Recent Event Highlights: A. D. Hershey & Martha Chase - The Hershey and Chase Experiments, and 6 more...
Created by MoGotti4102 on Nov 23, 2010
Last updated: 11/27/10 at 11:19 AM
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Maurice Wilkins and Rosalind Franklin, together with Ray Gosling, Alec Stokes and Herbert Wilson and other colleagues at the Randall Institute at King's, made crucial contributions to the discovery of DNA's structure in 1953. Wilkins began using optical spectroscopy to study DNA. In 1950, Wilkins and Gosling obtained the first crystalline X-ray diffraction patterns from DNA fibers and Alec Stokes suggested the patterns indicated DNA was helical in structure. Francis Crick and James Watson of Cambridge University obtained this photo, and together with some of Franklin's data, they were able to use this with their own deductions to build the first correct model of the DNA molecule.
At the age of 26, Rosalind Franklin had her PhD and she began working in x-ray diffraction -- using x-rays to create images of crystalized solids. She used this method in analyzing complex, unorganized matter. Franklin was invited to King's College in London to join a team of scientists studying living cells. She made advances in x-ray diffraction techniques with DNA. She adjusted her equipment to produce an extremely fine beam of x-rays where she extracted finer DNA fibers than ever before and arranged them in parallel bundles, and studied the fibers' reactions to humid conditions. All of this allowed her to discover crucial keys to the DNA structure.
When Watson and Crick made their first model, it failed. This caused the head of their unit to tell them to stop DNA research, but the subject kept coming up. Franklin found that her x-ray diffractions showed that the "wet" form of DNA had all the characteristics of a helix. Wilkins was frustrated. In 1953, he showed Franklin's results to Watson. Watson and Crick took a crucial step suggesting the molecule was made of two chains of nucleotides, one going up and the other going down. Crick added Chargaff's findings about base to the model, so matching base pairs interlocked in the middle of the double helix to keep distance between the chains constant. Watson and Crick showed each the DNA molecule strands was a template for the other. During cell division the two strands separate and on each strand a new "other half" is built, just like the one before. That way DNA can reproduce itself without changing structure.
Their experiment helped convince the world that DNA was the genetic material. Alfred Hershey and Martha Chase showed that the DNA of the phage virus, not the protein, contains the phage genes. After a phage particle attaches to a bacterium, its DNA enters through a tiny hole while its protein coat remains outside. The key to the success of the experiment was showing that viral infection was unaffected by violent agitation in a kitchen blender which removed the empty viral protein shells from the bacterial surface. The Hershey-Chase experiment became known as the "blender experiment."
Hydrogen and oxygen atoms on every fourth amino acid are joined together through hydrogen bonds, a type of weak chemical attraction. Pauling's background studying many types of biological and inorganic molecules gave him a distinct advantage seeing this helix structure. While many of his contemporaries tried to force an even number of amino acid residues into each turn of the helix, Pauling realized that a nonintegral number was the answer. Pauling and Corey's alpha helix and a second structure they defined, the beta sheet, have since been found in almost all proteins. The two structures are often the first line of description scientists give when discussing a protein's components. However, Pauling and Corey's work was not without mistakes: research since the 1951 protein papers found the beta sheet to be twisted, not flat as suggested in Corey and Pauling's research.
In 1944, Oswald and his team revised Frederick Griffith's experiment. But this time instead of using mice, they used test tubes. Avery used heat to kill virulent bacteria. He then extracted RNA, DNA, carbohydrates, lipids, and proteins from these dead cells, which were considered to be likely candidates for the carriers of genetic information. He then added each type of molecule to a culture of live non-virulent bacteria to determine which was responsible for changing them into virulent bacteria. Only the non-virulent cells which were given DNA from the dead virulent strain became virulent. Avery concluded that DNA must be the genetic material.
Erwin Chargaff's experiments showed that DNA, not amino acids in a cell, is the carrier of genetic information. His work changed the study of biology and heredity completely, and provided the foundation for the work of Francis Crick, James Watson, and Maurice Wilkins. Chargaff noticed a pattern in the amounts of the four bases: adenine, guanine, cytosine, and thymine. He took samples of DNA from different cells and found that the amount of adenine was almost equal to the amount of thymine, and that the amount of guanine was almost equal to the amount of cytosine. Thus you could say: A=T, and G=C. This discovery later became Chargaff’s Rule.
Frederick Griffith worked on an experiment that gave people the ability to find that DNA was a molecule of inheritance. His experiment involved injecting two types of pneumonia, a virulent and non-virulent, into two different mice. The mouse with the virulent pneumonia died whereas the mouse with the non-virulent pneumonia lived. Then Griffith heated up the virulent disease to kill it injected it into a mouse, and that mouse lived. Last he injected non-virulent pneumonia and virulent pneumonia, that had been heated and killed, into a mouse, this mouse ended up dead. Griffith thought the killed virulent bacteria passed on a characteristic to the non-virulent which made it virulent. This passing on of the inheritance molecule was what he called transformation.
P.A. Levene analyzed the components of the DNA molecule. He found it contained four nitrogenous bases: cytosine, thymine, adenine, and guanine; deoxyribose sugar; and a phosphate group. He concluded that the basic unit (nucleotide) was composed of a base attached to a sugar and the phosphate also attached to the sugar. He also mistakenly concluded that the proportions of bases were equal and that there was a tetranucleotide that was the repeating structure of the molecule. The nucleotide, however, remains as the fundamental unit of the nucleic acid polymer. There are four nucleotides: those with cytosine, with guanine, with adenine, and with thymine.