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Miescher
Contribution: Miescher identified what he called "nuclein" inside the nuclei of human white blood cells. "Nuclein" was later changed to "nucleic acid" and eventually to "deoxyribonucleic acid.
Method of discovery: Miescher's plan was to isolate and characterize protein components of white blood cells. He asked for and received used, pus-coated bandages from the local surgical clinic. -
Miescher
Method of discovery (continued): He planned to wash them, filter out the white blood cells, and extract and identify the many proteins within the white blood cells. He came across a substance from the cell nuclei that had chemical properties unlike any protein. It had a much higher phosphorous content and a resistance to protein digestion; he then realized that he discovered a new substance.
This was the first step in DNA discovery. -
Chargaff
Contribution: Chargaff uncovered details of DNA structure. He noted that nucleotide composition of DNA varies among species, and concluded that almost all DNA maintains certain properties, even as its composition varies. He also discovered the total amount of purines (A + G) and total amount of pyrimidines (C + T) are usually nearly equal; this is known as "Chargaff's rule." -
Chargaff (continued)
Method of Discovery: Chargaff developed a new paper chromatography method for separating and identifying small amounts of organic material.
Improved on Previous Model: The model aided in discovering that the same nucleotide bases do not repeat in the same order. -
Hershey & Chase
Contribution: Hershey and Chase did the Hershey-Chase blender experiment that proved that phage DNA, and not protein, was the genetic material.
Method of discovery: Chase tracked the transfer of proteins and DNA between a virus and its host. For their experiment, they chose to use the T2 bacteriophage as the vehicle for delivering genetic material. T2 is comprised of only a protein-based outer wall and a DNA core, its simple structure made it the perfect research candidate. -
Hershey & Chase (continued 1)
Method of Discovery (continued): The phage reproduces by injecting its genetic material into a bacterium. In doing so, it leaves its protein shell attached to the host. Then, through a microscopic takeover, the virus seizes control of the bacterium’s reproductive mechanisms and uses them to duplicate itself, destroying the host in the process. -
Hershey & Chase (continued 2)
Method of discovery (continued): After introducing to the phage culture to the bacterial sample, they used a Waring blender to violently disturb the infected bacteria, causing the protein shells to detach from their hosts. Then, using a centrifuge, they separated the bacterium from the phages and protein.
Improved previous model: This experiment would explain how a phage is able to reproduce within a bacterium without the entirety of the protein shell penetrating the bacterium’s membrane. -
Watson & Crick
Contribution: Watson and Crick found that DNA is a double-stranded helix, and that the two strands are connected by hydrogen bonds. They also discovered that most double helices are right-handed, and that the DNA double helix is anti-parallel. • They also learned that the outer edges of nitrogenous bases are exposed and available for potential hydrogen bonding. -
Watson & Crick (continued)
Method of Discovery: Watson and Crick used cardboard cutouts to represent the individual chemical components of the four bases and other nucleotide subunits. They shifted molecules around on their desktops, like they were putting together a puzzle. Each base pair was held together by hydrogen bonds, and structure also reflected Chargaff's rule.
How it improved previous: The model showed that DNA is a three-dimensional, double-helical structure. -
Franklin
Contribution: Franklin discovered the basic dimensions of DNA strands, and that the phosphates were on the outside of what was probably a helical structure.
Method of Discovery: Franklin had two sets of high-resolution photos of crystallized DNA fibers. She used two different fibers of DNA, one more highly hydrated than other, and that showed that DNA is three-dimensional. -
Franklin (continued)
Improved previous model: The experiment showed that DNA is a three-dimensional structure, and the dampness made it possible for her to discover two forms of DNA, a dry “A” form and a wetter “B” form. -
Meselson & Stahl
Contribution: Meselson & Stahl had the experimental proof for the semi-conservative replication of DNA. They also showed that recombination results from the splicing of DNA molecules. Meselson along with other scientists discovered the data necessary to prove the existence of mRNA. Meselson also went on to discover the enzymatic basis of host DNA protection, which is where the cell recognizes its own DNA by adding methyl groups to it. -
Meselson & Stahl (Continued 1)
Contribution (continued): He found that foreign DNA entering a host DNA's territory will be attacked and destroyed by restriction enzymes but host, methylated, DNA remains intact. Meselson also discovered the process of DNA mismatch repair, which allows cells to fix mistakes in DNA.
Method of discovery: He did this by inventing a new technique called density gradient centrifugation, which uses centrifugal force to separate molecules based on their densities -
Meselson & Stahl (continued 2)
Improved previous model: This invention proved the semi-conservative replication of DNA.
