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Nucleotides
P.A. Levene determined the basic structure of nucleotides that make up DNA. Nucleotides are the subunits of nucleic acids and consist of a five-carbon sugar, a phosphate group, and a nitrogenous base. The two nucleic acids found in living cells are DNA and RNA. DNA and RNA nucleotides contain the sugar deoxyribose, a phosphate, and one of four nitrogenous bases: adenine, guanine, cytosine, thymine for DNA, or uracil for RNA. -
Frederick Griffith
Griffith studied two strains of the bacteria Streptococcus pneumoniae, which causes pneumonia. He studied two strains, one with a sgaar coat and one without. The coated strain causes pneumonia and is known as th smooth strain. The noncoated strain does not cause pneumonia and is known as the rough strain. He experimented both strains using mice. The mouse injected with the smooth strain died. The mouse injected with the rough strain lived. -
Oswald Avery
Oswald and his colleagues identified the molecules that transformed a rough strain bacteria into a smooth strain by isolating the macromolecules, such as DNA, lipids, and proteins from killed "S" cells. When the "R" cells were exposed to the "S" strain DNA, they transformed into "S" cells. He concluded from Griffith's experiment that the DNA released from dead "S" cells were incorporated into the DNA of "R" cells and transformed them into "S" cells. -
Maurice Wilkins
Wilkins worked at King's College on a technique called X-ray diffusion that involved aiming X rays at the DNA molecule. He found evidence that DNA had a helical structure. His data would be used by Watson and Crick. -
Chargaff
Erwin Chargaff analyzed the amount of adenine, guanine, thymine, and cytose in the DNA of various species. A portion of Chargaff's data, known as the Chargaff's rule, was published in 1950. He found that the amount of guanine is equal to the amounnt of cytose, while the amount of thymine is equal to the ammount of adenine within a specie. Chargaff's rule: C=G and T=A. -
Rosalind Franklin
Rosalind joined the staff at King's College. She took the famous Photo 51 and collectected data that would be eventually used by Watson and Crick. Photo 51 indicated that the structure of DNA was double helix, or a twisted ladder shape, formed by two strands of nucleotides twisted around each other. -
Hershey and Chase
Their bacteriophage experiment produced results that provided a definitive evidence that DNA is the transforming factor. The bacteriophage used in the experiment were made up of DNA and proteins and the fact viruses cannot themselves were two components that made the experiment ideal for conforming that DNA is the genetic material. They had to determine which part was injected into the bacteria and which part was the genetic material. -
Hershey and Chase (Conclusion)
Their conclusion was that the viral DNA was injected into the cell and provided the genetic information needed to produce new viruses. This experiment provided strong evidence that DNA, not protein, was the genetic material that could be passed down from generations to generations in viruses. -
Linus Pauling
Pauling proposed a triple-stranded helix structure for DNA. He had the phosphate groups of each DNA strand facing into the helical core, with the nitrogenous bases facing out. Three strands would intertwined to make one helical DNA model. However, he forgot about the negative charges of oxygn in each phosphate group. These charges repel one anothr, making it impossible to hold together. -
Watson and Crick
Using Chargaff's data and Franklin's data, Watson and Crick measured the width of the helix and the spacing between the bases. They found out that the DNA structure is two alternating rails of ladders of deoxyribose and phosphate that form th double helix. In 1953, Watson and Crick surprised the scientific community with their published work that suggested a structure for DNA and hypothesized of replication for the molecule deduced from the structure.
