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Miescher extracted substances from white cells (pus from gauze bandages) and found a substance in the cell nucleus rich in phosphorus, different from proteins; he called it “nuclein”.
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Levene identified that DNA/RNA is made up of units that he called nucleotides (sugar + phosphate group + nitrogenous base). He also distinguished ribose and deoxyribose.
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Griffith showed that non-virulent bacteria can become virulent if exposed to dead virulent bacteria, suggesting that there is a “transforming principle.” (This principle would be DNA)
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They demonstrated that Griffith's transforming principle is DNA, not protein or RNA. When they degraded DNA, the transformation stopped.
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Chargaff discovered that the ratio of bases in DNA is not constant across species, and that the amounts of adenine/thymine and guanine/cytosine are equal (A = T, G = C). These data would be key to the double helix model.
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A very detailed image showing patterns consistent with a helical structure, with base steps, etc. This image was used by Watson and Crick to build their model.
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They used bacteriophages labeled with radioactive isotopes to see what enters the bacteria during infection: DNA or protein. They showed that DNA was the genetic material.
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They proposed the structure of DNA as a double helix with base pairs that allow the reproduction of genetic information.