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In 1928, British scientist Frederick Griffith was studying pnemonia. He isolated two slightly different types of pnemonia bacteria from mice. The disease-causing bacteria would kill the mice, but when heated, it would not. If he mixed the disease-causing bacteria with the harmless, the mouse would die. Griffith discovered transformation through this. The disease-causing bacteria had transferrred its ability to the harmless bacteria. His hyposthesized the transforming factor might be a gene.
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In 1944, a Canadian biologist named Oswald Avery repeated Griffith's work. He made an extract from the heat-killed bacteria and destroyed the lipids, proteins, carbohydrates, and other molecules in it, but transformation still occured. When he destroyed the DNA, transormation didn't occur. Therefore, DNA was the transforming factor. He discovered that the nucleic acid DNA stores and transmits the genetic info from one generation of an organism to the next, and genes are composed of DNA.
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An American biochemist, Chargoff discovered that the percentages of guanine and cytosine bases were almost equal in any sampe of DNA. The same went for thymine and adenine. He made a rule that [A]=[T] and [G]=[C]. Even though samples of bacteria and human DNA obeyed this rule, he still wasn't sure why.
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In 1951, Linus Pauling and Robert Corey discovered that the structure of a class of proteins is a helix.
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In 1952, a British scientist named Rosalind Franklin used X-ray diffraction to get info about the structure of the DNA molecule. She stretched DNA fibers in a thin glass tube so that the strands were parallel. She then aimed a beam at the concentrated DNA and took an X-ray photo of the pattern. Her photo indicated that the structure of DNA is helical. Her photo also suggested that the nitrogenous bases are near the center of the molecule.
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In 1952, two American scientists named Hershey and Chase studied viruses. They wanted to know if the protein coat of the DNA of a virus entered the cell because then they would learn whether genes were made of protein or DNAThey grew viruses in cultures, one containing bacteriophage with phosphorus-32 in DNA and the other bacteriophage with sulfur-35 in a protein coat. After their results, they found that genetic material of the bacteriophage was DNA, not protein.
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In 1953, American biologist, James Watson, and British physicist, Francis Crick, were attempting to build a 3D model of DNA out of cardboard and wire. After seeing Franklin's X-ray pattern, they used its clues to build a model that explained how DNA could carry info and how it could be copied. Their model was a double helix. Later, they also found that hydrogen bonds held the two strands together of nitrogenous bases. The concluded base pairing explained Chargoff's rules.
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In 1960, Sydney Brenner, a South African biologist discovered the existance of messenger RNA.
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In 1977, Walter Gilbert and his collegues developed methods to read the DNA sequence.
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In 2000, the Human Genome Project completed their attempts to sequence all human DNA.
