Rosalind Franklin didn't discover that DNA is two chains of molecules. That discovery was made by James Watson and Francis Crick in 1953.

However, Rosalind Franklin's work was crucial in helping them reach that conclusion.

Here's what Franklin did:

* X-ray Diffraction: Franklin used a technique called X-ray diffraction to capture images of DNA. These images, especially the famous "Photo 51," revealed the helical structure of DNA.

* Analyzing the Data: She analyzed the diffraction patterns to determine the dimensions and symmetry of the DNA molecule. This data showed that DNA had a double helix structure with a sugar-phosphate backbone on the outside.

While Franklin's data was instrumental, Watson and Crick are credited with building the first accurate molecular model of DNA and proposing that it consists of two complementary chains. Franklin's contributions were not fully acknowledged during her lifetime.