Only the pairing between a purine and pyrimidine and the antiparallel orientation of the two DNA strands can explain the uniform diameter. The diameter of the DNA double-helix is 2 nm, and it is uniform throughout. Therefore, 10 base pairs are present per turn of the helix. Each base pair is separated from the next base pair by a distance of 0.34 nm, and each turn of the helix measures 3.4 nm. The sugar and phosphate of the nucleotides form the backbone of the structure, whereas the nitrogenous bases are stacked inside, like the rungs of a ladder. The base pairs are stabilized by hydrogen bonds: adenine and thymine form two hydrogen bonds and cytosine and guanine form three hydrogen bonds. The two strands are anti-parallel in nature that is, the 3′ end of one strand faces the 5′ end of the other strand. Thus, adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. Base pairing takes place between a purine and pyrimidine on opposite strands, so that A pairs with T, and G pairs with C (suggested by Chargaff’s Rules). 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 previously 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. These are called the 5′ and 3′ ends of the chain. In a polynucleotide, one end of the chain has a free 5′ phosphate, and the other end has a free 3′-OH. The phosphate residue attached to the 5′ carbon of the sugar of one nucleotide forms a second ester linkage with 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 to produce phosphodiester bonds. The base is attached to the 1’carbon of the sugar. In RNA nucleotides, the 2′ carbon of the sugar ribose also contains a hydroxyl group. In DNA nucleotides, the 3′ carbon of the sugar deoxyribose is attached to a hydroxyl (OH) group. The phosphate, which makes DNA and RNA acidic, is connected to the 5′ carbon of the sugar by the formation of an ester linkage between phosphoric acid and the 5′-OH group (an ester is an acid + an alcohol). The carbon atoms of the five-carbon sugar are numbered 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”). The sugar is deoxyribose in DNA and ribose in RNA. Pyrimidines are smaller in size they have a single six-membered ring structure. The purines have a double ring structure with a six-membered ring fused to a five-membered ring. Each nucleotide is made up of a sugar, a phosphate group, and a nitrogenous base.
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