Using the search to find the structure of DNA as an example, describe the main observations which led up to the classic description and the process by which the double helix structure was arrived at - Did this constitute a revolution in science?
Using The Search To Find The Structure Of DNA As An Example, Describe The Main Observations Which Led Up To The Classic Description And The Process By Which The Double Helix Structure Was Arrived At. Did This Constitute A Revolution In Science?
The search for patterns of individual units of heredity, genes, started as early as the late 1800s when Gregor Mendel discovered a predictable transfer of characteristics from parents to offspring (Moore, 1963). From then until 1953 when Francis Crick and James Watson proposed the structure of DNA, which carries genetic information, it was an open field, many scientists were working on theories of its structure. It was only by postulating theories and disproving them that eventually a theory of the structure and description of DNA became established. I will try to account for the main observations which led to the description of the double helix and the process by which the structure was arrived at in this essay and also what effect the discovery had on genetic capabilities, scientific limitations and social risks - a revolution in science.
The search to find heredity spanned tens of years. At first, it was believed that proteins held the key to replication and this belief remained until the middle of the 20th century. In 1938 a textile physicist called William Thomas Astbury produced X-ray fiber photographs of a stretch dried film of DNA. Astbury and his colleague, Florence Bell, proposed the "a fold" structure for DNA but emphasised how its structure was related to proteins, giving the research into proteins more importance than that of DNA. In relation to proteins they suggested, "it is natural to assume, as a first working hypothesis at least, that they form the scroll on which is written the pattern of life" (Portugal and Cohen, 1979, p230). In a paper published in 1944, Journal of Experimental Medicine (Portugal and Cohen, 1979), O T Avery, C Macleod and M McCarthy, who had built on earlier work undertaken by Fred Griffith, reported that heredity traits could be transmitted from one bacterial cell to another by purified DNA molecules. Griffith discovered the transformation of pneumococcal types. He extended the work of J A Arkwright, studying the "characteristics of the virulent and attenuated strains of several bacteria" (Olby, 1974, p170). Griffith demonstrated the reversion of R forms (attenuated strains, rough, granular, flat and irregular) to S forms (virulent strains, smooth, dome-shaped and regular) in pneumococci. Although Avery was aware of the significance of this discovery, the revelation did not persuade the majority of scientist that DNA would provide the key to how genes determined heredity. Avery, himself, initially refused to accept the findings of Griffiths work, bacterial transformation, but went on with his associates to identify the transforming substance in bacteria which led to the "first evidence for the genetic role in DNA" (Olby, 1974, p181). The structure of DNA was still assumed to be in tetranucleotide form. The tetranucleotide hypothesis originated in 1909 from research undertaken by P A T Levene, which was then confirmed in 1914 by W Jones and remained in place until the late 1940s. This hypothesis postulated "that the four bases were present in equal proportions in the nucleic acids" (Portugal and Cohen, 1979, p80). By 1950, after years of chemical studies, Erwin Chargraff finally disproved the tetranucleotide hypothesis for the structure of nucleic acids. He had found that from research into the relative proportions of purine and pyrimidine bases, "the number of adenine (A) molecules was very similar to the number of thymine (T) molecules, while the number of guanine (G) molecules was very close to the number of cytosine (C) molecules" (Watson, 1968, p125). This work was to play a very important role later in the discovery of the structure of DNA by Francis Crick and James Watson.
The search for patterns of individual units of heredity, genes, started as early as the late 1800s when Gregor Mendel discovered a predictable transfer of characteristics from parents to offspring (Moore, 1963). From then until 1953 when Francis Crick and James Watson proposed the structure of DNA, which carries genetic information, it was an open field, many scientists were working on theories of its structure. It was only by postulating theories and disproving them that eventually a theory of the structure and description of DNA became established. I will try to account for the main observations which led to the description of the double helix and the process by which the structure was arrived at in this essay and also what effect the discovery had on genetic capabilities, scientific limitations and social risks - a revolution in science.
The search to find heredity spanned tens of years. At first, it was believed that proteins held the key to replication and this belief remained until the middle of the 20th century. In 1938 a textile physicist called William Thomas Astbury produced X-ray fiber photographs of a stretch dried film of DNA. Astbury and his colleague, Florence Bell, proposed the "a fold" structure for DNA but emphasised how its structure was related to proteins, giving the research into proteins more importance than that of DNA. In relation to proteins they suggested, "it is natural to assume, as a first working hypothesis at least, that they form the scroll on which is written the pattern of life" (Portugal and Cohen, 1979, p230). In a paper published in 1944, Journal of Experimental Medicine (Portugal and Cohen, 1979), O T Avery, C Macleod and M McCarthy, who had built on earlier work undertaken by Fred Griffith, reported that heredity traits could be transmitted from one bacterial cell to another by purified DNA molecules. Griffith discovered the transformation of pneumococcal types. He extended the work of J A Arkwright, studying the "characteristics of the virulent and attenuated strains of several bacteria" (Olby, 1974, p170). Griffith demonstrated the reversion of R forms (attenuated strains, rough, granular, flat and irregular) to S forms (virulent strains, smooth, dome-shaped and regular) in pneumococci. Although Avery was aware of the significance of this discovery, the revelation did not persuade the majority of scientist that DNA would provide the key to how genes determined heredity. Avery, himself, initially refused to accept the findings of Griffiths work, bacterial transformation, but went on with his associates to identify the transforming substance in bacteria which led to the "first evidence for the genetic role in DNA" (Olby, 1974, p181). The structure of DNA was still assumed to be in tetranucleotide form. The tetranucleotide hypothesis originated in 1909 from research undertaken by P A T Levene, which was then confirmed in 1914 by W Jones and remained in place until the late 1940s. This hypothesis postulated "that the four bases were present in equal proportions in the nucleic acids" (Portugal and Cohen, 1979, p80). By 1950, after years of chemical studies, Erwin Chargraff finally disproved the tetranucleotide hypothesis for the structure of nucleic acids. He had found that from research into the relative proportions of purine and pyrimidine bases, "the number of adenine (A) molecules was very similar to the number of thymine (T) molecules, while the number of guanine (G) molecules was very close to the number of cytosine (C) molecules" (Watson, 1968, p125). This work was to play a very important role later in the discovery of the structure of DNA by Francis Crick and James Watson.
