In the 1940s scientists discovered that DNA is made up of molecules called nucleotides. Each nucleotide consists of three different materials: a base, a sugar, and a phosphate. The key to understanding how DNA worked seemed to lie among the base materials. A chemist named Rosalind Franklin then discovered that DNA strands seemed to be in a spiral shape, but again, no one knew why. Watson and Crick described the shape they had found as a double helix. A double helix looks like a twisted ladder that spirals around and around. Each rung of the ladder is made of one of the four nucleotide bases. The secret of DNA, or what Crick called "the secret of life" is that every A molecule on one side of the rung always connects to a T on the other side, while every G molecule connects to a C. The key to understanding how DNA works is