B-DNA · RIGHT-HANDED
Two antiparallel strands wound around a single axis, held together by base pairs like the rungs of a twisted ladder. In the B form, one full turn spans 10.5 pairs.
Shortcuts (click or hover over the model first): ← → rotate ↑ ↓ tilt + − zoom Space pause R reset drag · wheel or pinch: zoom
02 · Workshop
Type in the top strand, and the complementary strand appears automatically according to the base-pairing rule.
Hover over a column, or reach it with Tab and the arrow keys — the pair lights up in the 3D model.
03 · Measurements
Base counts refer to the top strand; in a duplex A = T and G = C (Chargaff's rule). Length: number of pairs × 0.34 nm. The melting temperature from the Wallace rule is an approximation for short oligonucleotides (about 14–20 nt) — for longer sequences it comes out too high.
04 · Key
purine · two rings
Pairs with thymine through 2 hydrogen bonds.
pyrimidine · one ring
Pairs with adenine through 2 hydrogen bonds. In RNA it is replaced by uracil.
purine · two rings
Pairs with cytosine through 3 hydrogen bonds.
pyrimidine · one ring
Pairs with guanine through 3 hydrogen bonds.
Base-pairing rule: a purine always pairs with a pyrimidine — A with T, G with C. That makes every pair almost the same length (C1′–C1′ distance about 1.05 nm), so the helix keeps a constant diameter of about 2 nm.
05 · Challenges
Each challenge completes on its own when you do it on the model or in the workshop.
Unzip the helix and wait until two daughter helices form. To do
Click all five items on the “What the model shows” list one by one (or their markers on the model). To do
Build a strand of at most 20 bases for which the Wallace rule gives exactly 50 °C. To do
At least 8 pairs, with the complementary strand read 5′→3′ identical to the top one — like the GAATTC cut site. The examples from the list don't count. To do
06 · Fun facts