What does the instruction manual for life look like? On April 25, 1953, scientists James Watson and Francis Crick published a paper that answered this question. They described the shape of DNA as a double helix, which looks like a twisted ladder. DNA is the molecule inside every living cell that carries the instructions for how an organism grows and works. The two scientists did not work alone. A researcher named Rosalind Franklin used X-ray photography to capture images of DNA. Her most famous image, Photo 51, showed the telltale X-shape pattern of a helix. Watson and Crick used Franklin's work to build a physical model of DNA out of metal and wire. Their model showed that DNA has two strands that twist around each other. The rungs of the ladder are made of four chemical building blocks called bases. These bases pair up in a specific way: A always pairs with T, and C always pairs with G. This pairing rule is what allows DNA to copy itself precisely when cells divide. The discovery earned Watson and Crick the Nobel Prize in 1962.