The History of the Discovery of DNA

Griffith – 1928 Studied Streptococcus pneumoniae, the bacterium that causes pneumonia. The outer coat of the bacterium is either smooth (S) or rough (R). Griffith injected both strains into mice and found that only the S bacteria were deadly; live R and heat-killed S were harmless individually. BUT – if the live R and heat-killed S were injected together, mice died and living S bacteria appeared. Griffith concluded that something from the dead S bacteria converted R bacteria into the S form. He called it the “transforming principle.”

Avery – 1943 Avery put heat-killed S bacteria into a test tube. When he added enzymes that destroy proteins, the S bacteria were still able to transform R bacteria. But when DNA destroying enzymes were added, the R bacteria couldn’t be transformed. Avery concluded that DNA was the “transforming” substance.

Chargaff – 1950 Chargaff studied the chemical composition of the DNA molecule. Chemical studies of DNA had revealed that it contained a type of sugar called deoxyribose, plus a phosphate group and four different molecules, or bases, called adenine, thymine, guanine, and cytosine. Chargaff realized that the amount of adenine in a DNA sample always equaled the amount of thymine, and the amount of cytosine always equaled the amount of guanine. This relationship, called Chargaff’s law, helped scientists who came after him decipher the three-dimensional structure of the molecule.

Hershey and Chase – 1952 Hershey and Chase worked with bacteriophages. They knew that the viruses were transferring hereditary information to infected bacteria, but they weren’t sure if the information was protein or DNA. So they infected bacteria with viruses that had either radioactively labeled proteins or radioactively labeled DNA. Infected bacteria became radioactive only from the DNA, proving that DNA is the molecule of heredity.

Franklin – 1952 Franklin was an expert at x-ray diffraction, a technique that uses focused beams of x-rays to produce photographs of the crystallized forms of molecules. As the x-rays pass through the molecule, they bounce off— or diffract—before striking photographic film. The patterns produced on the film by those bouncing x-rays can then be used to reconstruct the three-dimensional structure of the molecule. Franklin’s images of DNA ultimately revealed its exact structure.

Watson and Crick – 1953 Watson and Crick had been tinkering with large three-dimensional models of DNA, trying to determine the structure of the molecule. After viewing on of Franklin’s x-ray diffraction images, which they did without her knowledge, they figured out that DNA is made up of two mirror-image strands composed of stacked DNA nucleotides linked together into a twisting double helix. The double helix resembles a ladder, with the sugar and phosphate as the rails and the nitrogen bases held by hydrogen bonds as the rungs. For the discovery, they received the Nobel Prize in Chemistry. Franklin’s colleague, Wilkins, also received the award, but not Franklin as she had died.

Sanger – 1975 The two-time Nobel Prize winner developed a method of sorting DNA fragments so that the exact sequence of nitrogen bases can be determined.

Genentech – 1977-1979 The world’s first genetic engineering company was the first to genetically modify bacteria to produce human hormones, including insulin and human growth hormone.

Mullis – 1985 Mullis published the first paper describing polymerase chain reaction (PCR), a chemical reaction that can be used to make many copies of even tiny amounts of DNA quickly.

National Institutes of Health – 1990 The first federally approved gene replacement therapy was conducted on a 4- year-old girl with a rare form of immune deficiency disease.

Calgene, Inc. – 1993 The California biotech company produced the first commercially grown genetically engineered food—a tomato—that was granted a license for human consumption from the US FDA.

Roslin Institute – 1996 Dolly the sheep, the first mammal to ever be cloned from the cells of an adult animal, was born at the Roslin Institute in Scotland.

International Human Genome Sequencing Consortium – 2003 Scientists with the Human Genome Project, begun in 1990, announced the sequencing of the human genome to an accuracy of 99.99%.