Nucleic acids are the repositories and transmitters of genetic information for every cell, tissue, and organism. These include DNA and RNA which are polymers comprised of five different monomers: adenine, thymine (DNA only), uracil (RNA only), cytosine, and guanine. These individual monomers are composed of a sugar (ribose for RNA, deoxyribose for DNA), a base (purine or pyrimidine), and a phosphate linker. When the individual monomer contains all three components, it is referred to as a nucleotide, and when it lacks the phosphate, it is a nucleoside. These five bases can be isolated in trace amounts from plants, as well as a number of unusual bases with closely related strucutres. 5-Methylcy-tosine is found in the DNA of wheat germ. The pyrimidine glycosides, vicine and convicine, have been found in certain legume seeds. The methylated purines, theobromine and caffeine, occur regularly in plants and are valued for their
Mrlh^p-i.-cllrlitpinr FIGURE 1.29 Common alkaloids.
stimulant effects. Substituted purines constitute the cytokinins, which act as plant growth regulators and initiators of cell division. The purines and pyrim-idines are only slightly soluble in water. The nucleosides are shown in Figure 1.30 and the purines and pyrimidines in Figure 1.31.
There are two major nucleic acids: Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These are chain-like macromolecules that store and transfer genetic information. They are major components of all cells, comprising up to 15% of their dry weight. The monomeric units of DNA are the deoxynucleotides and for RNA, the ribonucleotides. Each of these nucleotides in turn consists of three main components: (1) a nitrogenous heterocyclic purine or pyrimidine
base, (2) a pentose sugar, and (3) a molecule of phosphoric acid. DNA contains two pyrimidine bases (cytosine and thymine) and two purines (adenine and guanine). RNA has the same nitrogenous bases except that uracil replaces thymine. When the phosphate group of a nucleotide is absent, the remaining structure is called a nucleoside. Like the free purines and pyrimidines, free nucleosides occur only in trace amounts in most plant cells. The nucleotides can be present in significant amounts.
In addition to the common bases listed above, numerous other purine and pyrimidine derivatives also have been isolated. The function of many of these rare bases is not well understood though it has been found that transfer RNA may contain up to 10% of these minor components.
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