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Episode 1 : The Secret of LifeEpisode 1 : The Secret of Life

Special Report
DNA: The Ugly Duckling of Genetics, by Magdalena Eriksson: Page 2 of 2
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Meischer made his discovery at a time when medicine was undergoing a revolution. Theories about the four humors -- a balance among four supposedly elemental fluids, including blood, yellow bile, phlegm, and black bile -- were under scrutiny. For thousands of years, the medical field had relied upon the practice of bloodletting, the intentional bleeding of patients, to restore a balance of the humors. Bloodletting was gradually losing popularity among physicians. Still, few scientists were prepared to subscribe to the hereditary importance of nuclein.

Thomas Hunt Morgan, a Kentucky-born biologist active at Columbia University in the early 20th century, picked up where others left off. He localized genes to specific locations on chromosomes. Morgan also concluded that gene s were the hereditary unit Mendel had described, and the key element in Darwinian evolution.

Morgan's specific mapping of genes onto chromosomes made biology accessible for experiments. Many new questions could be addressed. The basic structure and chemical properties of genes were still unknown, and their function was far from understood. How do genes function and duplicate? What is the basis of genetic disease and the role of mutations? But perhaps the most thrilling problems were: how do genes contain and forward hereditary information, and how do they direct the development of entire organisms?

Another clue came from Great Britain, where Fred Griffith at the Ministry of Health studied different strains of pneumococci, the bacteria that can cause pneumonia. One strain, the S-form, has a capsule. To Griffith, it looked smooth under the microscope, whereas the R-form doesn't have a capsule and therefore appeared rough. The S-form was virulent and killed infected mice, while the R-form had no serious effect. Griffith concluded that the immune system failed to break through the cell wall of the smooth bacterium, but destroyed the rough one and rendered it harmless.

Test tubes
Griffith concluded that the immune system failed to break through the cell wall of the smooth bacterium, but destroyed the rough one and rendered it harmless.

Griffith experimented with mixtures of the two forms, and injected mice with heat-killed S, and normal R bacteria. He expected the mixture to be harmless since the dangerous S-form bacteria had been killed. But the mice died, and Griffith found living S bacteria in their dead bodies. He concluded that something had been transferred between the two types of bacteria -- a genetic change had occurred in the R-form.

Like Miescher's work, Griffith's discovery drew little attention from his contemporaries. He didn't realize the full implications of his result and failed to convince his colleagues of its importance before he died in a London bomb raid in 1941. Fifteen years later, however, Griffith's pneumococcus experiment was repeated at Rockefeller Institute for Medical Research in New York City by Oswald Avery, who for years had focused his research on bacteria from pneumonia patients.

Avery and his colleagues, Colin MacLeod and Maclyn McCarty, went about their quest for the transforming principle in a systematic manner. They showed that proteases, enzymes that degrade proteins, had no effect on the heat-treated S pneumococci. Only when the bacteria were treated with the enzyme DNase, which degrades DNA, could the S-form no longer induce its virulence on the R-form. They had made it clear that DNA is the transforming principle.

Yet even this discovery was received with skepticism. To many, DNA seemed too simple for the task of holding such vast amounts of biological information. Proteins, with their 20 different amino acids, had for so long been favored as the most likely carrier of genetic information. And for years there was a looming suspicion that the DNA that Avery had claimed as the transforming principle had been contaminated by proteins that might have carried the genetic information. It wasn't until 1952, when another team independently showed that DNA, safely separated from proteins, brought genes into bacteria, that the last skeptics were persuaded. DNA won general acceptance as the carrier of genes and the race for the structure was on. The rocket into modern biology was launched.

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Francis Crick with DNA model
Francis Crick
Chromosomes
Chromosomes
X-ray diffractions of DNA
An example of Rosalind Franklin's X-ray diffraction work

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