Answer:
The photo called photo 51 A was produced by the Rosalind Franklin who was working on the structure of DNA.
She produced the picture of DNA with the help of the X-ray diffraction technique and found that DNA structure is helical in appearance.
The photo was later used by the Watson and Crick who gave the structure of the DNA using the photo as reference. They showed the chemical bonds formed between the two strands of DNA.
They used the Chargaff rule of complementary base pairing that A will bind T and G will bind C which if not followed or lacks will produce an alignment which will not allow the formation of a helical structure. The lack of base pairing will allow the formation of just single-stranded structure and not double-helical structure.