In the early 1950s, it was a race to find the structure of DNA.  Many scientists around the world were testing the chemical structure of DNA & trying physical models that would fit the chemical properties.  Many DNA models were attempted, but found to be unstable & ended up breaking apart.
During that time, two scientist Watson (in the U.S.) & Crick (in England) began putting their pieces together & therefore getting closer & closer to solving the puzzle of the DNA structure.  They did not put ALL the pieces of the puzzle together until they saw the following picture (shared by a fellow male scientist).

This famous picture, named "photo 51" was taken by a female scientist named Rosalind Franklin.  Franklin was perfecting her technique in X-ray diffraction with several types of chemicals.  At the time, there were not many female scientists and therefore were not respected in their fields by the male counterparts.  Franklin was working in a lab with Wilkins who decided to take her "photo 51" & happen to share it with Watson & Crick at a conference unbeknownst by Franklin.
At this point, the rest is history.  DNA was found to be double stranded & known as the double-helix which looks like a "twisted-ladder".