T.H. Morgan and his group of scientists discovered genes are the chromosomes as well as determined proteins and DNA as candidates for genetic material.
Frederick Griffith discovered the genetic role of DNA in 1928. He made the famous experiment of using two strains of one bacteria known as Streptococcus pneumoniae. The two strains consist of a harmless R strain and a pathogenic S Strain. A mouse would survive from a harmless R strain as well as a dead S strain. However, when the two harmless elements were fused together and injected into the mouse, the mouse died from a pathogenic S strain. The newly discovered the phenomenon, transformation, was responsible. This is the result of a change in genotype and phenotype due to the assimilation of a foreign substance (now known as DNA) by a cell.
In 1947, after studying the DNA composition in organisms, Erwin Chargaff developed a series of rules. He found out that the DNA composition varies from species to species. Furthermore, although the 4 bases (A,T,G,C) are found in characteristic in any one species, the bases do not appear in equal amounts nor ratios. Chargaff's Rules was developed to reflect his findings on the ratio of nucleotide bases. Adenine equals approximately to Thymine, while Guanine equals approximately to cytosine.
James D. Watson and Francis Crick were a student-teacher group working to solve the 3-D DNA structure. By the 1950s, numerous groups of scientists were in the race to discover the true structure of DNA. After receiving the findings of Wilkins and Franklin, Watson managed to learn that the DNA was of helical shape. Watson deduced the width of helix and the spacing of bases to produce the first successful 3-D DNA structure.
Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study the structure of DNA. Unfortunately, Franklin died at an early age to to excessive radiation exposure.
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