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Once pure DNA fibers could be isolated, biophysicists and biochemists examined the DNA for hints about its structure. The evidence eventually used to solve DNA’s structure included crucial data obtained using X-ray crystallography and a thorough characterization of the chemical composition of DNA.

PHYSICAL EVIDENCE FROM X-RAY DIFFRACTION Some chemical substances, when they are isolated and purified, can be made to form crystals. The positions of atoms in a crystallized substance can be inferred from the diffraction pattern of X rays passing through the substance. In the early 1950s the New Zealand–born biophysicist Maurice Wilkins discovered a way to make highly ordered fibers of DNA that were suitable for X-ray diffraction studies. His samples were analyzed by Rosalind Franklin of Kings College, London (Figure 13.5). Franklin’s data suggested that DNA was a double (two-stranded) helix with ten nucleotides in each full turn, and that each full turn was 3.4 nanometers (nm) in length. The molecule’s diameter of 2 nm suggested that the sugar–phosphate backbone of each DNA strand must be on the outside of the helix.

CHEMICAL EVIDENCE FROM BASE COMPOSITION Biochemists knew that DNA was a polymer of nucleotides. Each nucleotide consists of a molecule of the sugar deoxyribose, a phosphate group, and a nitrogen-containing base (see Figure 4.1). The only differences among the four nucleotides of DNA are their nitrogenous bases: the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C) and thymine (T).

In the early 1950s, biochemist Erwin Chargaff and his colleagues at Columbia University reported that DNA from many different species—and from different sources within a single organism—exhibits certain regularities. This led to the following rule: In any DNA sample, the amount of adenine equals the amount of thymine (A = T), and the amount of guanine equals the amount of cytosine (G = C). As a result, the total abundance of purines (A + G) equals the total abundance of pyrimidines (T + C):

Chargaff’s rule provided an important clue about the way the bases are arranged in a DNA double helix. While Chargaff and colleagues found that this rule held for every organism they examined, they noted that the relative abundances of A + T versus G + C vary among organisms. In human DNA, A and T each account for 30 percent of the nitrogenous bases present, while G and C each account for 20 percent. Put another way, in human DNA there is a ratio of 60 percent (A + T) to 40 percent (C + G).

WATSON AND CRICK’S MODEL If you have taken chemistry courses, you may be familiar with model building, where balls (atoms) and sticks (bonds) are used to put together molecules based on known physical and chemical properties and bond angles. A physicist, Francis Crick, and a geneticist, James D. Watson (Figure 13.6A), who were then at the Cavendish Laboratory of Cambridge University, used model building to solve the structure of DNA. They used the physical and chemical evidence we just described:

Crick and Watson built their tin model of DNA in late February 1953. This structure explained the known chemical properties of DNA, and it opened the door to understanding its biological functions.

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Question

Hydrogen bonds occur between the bases on opposite strands, within base pairs. Covalent bonds occur between the atoms that make up nucleotides and between the nucleotides in a DNA strand. van der Waals forces occur between the flat bases that stack on top of each other within the double helix, stabilizing them in the stacking.