Figure 2.13. Seven bases found in nucleotides.


Figure 2.12a shows a nucleoside called adenosine. It is composed of ribose coupled to a nitrogen-rich compound called adenine. The numbers on the sugar—1',2'; and so forth—are the same numbering system we have seen before; the 'symbol is pronounced "prime" and is there to indicate that we are identifying the atoms of the sugar, not the atoms of the adenine. The name nucleoside reflects the fact that phosphorylated nucleosides (see below) are the building blocks of the nucleic acids that form the genetic material in the nucleus. However, nucleosides also play important roles in other places inside and outside the cell. Seven different compounds can be used to generate nucleosides (Fig. 2.13). All seven contain many nitrogen atoms and one or more ring structures. They are the three purines called adenine, guanine and hypoxanthine, the three pyrimidines called cytosine, thymine, and uracil, and an odd man out called nicotinamide. These ring compounds are called bases. Historically, the name arose because the compounds are indeed bases in the sense used earlier in this chapter—they will exchange an H+ with water. The roots of the name are now forgotten by most molecular biologists, who now use the word base to mean a purine,

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