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Mendel's ideas were so unorthodox that they were ignored for 35 years. But by the time the Mendelian concept was rediscovered at the turn of the century, scientists were better prepared for it. They already suspected that genetic information was hidden inside pairs of tiny, threadlike strands in cell nuclei called chromosomes, or colored bodies (for their ability to pick up dyes). During cell division they always split lengthwise, thereby giving each daughter cell a full share of what was presumed to be hereditary material.
A few years later, the suspicions were dramatically confirmed by the pioneering geneticist Thomas Hunt Morgan in Columbia University's famed "Fly Room." Through ingenious crossbreeding experiments with the fruit fly Drosophila melanogaster, Morgan and his students were able to map the relative positions of the genes along the insect's four pairs of chromosomes. Still, the gene's physical nature remained as great a mystery as ever. DNA had been discovered in the nuclei of cells by the Swiss biochemist Friedrich Miescher a few years after Mendel did his work on peas. But since the chromosomes in which the DNA was found also contained proteins—the basic building blocks of life—few scientists had any inkling that DNA might be playing an even more central role to life.
By the 1940s, however, the molecular biologists had come on the scene, and they insisted that fundamental life processes could be fully understood only on the molecular level. In their investigations, some used the electron microscope, which revealed details of structure invisible to ordinary optical instruments. Others specialized in X-ray crystallography, a technique for deducing a crystallized molecule's structure by taking X-ray photographs of it from different angles. Physicist Max Delbrück turned to nature for his investigative tools: bacteriophages (literally, "bacteria eaters"), tiny parasitic viruses that invade their host bacteria and rob them of their genetic heritage.
BUT THE HONORS for making the breakthrough discovery went to a traditional bacteriologist. Taking purified DNA extracted from the chromosomes of dead pneumonia bacteria, Rockefeller Institute's Oswald T. Avery and his associates showed that it could transform other, normally harmless bacteria into virulent ones. The experiment indicated that it was DNA, and not protein, that carried the genetic message. So unexpected was that finding that even Avery was at first unwilling to accept it. Eight years later, Alfred Hershey and his assistant Martha Chase demonstrated that a virus' DNA could, by taking over a bacterium, also nullify the cell's genetic instructions and replace them with its own. Only then was DNA finally accepted as the magic substance of the genes.