How did Griffith, Hershey-Chase, Rosalind Franklin, Watson and Crick contribute to our understanding of the structure and function of DNA?

Several scientists contributed to our understanding of the structure and function of DNA. 

Frederick Griffith (1928): Griffith experimented with a bacteria called Streptococcus pneumoniae. Griffith used two strains of this bacteria in his experiments.

• The first strain was called the "S strain" because the presence of a polysaccharide capsule in these bacteria produced colonies with a smooth appearance. The S strain bacteria were lethyl to mice.

• The second strain was called the "R strain." This strain did not have a polysaccharide capsule and produced colonies with a rough appearance. The R strain bacteria were not lethyl to mice.

Griffith conducted the following experiments:

• First, Griffith killed some of the lethyl S strain bacteria by boiling them. He then injected the dead cells into mice. The mice survived. This proved that the outer shell of the bacteria was not the lethyl factor that was responsible for the death of the mice. 

• Second, Griffith injected dead S strain bacteria with live nonvirulent R cell bacteria into the mice. These mice died. Live cells were recovered from the dead mice and cultured. S strain bacteria grew in the cultures. When these S strain bacteria were injected into mice, the mice died. This showed that something in the dead S cells was able to transform the R cells into S cells.

Oswald Avery, C. M. MacLeod, and M. McCarty (1944): These scientists continued experiments with the S and R strains of the bacteria Streptococcus pneumoniae. They separated the molecules found in the dead S strain bacteria and injected them individually into mice together with live R strain bacteria. The only molecule that was able to transform the R strain bacteria into S strain bacteria was DNA. They concluded that the DNA molecule was able to transfer S strain genes into the R strain bacteria.

Erwin Chargaff (1950): After studying the DNA of several organisms, Chargaff was able to determine that the total amount of thymine and cytosine in DNA always equals the total amount of adenine and guanine.

Alfred Hershey and Martha Chase (1952): Hershey and Chase experimented with a phage virus called T2. The virus T2 is able to infect E. coli bacteria. The phage virus is almost entirely composed of protein and DNA. The protein contains S atoms, but not P atoms. The DNA contains P atoms, but not S atoms. Hershey and Chase radioactively labeled the S and P atoms in the virus. When the labeled virus infected the bacteria, most of the labeled P atoms from the DNA were located inside the bacterial cells, while most of the labeled S atoms from the protein remained outside of the bacteria. This proved that DNA is the hereditary material.

Rosalind Franklin and Maurice Wilkins (1953): Franklin and Wilkins fired X rays at DNA fibers. When the X rays hit the DNA fibers, they scattered. The scattered rays were detected by photographic film. This data indicated that the DNA molecule was helical. 

James Watson and Francis Crick (1953): Using the discoveries of Chargoff, Franklin, and Wilkins, Watson and Crick were able to create a three dimensional model of the DNA molecule. This model described DNA as a double helix. The backbones of the double helix strands were held together by phosphodiester bonds. The strands themselves were held together by hydrogen bonds between pairs of adenine and thymine bases and pairs of cytosine and guanine bases.