Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a right-handed helix. (credit a: modification of work by Marjorie McCarty, Public Library of Science) Scientist Rosalind Franklin discovered (b) the X-ray diffraction pattern of DNA, which helped to elucidate its double helix structure. The work of pioneering scientists (a) James Watson, Francis Crick, and Maclyn McCarty led to our present day understanding of DNA. Unfortunately, by then Franklin had died, and Nobel prizes are not awarded posthumously.įigure 2. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Watson and Crick were able to piece together the puzzle of the DNA molecule on the basis of Franklin’s data because Crick had also studied X-ray diffraction (Figure 2). In Wilkins’ lab, researcher Rosalind Franklin was using X-ray diffraction methods to understand the structure of DNA. Pauling had discovered the secondary structure of proteins using X-ray crystallography. Other scientists like Linus Pauling and Maurice Wilkins were also actively exploring this field. In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the University of Cambridge, England. The phosphate residue is attached to the hydroxyl group of the 5′ carbon of one sugar of one nucleotide and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, thereby forming a 5′-3′ phosphodiester bond. The nucleotides combine with each other by covalent bonds known as phosphodiester bonds or linkages. The carbon atoms of the five-carbon sugar are numbered 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”). Pyrimidines are smaller in size they have a single six-membered ring structure. Purines have a double ring structure with a six-membered ring fused to a five-membered ring. The sugar is deoxyribose in DNA and ribose in RNA. Each nucleotide is made up of a sugar, a phosphate group, and a nitrogenous base.