88 - The Cell 



NH, 



N 



HC 



^ v 



**> 



C 



II 



N' 



N 



TT 



CH OH H H 



III 



-C — c — c 



I 



H 



base 

 (adenine) 



OH H 



C C— -C—C- 



I I I I 

 OH OH H H 



pentose sugar 

 (ribose) 



a nucleoside 



-O-P— OH 



I 



OH 



phosphate 



a nucleotide 

 (adenosine monophosphate) 



Fig. 4-18. Adenosine monophosphate, a typical nucleotide. A variety of nucleotide units, chemically linked in 

 varying sequence into long chainlike macromolecules, determines the structure of the nucleic acids (Fig. 4-20). 



changes, the proteins of the cell become in- base. There are, however, two general and 



active or, in other words, they become de- five specific kinds of organic bases, as may be 



natured. seen in Figure 4-19. The pentose sugar re- 



Simple vs. Conjugated Proteins. Proteins peatedly represented in deoxyribonucleic 



that yield only amino acids upon complete acid (DNA) is always deoxyribose (Fig. 4-8); 



hydrolysis are designated as simple proteins. whereas that of ribonucleic acid (RNA) is 



But if other substances are also liberated, one always ribose — as is suggested, of course, by 



is dealing with a conjugated protein. Glucose, the names. 



for example, can be derived from the glyco- A very large number of specifically differ- 



proteins; lipid, from the lipoproteins; or ent nucleic acids is present in any cell. Each 



nucleic acid, from the nucleoproteins. time the nucleotides are combined in differ- 



Nucleoproteins. By far the most important ent sequence and proportion, the properties 



of the conjugated proteins are the nucleo- 

 proteins — not only the DNA proteins, or 

 genie materials, which generally remain in- 

 side the nucleus, but also the RNA proteins, 

 which carry on many of their functions in 

 the cytoplasm. Essentially each nucleopro- 

 tein represents a gigantic macromolecule 



of the resulting molecule are changed. In 

 DNA. for example, the four different bases 

 (adenine, cytosine, guanine, and thymine) 

 may occur in many different sequences, as is 

 indicated in Figure 4-20. Moreover, the 

 sugar-phosphate skeleton of a DNA is usu- 

 ally several thousand units in length. This 



formed by a union between a specific protein means that an almost infinite variety of such 



and nucleic acid. macromolecules can be formed. 



The nucleic acids also are very long- It is not possible to overemphasize the 



chained molecules. The chain of a nucleic importance of DNA substances in the life of 



acid, however, is constituted by a sequence the organism. Between each cell division, 



of complex units, called nucleotides (Figs. each DNA unit replicates itself in very pre- 



1-18. 4-19, and 4-20). Each nucleotide, in cise manner. Moreover, as will be seen in 



turn, is formed by chemical union between Chapter 7, each unit of DNA carries a set of 



three constituents, namely: (1) phosphoric coded instructions and this code, transmitted 



acid; (2) a pentose sugar; and (3) an organic through the agency of RNA, serves to de- 



