. Cytology. Cytology. Substrate ~J^â[bT â ». Enzyme-substrate complex Figure 2-3. Diagram Showing the Formation of Substrate - Enzyme Com- plex as the Result of Coupling Between Two Reactive Sites of the Substrate and Enzyme. (From Harrow, B. and Mazur, A., 1958. "Textbook of Bio- chemistry," 7th ed., W. B. Saunders Co., Philadelphia, Pa., p. 130.) Nucleic Acids Although nucleic acids have long been known to be a constant con- stituent of living tissues, it is only during the past twenty years that any considerable attention has been paid to their distribution and role in the cell.
. Cytology. Cytology. Substrate ~J^â[bT â ». Enzyme-substrate complex Figure 2-3. Diagram Showing the Formation of Substrate - Enzyme Com- plex as the Result of Coupling Between Two Reactive Sites of the Substrate and Enzyme. (From Harrow, B. and Mazur, A., 1958. "Textbook of Bio- chemistry," 7th ed., W. B. Saunders Co., Philadelphia, Pa., p. 130.) Nucleic Acids Although nucleic acids have long been known to be a constant con- stituent of living tissues, it is only during the past twenty years that any considerable attention has been paid to their distribution and role in the cell. Nucleic acids are organic polymers composed of repeating units called nucleotides, each of which consists of a pentose sugar, an organic nitrogenous base, and phosphoric acid bound together by sugar-phos- phate linkages (Formula (2-5)). Two kinds of nucleic acids are recog- base base base 1 o 1 o 1 II 1 OH "0- II ^p^ 1 OH O- ^Cs' (2-5) nized on the basis of the pentose sugar component they contain and to some extent by the differences in bases making up the individual nucleo- tide units. The pentoses concerned are D-ribose characteristic of ribo- nucleic acid (RNA) and deoxyribose characteristic of deoxyribose nucleic acid (DNA). The structural difference between these two sugars involves carbon 2 as is shown in Formula (2-6). This difference appears to be rather small but it is quite sufficient to distinguish one sugar from the other by chemical means. The nitrogenous bases consist of two types, namely, the pyrimidines and purines. The former are monocyclic, the latter dicyclic. The most commonly occurring pyrimidines are thymine, cytosine, and uracil while 16 / CHAPTER 2. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Wilson, G. B. (George Bernard), 1914-; Morrison, John H. (John Herbert), 1927-.
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