DNA (deoxyribonucleic acid), the giant molecule that carries genetic information in living things, is made up of just a few chemical building blocks that bond together in very particular ways.
A typical molecule of DNA consists of two strands that are linked together. A segment can be visualized as a ladder-like structure:
(Actually DNA looks like a twisted ladder or a spiral staircase, a shape commonly called a helix or, since there are two strands in the structure, a double helix. Here we are focusing just on the structure of the ladder.)
In our flat unrealistic model picture we are visualizing the DNA as made of two strands, a left and a right, which are linked in the middle. In reality, since the ladder is twisted, there really is no "right" or "left," but we use that terminology here since it applies to the model graphs. The square shapes and diamonds you see in the middles of the "rungs" are the links, which are meant to represent the different chemical bonds between the two strands of DNA.
Each of the two strands of DNA consists of a sequence of nitrogenous bases, and any one of four such bases can appear on a strand. (The DNA also contains other molecular building blocks like sugars and phosphate groups, but the nitrogenous bases are the genetically important part.) The four bases and our geometric representation of each are as follows:
Base
Representation
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