Erwin Chargaff’s Experiment (1950)
The base ratio experiments performed by Chargaff. DNA was extracted from various organisms and treated with acid to hydrolyze the phosphodiester bonds and release the individual nucleotides. Each nucleotide was then quantified by chromatography. The data show some of the actual results obtained by Chargaff. These indicate that, within experimental error, the amount of adenine is the same as that of thymine, and the amount of guanine is the same as that of cytosine.
The Hershey-Chase Experiment
Legend:
Illustration of the 1952 experiment connecting DNA and heredity.
Side by side experiments are performed with separate bacteriophage (virus) cultures in which either the protein capsule is labeled with radioactive sulfur or the DNA core is labeled with radioactive phosphorus.
1. The radioactively labeled phages are allowed to infect bacteria.
2. Agitation in a blender dislodges phage particles from bacterial cells.
3. Centrifugation concentrates cells, separating them from the phage particles left in the supernatant.
Results:
1. Radioactive sulfur is found predominantly in the supernatant.
2. Radioactive phosphorus is found predominantly in the cell fraction, from which a new generation of infective phage can be isolated.
Conclusion: The active component of the bacteriophage that transmits the infective characteristic is the DNA. There is a clear correlation between DNA and genetic information.
ROSALIND ELSIE FRANKLIN (1920-1958)
Rosalind Franklin produced the X-ray crystallography pictures of BDNA which Watson and Crick used to determine the structure of double-stranded DNA. She was born in London, England. Her family was well-to-do and both sides were very involved in social and public works. Franklin's father wanted to be a scientist, but World War I cut short his education and he became a college teacher instead. Rosalind Franklin was extremely intelligent and she knew by the age of 15 that she wanted to be a scientist. Her father actively discouraged her interest since it was very difficult for women to have such a career. However, with her excellent education from St. Paul's Girls' School, one of the few institutions at the time that taught physics and chemistry to girls, Franklin entered Cambridge University in 1938 to study chemistry.
When she graduated, Franklin was awarded a research scholarship to do graduate work. She spent a year in R.G.W. Norrish's lab without great success. Norrish recognized Franklin's potential but he was not very encouraging or supportive toward his female student. When offered the position as an assistant research officer at the British Coal Utilization Research Association (CURA), Franklin gave up her fellowship and took the job.
CURA was a young organization and there was less formality in the way research had to be done. Franklin worked fairly independently, a situation that suited her. Franklin worked for CURA until 1947 and published a number of papers on the physical structure of coal.
Franklin's next career move took her to Paris. An old friend introduced her to Marcel Mathieu who directed most of the research in France. He was impressed with Franklin's work and offered her a job as a "chercheur" in the Laboratoire Central des Services Chimiques de l'Etat. Here she learned X-ray diffraction techniques from Jacques Mering.
In 1951, Franklin was offered a 3-year research scholarship at King's College in London. With her knowledge, Franklin was to set up and improve the X-ray crystallography unit at King's College. Maurice Wilkins was already using X-ray crystallography to try to solve the DNA problem at King's College. Franklin arrived while Wilkins was away and on his return, Wilkins assumed that she was hired to be his assistant. It was a bad start to a relationship that never got any better.
Working with a student, Raymond Gosling, Franklin was able to get two sets of high-resolution photos of crystallized DNA fibers. She used two different fibers of DNA, one more highly hydrated than the other. From this she deduced the basic dimensions of DNA strands, and that the phosphates were on the outside of what was probably a helical structure.
She presented her data at a lecture in King's College at which James Watson was in attendance. In his book The Double Helix, Watson admitted to not paying attention at Franklin's talk and not being able to fully describe the lecture and the results to Francis Crick. Watson and Crick were at the Cavendish Laboratory and had been working on solving the DNA structure. Franklin did not know Watson and Crick as well as Wilkins did and never truly collaborated with them. It was Wilkins who showed Watson and Crick the X-ray data Franklin obtained. The data confirmed the 3-D structure that Watson and Crick had theorized for DNA. In 1953, both Wilkins and Franklin published papers on their X-ray data in the same Nature issue with Watson and Crick's paper on the structure of DNA.
Franklin left Cambridge in 1953 and went to the Birkbeck lab to work on the structure of tobacco mosaic virus. She published a number of papers on the subject and she actually did a lot of the work while suffering from cancer. She died from cancer in 1958.
In 1962, the Nobel Prize in Physiology or Medicine was awarded to James Watson, Francis Crick, and Maurice Wilkins for solving the structure of DNA. The Nobel committee does not give posthumous prizes.
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The experiment carried out by Meselson and Stahl involved growing a culture of Escherichia coli in a medium containing 15NH4Cl (ammonium chloride labeled with the heavy isotope of nitrogen). Cells were then transferred to normal medium (containing 14NH4Cl) and samples taken after 20 minutes (one cell division) and 40 minutes (two cell divisions). DNA was extracted from each sample and the molecules analyzed by density gradient centrifugation. After 20 minutes all the DNA contained similar amounts of 14N and 15N, but after 40 minutes two bands were seen, one corresponding to hybrid 14N-15N-DNA, and the other to DNA molecules made entirely of 14N. (B) The predicted outcome of the experiment is shown for each of the three possible modes of DNA replication. The banding pattern seen after 20 minutes enables conservative replication to be discounted because this scheme predicts that after one round of replication there will be two different types of double helix, one containing just 15N and the other containing just 14N. The single 14N-15N-DNA band that was actually seen after 20 minutes is compatible with both dispersive and semiconservative replication, but the two bands seen after 40 minutes are consistent only with semiconservative replication. Dispersive replication continues to give hybrid 14N-15N molecules after two rounds of replication, whereas the granddaughter molecules produced at this stage by semiconservative replication include two that are made entirely of 14N-DNA.