Question

It was Rosalind Franklin that helped Watson and Crick realize that the sugar-phosphate backbone of DNA would be facing outwardly and the nitrogen bases would be facing inwardly in the double helix. What properties of these molecules made her so sure of that?

Answer 1

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The DNA structure was not known, the basic building blocks of DNA had been known for many years. The basic elements of DNA had been isolated and determined by partly breaking up purified DNA. These studies showed that DNA is composed of only four basic molecules called nucleotides, which are identical except that each contains a different nitrogen base. Each nucleotide contains phosphate, sugar (of the deoxyribose type), and one of the four bases.

When the phosphate group is not present, the base and the deoxyribose form a nucleoside rather than a nucleotide.

The four bases are adenine, guanine, cytosine, and thymine. Two of the bases, adenine and guanine, are similar in structure and are called purines.

The other two bases, cytosine and thymine, also are similar and are called pyrimidines.

• Watson and Crick in 1953—worked from two kinds of clues. First, Rosalind Franklin and Maurice Wilkins had amassed X-ray diffraction data on DNA structure. In such experiments, X rays are fired at DNA fibers, and the scatter of the rays from the fiber is observed by catching them on photographic film, where the X rays produce spots. The angle of scatter represented by each spot on the film gives information about the position of an atom or certain groups of atoms in the DNA molecule. This procedure is not simple to carry out (or to explain), and the interpretation of the spot patterns is very difficult. The available data suggested that DNA is long and skinny and that it has two similar parts that are parallel to each other and run along the length of the molecule. The X-ray data showed the molecule to be helical (spiral-like). Other regularities were present in the spot patterns, but no one had yet thought of a three-dimensional structure that could account for just those spot patterns.

•The second set of clues available to Watson and Crick came from work done several years earlier by Erwin Chargaff. Studying a large selection of DNAs from different organisms (Table 8-1), Chargaff established certain empirical rules about the amounts of each component of DNA:

1.The total amount of pyrimidine nucleotides (T + C) always equals the total amount of purine nucleotides (A + G).

2.The amount of T always equals the amount of A, and the amount of C always equals the amount of G. But the amount of A + T is not necessarily equal to the amount of G + C.This ratio varies among different organisms.

•The structure that Watson and Crick derived from these clues is a double helix, which looks rather like two interlocked bedsprings. Each bedspring (helix) is a chain of nucleotides held together by phosphodiester bonds, in which a phosphate group forms a bridge between −OH groups on two adjacent sugar residues. The two “bedsprings” (helices) are held together by hydrogen bonds, in which two electronegative atoms “share” a proton, between the bases. Hydrogen bonds form between hydrogen atoms with a small positive charge and acceptor atoms with a small negative charge. For example,

The double helix accounted nicely for the X-ray data and tied in very nicely with Chargaff’s data. Studying models that they made of the structure, Watson and Crick realized that the observed radius of the double helix (known from the X-ray data) would be explained if a purine base always pairs (by hydrogen bonding) with a pyrimidine base .

Such pairing would account for the (A + G) = (T + C) regularity observed by Chargaff, but it would predict four possible pairings: T···A, T···G, C···A, and C···G. Chargaff’s data, however, indicate that T pairs only with A and C pairs only with G. Watson and Crick showed that only these two pairings have the necessary complementary “lock and key” shapes to permit efficient hydrogen bonding.