Assembling the DNA double helix
Rotate an idealized DNA ladder: identify the sugar–phosphate backbone, the A–T and G–C rungs, and see why the two strands must run in opposite directions.
Goal
Locate the two backbone spirals and the base-pair rungs; count the rise and twist per base pair (≈0.34 nm, ≈36°) in the model.
Apparatus and reagents
Virtual model kit: a ten–base-pair helix built from backbone spheres and lettered bases; nothing else needed.
Procedure
- Rotate until you look straight down the helix axis: the ten rungs stack like a spiral staircase.
- Find a rung labelled A–T and one labelled G–C: note that every rung pairs a purine (blue, two rings) with a pyrimidine (red, one ring) so the width stays constant.
- Follow one orange strand from bottom to top, then the other: they spiral in opposite directions (antiparallel).
- Count the base pairs in one full 360° turn of the model — about ten, matching real B-DNA.
What to observe
- The backbone (orange) is on the outside and the base pairs are buried inside, protected from water.
- All rungs have the same length: a purine–pyrimidine pair spans the helix evenly, which is why A always pairs with T and G with C.
- Near one rung, the two strands point in opposite directions — the signature of antiparallel strands.
Explanation
DNA stores information because the backbones are generic (sugar–phosphate repeats) while the rungs are specific: A pairs with T through two hydrogen bonds and G with C through three, so each sequence on one strand dictates its complement. The regular 2 nm diameter comes from always pairing a two-ring purine with a one-ring pyrimidine — a mispair like A–C would bulge and be caught by repair enzymes.
History of the experiment
Chemists behind it
Related topics
Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.