In 1868, Johann Friedrich Miescher graduated from medical school in Switzerland. Influenced by an uncle who believed that the key to understanding disease lay in the chemistry of tissues, Miescher traveled to Tübingen, Germany, to study under Ernst Felix Hoppe-Seyler, an early leader in the emerging field of biochemistry. Under Hoppe-Seyler’s direction, Miescher turned his attention to the chemistry of pus, a substance of clear medical importance. Pus contains white blood cells, which have large nuclei. Miescher isolated a novel substance from these nuclei that was slightly acidic and high in phosphorus.

By 1887, several researchers had independently concluded that the physical basis of heredity lies in the nucleus. The material that Miescher had isolated (chromatin) was shown to consist of nucleic acid and proteins, but which of these substances was actually the genetic information was not clear. In the late 1800s, Albrecht Kossel carried out further work on the chemistry of DNA and determined that it contains four nitrogenous bases: adenine, cytosine, guanine, and thymine (abbreviated A, C, G, and T).

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Phoebus Aaron Levene later showed that DNA consists of a large number of linked, repeating units called nucleotides; each nucleotide contains a sugar, a phosphate, and a base.

Levene incorrectly proposed that DNA consists of a series of four-nucleotide units, each containing all four bases—adenine, guanine, cytosine, and thymine—in a fixed sequence. This concept, known as the tetranucleotide hypothesis, implied that the structure of DNA was not variable enough to be the genetic material. The tetranucleotide hypothesis contributed to the idea that protein is the genetic material because the structure of protein, with its 20 different amino acids, could be highly variable.

As additional studies of the chemistry of DNA were completed in the 1940s and 1950s, this notion of DNA as a simple, invariant molecule began to change. Erwin Chargaff and his colleagues carefully measured the amounts of the four bases in DNA from a variety of organisms and found that DNA from different organisms varies greatly in base composition. This finding disproved the tetranucleotide hypothesis. They discovered that, within each species, there is some regularity in the ratios of the bases: the amount of adenine is always equal to the amount of thymine (A = T), and the amount of guanine is always equal to the amount of cytosine (G = C; Table 8.1). These findings became known as Chargaff’s rules.