The double helix.

Step 1 of 5

Two strands and a ladder

Every cell in your body holds a copy of the instructions for building the whole body, and they are written in DNA. A DNA molecule is two long strands twisted round each other into a double helix, like a rope ladder someone has twisted. The sides of the ladder are a backbone of sugar and phosphate, and the rungs are pairs of bases.

There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). The instructions are written in their order along a strand, like words in an alphabet of four letters. Each rung holds one base from each strand, and the pairing rule decides the pair.

Turn the Scene and look at the rungs. Set Highlight to A–T and then to G–C, and check what sits opposite what.

Step 2 of 5

Hydrogen bonds

The two strands are not tied together by strong bonds. Each base pair is held by hydrogen bonds, weak attractions between atoms, drawn in the Scene as short lines across the gap. An A–T pair is held by two bonds, and a G–C pair by three.

One hydrogen bond is very weak, but along a strand of millions of pairs they add up to a firm hold. And because each one is weak, the helix can be opened like a zip without breaking the strands themselves.

G–C pairs is fixed now, and the G–C pairs are highlighted. Count the bonds.

Step 3 of 5

Chargaff’s rule

In $1950$⁠, before anyone knew how DNA is built, Erwin Chargaff measured the amounts of the bases in the DNA of different living things. For every one of them he found the same thing: exactly as much A as T, and exactly as much G as C. The balance of A–T against G–C changes from one living thing to the next, but the equalities do not.

The double helix explains this at once: every A sits opposite a T, so they come in pairs. From that, all four bases can be worked out from one number.

Step 4 of 5

A sturdier helix

Change G–C pairs and watch the number of bond lines change. GC-rich DNA is held by more bonds, so it takes more heat to open it. Lab tests such as PCR heat DNA until the strands come apart, and GC-rich DNA needs a higher temperature for that.

Step 5 of 5

One molecule becomes two

Before a cell divides it must copy its DNA, so that each daughter cell gets a whole copy. The pairing rule is what makes this possible: each strand holds enough information to build the other. Watson and Crick wrote about it in $1953$⁠, in a sentence that became famous, saying it had “not escaped our notice” that the pairing rule suggests a way of copying.

Look inside is fixed at Double helix now. Once you choose an answer, the Scene will open the helix and copy it.