying without breaking the sequence.

  • Direction matters

    The strands are antiparallel: one runs 5′ → 3′, the other 3′ → 5′. Enzymes that copy DNA can only add to a 3′ end — you'll need this for DNA replication.

The four bases

BaseTypePairs withH-bonds
Adenine (A)PurineThymine (T)2
Guanine (G)PurineCytosine (C)3
Thymine (T)PyrimidineAdenine (A)2
Cytosine (C)PyrimidineGuanine (G)3
How we know

Who figured out the double helix?

In 1950, Erwin Chargaff showed that DNA always contains equal amounts of A and T, and of G and C. In 1952, Rosalind Franklin's X-ray diffraction image — known as Photo 51, taken with her student Raymond Gosling — revealed a helix of constant width. Building on both, James Watson and Francis Crick published their double-helix model in 1953. Chargaff's ratios explained the pairing; Franklin's data revealed the shape.

Quick recap

  1. 1

    Double helix — Two strands twisted into a right-handed helix, ~2 nm wide, ~10 base pairs per turn.

  2. 2

    Nucleotide — Phosphate + deoxyribose sugar + nitrogenous base. Base on the 1′ carbon, phosphate on the 5′ carbon.

  3. 3

    Backbone — Alternating sugars and phosphates joined by covalent phosphodiester bonds (3′ –OH to 5′ phosphate).

  4. 4

    Base pairing — A=T (2 H-bonds), G≡C (3 H-bonds). A purine always pairs with a pyrimidine, so %A = %T and %G = %C.

  5. 5

    DNA vs. RNA — RNA uses ribose (with a 2′ –OH), has uracil (U) instead of thymine, and is usually single-stranded.