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chap16 studyguide

chap16 studyguide - Chapter 16 The Molecular Basis of...

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Chapter 16 The Molecular Basis of Inheritance Study Guide Overview: Life’s Operating Instructions In April 1953, James Watson and Francis Crick shook the scientific world with an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA. Your genetic endowment is the DNA you inherited from your parents. Nucleic acids are unique in their ability to direct their own replication. The resemblance of offspring to their parents depends on the precise replication of DNA and its transmission from one generation to the next. It is this DNA program that directs the development of your biochemical, anatomical, physiological, and (to some extent) behavioral traits. Concept 16.1 DNA is the genetic material The search for genetic material led to DNA. Until the 1940s, the great heterogeneity and specificity of function of proteins seemed to indicate that proteins were the genetic material. However, this was not consistent with experiments with microorganisms, such as bacteria and viruses. By 1947, Erwin Chargaff had developed a series of rules based on a survey of DNA composition in organisms. ° He already knew that DNA was a polymer of nucleotides consisting of a nitrogenous base, deoxyribose, and a phosphate group. ° The bases could be adenine (A), thymine (T), guanine (G), or cytosine (C). Chargaff noted that the DNA composition varies from species to species. In any one species, the four bases are found in characteristic, but not necessarily equal, ratios. He also found a peculiar regularity in the ratios of nucleotide bases that are known as Chargaff’s rules. In all organisms, the number of adenines was approximately equal to the number of thymines (%T = %A). The number of guanines was approximately equal to the number of cytosines (%G = %C). Human DNA is 30.9% adenine, 29.4% thymine, 19.9% guanine, and 19.8% cytosine.
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The basis for these rules remained unexplained until the discovery of the double helix. Watson and Crick discovered the double helix by building models to conform to X-ray data. By the beginnings of the 1950s, the race was on to move from the structure of a single DNA strand to the three-dimensional structure of DNA. ° Among the scientists working on the problem were Linus Pauling in California and Maurice Wilkins and Rosalind Franklin in London. Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study the structure of DNA. ° In this technique, X-rays are diffracted as they passed through aligned fibers of purified DNA. ° The diffraction pattern can be used to deduce the three-dimensional shape of molecules. James Watson learned from their research that DNA was helical in shape, and he deduced the width of the helix and the spacing of nitrogenous bases. ° The width of the helix suggested that it was made up of two strands, contrary to a three-
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chap16 studyguide - Chapter 16 The Molecular Basis of...

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