The Watson–Crick model

Putting all this information together, James Watson and Francis Crick produced the first acceptable model:

• Two strands of DNA form a ladder shape. The sides of the ladder are the sugar–phosphate backbones; the rungs are pairs of bases.
• Each rung is A paired with T, or G paired with C (complementary base pairing). Either base of a pair may be on either strand.
• The base pairs are held together by hydrogen bonds: two hydrogen bonds between A and T; three between G and C.
• Because each rung is one double-ring purine paired with one single-ring pyrimidine, all rungs have the same width, so the two backbones stay at an equal distance along the whole molecule.
• For the hydrogen bonds to form properly, the two strands must run in opposite directions (they are antiparallel): the 5′ end of one strand is next to the 3′ end of the other.
• The whole ladder is twisted into a spiral with about ten base pairs per turn; because the spiral has two strands wound round each other, DNA is a double helix.