Rosalind Franklin discovered the molecular structure of DNA (specifically the DNA double helix).
In 1953, James Watson and Francis Crick started to build a molecular model of DNA, most of their data was derived directly from research done by Franklin. Her data was critical to the completion of Watson and Crick’s research. Franklin never built a model, she did not want to embark on that until the structure of DNA was known so as to eliminate any incorrect possibilities. Franklin died in 1958.
Franklin worked with Maurice Wilkins, who oversaw much of her research. Franklin changed universities, and once she left Watson and Crick received a letter from Wilkins advising them that Franklin was “finally leaving" and that they could put “all hands to the pump”.
For her discovery, three men, James Watson, Francis Crick and Maurice Wilkins were awarded the Nobel Prize in Physiology or Medicine. Franklin was never nominated and was not recognised for her contribution to the discovery of the structure of DNA until after she died. Nobel rules prohibit posthumous nominations or splitting of Prizes more than three ways.
Aaron Klug, Franklin’s colleague and beneficiary in her will, was the sole winner of the Nobel Prize in Chemistry in 1982. The work that he was awarded for was exactly what Franklin had started, and she introduced this to Klug. It is widely considered in the scientific community that, were she alive, she would have been awarded the Nobel Prize.