The Discovery of DNA as hereditary material
1868 - Friedrich Miescher tried to isolate the nucleus (plural is nuclei) from the cell. He subjected the purified nuclei to an alkaline extraction followed by acidification, resulting in the formation of a precipitate that Miescher called nuclein (now known as DNA). He found that this contained phosphorus and nitrogen, but not sulfur. Miescher and his students researched much nucleic acid chemistry, but its function remained unknown.
1928 - Frederick Griffith was searching for a vaccine for pneumonia (a vaccine against Pneumonococci bacteria). Pneumococci has two general forms—rough (R) and smooth (S). The S form is virulent and has a capsule, which is a slippery polysaccharide coat that improves bacterial evasion of efficient phagocytosis by the host's innate immune cells.
Griffith concluded that a "principle" from the virulent bacteria had entered the non-virulent bacteria and had changed its characteristics. He referred to this process as transformation, although he didn't know what was the actual chemical compound that was involved.
1944 - Oswald Avery continued the research done by Griffith. The results published in 1944 were a culmination of work started in the 1930's. Avery and his team purified the different chemical component found in the S cells (carbohydrates, proteins, lipids, DNA and RNA) and then they mixed each purified component with R cells to see which one resulted in a transformation. Their work showed that DNA was the transformational material.
The following animation provides a good, thorough explanation, though it is a bit slow going:
1952 - The Hershey–Chase experiments were a series of experiments conducted in 1952 by Alfred Hershey and Martha Chase that helped to confirm that DNA is the genetic material. Hershey and Chase needed to be able to examine different parts of the phages they were studying separately, so they needed to isolate the phage subsections. Viruses were known to be composed of a protein shell and DNA, so they chose to uniquely label each with a different elemental isotope. This allowed each to be observed and analyzed separately. Since phosphorus is contained in DNA but not amino acids, radioactive phosphorus-32 was used to label the DNA contained in the T2 phage. Radioactive sulfur-35 was used to label the protein sections of the T2 phage, because sulfur is contained in amino acids but not DNA.
1953 - James Watson and Francis Crick - They determined the double helix structure of DNA and also came up with a hypothesis of how DNA replication occurs. They also suggested how protein synthesis occurs (mRNA transcription translation). Their work was made possible by the accomplishments of others, such as the realization by Erwin Chargaff that the number of purines was always equal to the number of pyrimidines (and specifically that A=T and G=C), and x-ray crystallography work of Rosalind Franklin and Maurice Wilkins.
The following sites provide more information if something is still not clear or you want to know more:
http://www.nobelprize.org/educational/medicine/dna_double_helix/readmore.html
http://www.dnaftb.org/19/animation.html
For more information (if you are really interested, but otherwise don't worry about it!) about who discovered what and when and all that, check out the following article: http://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397
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