Our modern understanding of DNA's role in heredity has led to a variety of practical applications, including forensic analysis, paternity testing, and genetic screening. Thanks to these wide-ranging uses, today many people have at least a basic awareness of DNA.
It may be surprising, then, to realize that less than a century ago, even the best-educated members of the scientific community did not know that DNA was the hereditary material!
In this article, we'll look at some of the classic experiments that led to the identification of DNA as the carrier of genetic information.
Protein vs. DNA
The work of Gregor Mendel showed that traits (such as flower colors in pea plants) were not inherited directly, but rather, were specified by genes passed on from parents to offspring. The work of additional scientists around the turn of the 20th century, including Theodor Boveri, Walter Sutton, and Thomas Hunt Morgan, established that Mendel's heritable factors were most likely carried on chromosomes.
Scientists first thought that proteins, which are found in chromosomes along with DNA, would turn out to be the sought-after genetic material. Proteins were known to have diverse amino acid sequences, while DNA was thought to be a boring, repetitive polymer, due in part to an incorrect (but popular) model of its structure and composition^1
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Today, we know that DNA is not actually repetitive and can carry large amounts of information, as discussed further in the article on discovery of DNA structure. But how did scientists first come to realize that "boring" DNA might actually be the genetic material?
Frederick Griffith: Bacterial transformation
In 1928, British bacteriologist Frederick Griffith conducted a series of experiments using Streptococcus pneumoniae bacteria and mice. Griffith wasn't trying to identify the genetic material, but rather, trying to develop a vaccine against pneumonia. In his experiments, Griffith used two related strains of bacteria, known as R and S.
R strain. When grown in a petri dish, the R bacteria formed colonies, or clumps of related bacteria, that had well-defined edges and a rough appearance (hence the abbreviation "R"). The R bacteria were nonvirulent, meaning that they did not cause sickness when injected into a mouse.
S strain. S bacteria formed colonies that were rounded and smooth (hence the abbreviation "S"). The smooth appearance was due to a polysaccharide, or sugar-based, coat produced by the bacteria. This coat protected the S bacteria from the mouse immune system, making them virulent (capable of causing disease). Mice injected with live S bacteria developed pneumonia and died.
As part of his experiments, Griffith tried injecting mice with heat-killed S bacteria (that is, S bacteria that had been heated to high temperatures, causing the cells to die). Unsurprisingly, the heat-killed S bacteria did not cause disease in mice.
The experiments took an unexpected turn, however, when harmless R bacteria were combined with harmless heat-killed S bacteria and injected into a mouse. Not only did the mouse develop pnenumonia and die, but when Griffith took a blood sample from the dead mouse, he found that it contained living S bacteria!
Griffith concluded that the R-strain bacteria must have taken up what he called a "transforming principle" from the heat-killed S bacteria, which allowed them to "transform" into smooth-coated bacteria and become virulent.

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