By the middle of the 20th century, most scientists assumed that genes, the agents for the transmission of genetic information, had to be a protein. Proteins are large molecules made up from 20 different smaller molecules called amino acids. The particular sequence of a long chain of amino acids making up a protein, the DNA structure, could be the genetic code

There was one problem with this idea. Chromosomes, the small threadlike structures in a cell's nucleus that appeared to be the physical units of heredity, are not composed of proteins. Investigation into their chemical composition found that chromosomes instead consist of a large molecule called DNA (deoxyribonucleic acid). DNA itself consists of only four chemical bases, adenine (A), cytosine (C), guanine (G), and thymine (T). Assuming DNA was the chemical that contained the genetic code, how could these four different bases contain the complex genetic information necessary to produce a human being? The answer was in the DNA structure of the molecule.

In 1953, Francis Crick and James Watson, supported by Rosalind Franklin and Maurice Wilkins, discovered the structure of DNA. X-ray images of the molecule from Franklin and Williams suggested that DNA was a helix. Working with models, the group finally came up with the DNA structure, a ladder with the four bases making up its rungs.

The ladder was coiled like a spiral staircase so that a very long molecule could be composed into the tiny microscopic chromosomes. This is the now famous double helix structure of DNA. It is the order of the bases, the rungs of the ladder, that is the genetic code.

Mathematically, a sequence of at least three bases is required to specify one of the 20 amino acids. The first three-base code, for the amino acid phenylalanine, was discovered in 1961; by 1966 the entire genetic code had been discovered. Our genetic code consists of an alphabet of four letters: A, G, C, and T. These four letters are combined into three-letter "words." That is the dictionary of all living things.

So what are these bases A, C, G, or T, the four letters that form our genes? The answer lies in the DNA model itself. Each "rung" of the ladder is made up of two bases. If we split the ladder down the middle to form two half ladders, a small part of the outer rail of the ladder and one of the two bases together to form a nucleotide. Reassembling the ladder, we have two nucleotides that together form the rung and complete the ladder a base pair.

The four bases in human DNA form tow types of rungs: the T-A rung and the G-C rung. That is, when a T appears on one side of a single rung, an A will be on the other; when one side of a rung has a G, the other side will have a C. This is the principle of complementary base pairs. This is an important principle because it allows the DNA molecule to replicate and helps ensure the integrity of the base sequences.