DNA is a long strand of smaller molecules called bases.
The four bases are called adenine (A), thymine (T), cytosine (C), and Guanine (G). A only bonds to the T and the C only bonds to the G. Just these four molecules form the code that provides the instructions for all living things.
The bases are held in place by a backbone of sugar and phosphate molecules (the sides of the ladder). These molecules form strong bonds that keep the DNA bases in the correct order.
How fast are you at copying DNA? Try DNA - The Double Helix game (Nobel Prize)
This animation shows how the two sides of the DNA ladder fit together. (Howard Hughes Medical Inst)
