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The History of DNA
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Mendel's Pea Plant Experiments
At this time Mendel finished a series of experiments with peas. His observations turned out to be closely connected to the finding of nuclein. Mendel was able to show that certain traits in the peas, such as their shape or color, were inherited in different packages. These packages are what we now call genes. -
Friedrich Miescher
Friedrich Miescher,isolated something no one had ever seen before from the nuclei of cells. He called the compound "nuclein." This is today called nucleic acid, the "NA" in DNA (deoxyribo-nucleic-acid) and RNA (ribo-nucleic-acid). -
Oswald Avery
American scientist Oswald Avery managed to transfer the ability to cause disease from one strain of bacteria to another. The previously harmless bacteria could also pass the trait along to the next generation. What Avery had moved was nucleic acid. This proved that genes were made up of nucleic acid. -
Advances in DNA Structure
In the late 1940's, the members of the scientific community were aware that DNA was most likely the molecule of life, even though many were skeptical since it was so "simple." They also knew that DNA included different amounts of the four bases adenine, thymine, guanine and cytosine (usually abbreviated A, T, G and C), but nobody had the slightest idea of what the molecule might look like. -
Erwin Chargoff
In 1949 Erwin Chargoff showed that even though different organisms have different amounts of DNA, the amount of adenine always equals the amount of thymine. The same goes for the pair guanine and cytosine. For example, human DNA contains about 30 percent each of adenine and thymine, and 20 percent each of guanine and cytosine. -
Further Advances
Watson and Crick used stick-and-ball models to test their ideas on the possible structure of DNA. Other scientists used experimental methods instead. Among them were Rosalind Franklin and Maurice Wilkins, who were using X-ray diffraction to understand the physical structure of the DNA molecule.
When you shine X-rays on any kind of crystal – and some biological molecules, such as DNA, can form crystals if treated in certain ways – the invisible rays bounce off the sample. The rays then create com -
Watson & Crick's Three-Helical Model
Watson and Crick had worked out a three-helical model for the structure of DNA, in 1951. But their theory was wrong. Their mistake was partly based on Watson having misremembered a talk by Rosalind Franklin where she reported that she had established the water content of DNA by using X-ray crystallographic methods. But Watson did not take notes, and remembered the numbers incorrectly. -
Photograph 51
Franklin's famous "photograph 51" that finally revealed the helical structure of DNA to Watson and Crick in 1953. This picture of DNA that had been crystallized under moist conditions shows a fuzzy X in the middle of the molecule, a pattern indicating a helical structure. -
Watson's Sugar Phosphate Backbone Discovery
Watson saw that the adenine-thymine bond was exactly as long as the cytosine-guanine bond. If the bases were paired in this way, each rung of the twisted ladder in the helix would be of equal length, and the sugar-phosphate backbone would be smooth. -
Linus Pauling
The scientist Linus Pauling was eager to solve the mystery of the shape of DNA. He became a Nobel Laureate in Chemistry for his ground-breaking work on chemical bonds and the structure of molecules and crystals. In early 1953 he had published a paper where he proposed a triple-helical structure for DNA.
