 We explain how scientists figured out DNA's structure to unlock its mysteries. Until the early 1950s, the structure of DNA remained a mystery. At Cambridge University, Francis Crick and James Watson worked on making physical models of what DNA may look like. Meanwhile at King's College in London, Maurice Wilkins and Rosalind Franklin were also studying DNA by examining X-rated fraction images of DNA. In short, this means that when shining X-rays through DNA molecules the X-rays made a shadow of the molecule's structure. Over time, different researchers made important but seemingly unconnected findings about the composition of DNA. For example, Alexander Todd discovered that the backbone of the DNA molecule contained repeating phosphate and deoxyribose sugar groups. Linus Pauling discovered the single-stranded alpha helix, prompting biologists to think of helical forms. He also pioneered the method of model building in chemistry. Maurice Wilkins and Rosalind Franklin had obtained high-resolution X-ray images of DNA fibers. It suggested a helical, corkscrew-like shape. Franklin suspected that all DNA was helical. Phoebus Levine and others discovered that DNA was composed of subunits, called nucleotides. A nucleotide is made up of a sugar, a phosphate group, and one of four nitrogenous bases. These are adenine, thymine, guanine and cytosine. Irwin Chargaff had found that adenine and thymine always appeared in ratios of 1 to 1, as did guanine and cytosine. In the end, it was Watson and Crick who eventually unified all these findings to reveal DNA's structure. They used Pauling's method of model building in chemistry to uncover the structure of DNA. Franklin's double helix idea and Chargaff's findings about base pairs were incorporated into Watson and Crick's model. This meant that matching base pairs interlocked in the middle of the double helix to keep the distance between the latter legs, or backbones, constant. Finally, Watson and Crick realized that A always paired with T, and likewise C with G. The base is connected to the two helical sugar phosphate backbones of DNA at right angles, so the legs retained their regular double helix shape. Similarly, the complementary pairing of the bases meant that the backbones ran in opposite direction to each other, one up, the other down. And so, Watson and Crick showed that each strand of the DNA molecule was a template for the other. During cell division the two strands separate and on each strand a new other half is built. This way DNA can reproduce itself without changing its structure. This discovery opened many doors in biological research.