Answer:
<em>Chargaff: </em>The work of Erwin Chargaff has major contributions towards understanding of DNA structure as a double helix. Although, he didn't propose the DNA as a double helix, but he was the first scientist two establish two rules that lead to the discovery of DNA as a double helix. Briefly, he proposed that (1) in any DNA, the amount of Guanine are always equal to the amount of Cytosine and likewise, the amount of Adenine are always equal to the amount of Thymine; and (2) the amount of guanine, cytosine, adenine and thymine bases are different in each species. This observation supported the idea later that DNA is the heriditary material rather than proteins.
<em>Franklin</em><em>:</em> Rosalind E. Franklin is known for her work on understanding the molecular structure of DNA via X-Ray diffraction. For that, she exposed crystallized DNA moleculeto X-rays and some of the rays were deflected back thus forming a diffraction pattern. She was the first scientist that produces real images of DNA explaining its molecular structure. As an example, I have attached the Photo 51 she took while working at King's College London. Her work led to the actual discovery of DNA as a double helix (Watson and Crick Model) which we know today.
<em>Watson and Crick</em>: James Watson and Francis Crick proposed the 3D structure and model of the DNA helix which we know as of today. Precisely, their work explained that DNA is a double-stranded, antiparallel, right-handed helix. Further, the nitrogenous basis proposed earlier (A, T, G, C) are attached to sugar-phosphate backbone via hydrogen bonds. It is one of the major discoveries in molecular biology.
