Type a sequence (A, C, G, T; 6 to 60 bases), watch the helix build itself, then heat it. Educational model only: not medical or diagnostic advice.
The science
Geometry (B-DNA). Base pairs are stacked 0.34 nm apart and each is twisted 34.3° from the last, so one turn takes 360 / 34.3 = 10.5 base pairs and 3.57 nm (the pitch). The backbone radius is 1.0 nm (diameter about 2 nm). The two strands are offset by 140° around the axis, which makes a narrow minor groove and a wide major groove; the exact offset is a drawing choice here.
Nearest-neighbour thermodynamics. The stability of a duplex is the sum of its 10 distinct stacked pairs (ΔH, ΔS from the unified SantaLucia 1998 table, 1 M NaCl), plus initiation terms and a symmetry term for self-complementary sequences. Sodium correction: ΔS = ΔS(1 M) + 0.368 (N − 1) ln[Na+]. Melting temperature: Tm = ΔH / (ΔS + R ln(CT / x)) − 273.15 °C with x = 4 for two different strands and 1 for self-complementary ones, R = 1.987 cal/(mol K). Table values were typed from memory of the published table: spot-check them before quoting.
Melting curve. The duplex fraction follows from the two-state mass-action equilibrium K = exp(−ΔG / RT). The page checks the closed-form Tm against a bisection search of this curve at α = 0.5.
Honest simplifications. Two-state model (no hairpins, mismatches, dangling ends or partial melting in the curve); only the 3D opening along the helix uses a local, windowed stability to decide which pairs open first (AT-rich and end pairs). The sugar-phosphate beads, base rods, chromosome, mitochondria, nucleus and nucleosome are Blender models, drawn with Three.js; decorations are not to scale (a chromosome is about 1.4 µm long, a helix 2 nm wide).