Pioneers of DNA Fredrick Griffith Griffith found that one type of bacteria could give information to another type of bacteria and transform it. His findings led to the search for the identity of the transforming agent. Griffith worked with a form of bacteria, known as Streptococcus pneumoniae. One type (S strain) caused pneumonia and the other strain (R strain) did not. His findings are as follows: 1. S strain alone killed the mouse 2. R strain alone caused the mouse to survive. 3. When the S strain was heated and killed, the mouse lived 4. When the killed S strain was mixed with a living R strain, the mouse died. Oswald Avery Avery’s experiments in 1944, led to the identification of DNA as Griffith’s transforming agent. However, his conclusions were not widely accepted as many scientists still believed that proteins were the transforming agent. Erwin Chargaff In 1950, Erwin Chargaff analyzed the nitrogenous bases in the nucleotides of DNA. The bases are cytosine, guanine, adenine, and thymine. Chargaff found that the amount of adenine nearly equaled the amount of thymine and the amount of cytosine nearly equals the amount of guanine. What do Chargaff’s findings mean? James Watson and Francis Crick Watson and Crick are credited with proposing the structure of DNA. Their model included: • Two outside strands of deoxyribose and phosphate • Cytosine and guanine paired by 3 hydrogen bonds • Thymine and adenine paired by two hydrogen bonds. Watson and Crick’s DNA model In 1953, Watson and Crick published their findings regarding the structure of DNA Even though Watson and Crick are credited with discovering the structure of DNA, their finding were based on the research of CHARGAFF, FRANKLIN and WILKINS Rosalind Franklin and Maurice Wilkins Franklin and Wilkins worked on a team at King’s College in London. They used a technique called X-RAY DIFFRACTION to take pictures of the DNA molecule. These pictures were used by Watson and Crick to create the DNA structure Alfred Hershey and Martha Chase In 1952, Hershey and Chase proved that DNA is a transforming factor, and not proteins. They used bacteriophages (viruses) with radioactive tags on the proteins and DNA to examine what was passed down. .