Chemical Nature of Genes 



303 



6. Through the use of labeled atoms, tagged 

 because they are radioactive or have an 

 abnormal weight, it is found that parts of 

 many cellular components are constantly 

 being replaced. In these cases, then, there 

 is an atomic turnover even though no 

 addition is being made to the total amount 

 of substance demonstrating the turnover. 

 DNA is unusual in that it shows little, if 

 any, turnover. 



7. DNA is a long, linear, unbranched poly- 

 mer, as would be expected if it represents 

 a string of recons. Each deoxyribonucleo- 

 tide is bipolar, in the respect that it 

 typically can join only to two other 

 deoxyribonucleotides via its 3' and 5' 

 sugar linkages to phosphate, as would be 

 expected if each deoxyribonucleotide was 

 the equivalent of a nonterminal recon (cf. 

 p. 197). 



We have discussed briefly the location of 

 DNA, the amount and behavior of DNA be- 

 fore, during, and at the conclusion of mitosis 

 (1), meiosis and fertilization (2), the quantity 

 of DNA in polyploid (3) and polytene (4) 

 chromosomes, the relation of ultraviolet light 

 mutability to DNA absorbence (5), and the 

 constancy or stability with which DNA main- 

 tains its integrity at the molecular level (6). 

 In all these respects, the observations are 

 consistent with the view that DNA either is 

 the genetic material or is intimately associ- 

 ated with the genetic material. Furthermore, 

 the linear arrangement of recons has a parallel 

 in the linear arrangement of deoxyribonucleo- 

 tides in the DNA polymer (7). 



Chemical Composition of RNA 



Besides DNA, there is another type of nu- 

 cleic acid found in the nucleus. This is 

 called ribonucleic acid, or RNA. RNA is nor- 

 mally found in combination with protein in 

 the form of ribonucleoprotein. Because the 

 RNA content of chromosomes varies within 

 and among diploid cells of the same organ- 

 ism, according to the metabolic activity of 



the cell, it can be concluded that RNA is un- 

 likely to be the chemical basis of genes in 

 typical (DNA-containing) chromosomes. 

 Nevertheless, let us take this opportunity to 

 discuss the chemical composition of RNA, 

 noting in particular how it compares with 

 DNA. 



Chromosomal RNA, like DNA, is a long, 

 unbranched polymer of a basic unit called a 

 ribonucleotide. The ribonucleotide is like the 

 deoxyribonucleotide in being a combination 

 of a base + sugar + phosphate; one way in 

 which it differs is that the sugar is D-rihose 

 (Figure 33-5). Another difference is found 

 in the pyrimidine bases which it may contain. 

 The two pyrimidines commonly found in 

 RNA are cytosine (also common in DNA) 

 and wrac/V (2.6-oxypyrimidine; not found in 

 DNA). Uracil's structure is shown in Figure 

 33 3. The two purines commonly found 

 in DNA, adenine and guanine, are also com- 

 mon in ribonucleotides. In RNA, the 

 base 4- sugar combination is called a riboside. 

 Ribonucleotides are joined together by 

 phosphates joined both at the 3' and 5' posi- 

 tions of the sugar, just as in DNA, so that 

 Figure 33-9 would equally well represent a 

 polyribonucleotide if an O was added at each 

 2' position (making each sugar D-ribose), and 

 if uracil was substituted for thymine as one 

 of the bases usually included. It should be 

 noted, finally, that RNA also absorbs ultra- 

 violet light of 2600 A, but can be removed 

 from the chromosome by treatment with 

 ribonucleases, or RNAases. 



In summary, we can say that the chromo- 

 somes contain two nucleic acids, DNA and 

 RNA. These normally occur in combination 

 with protein to form nucleoproteins (deoxy- 

 ribonucleoprotein and ribonucleoprotein, re- 

 spectively), in which these acids occur 

 as polynucleotides (polydeoxyribonucleotides 

 and polyribonucleotides, respectively), each 

 of which is built of (mono-) nucleotides 

 (deoxy- and ribonucleotides, respectively), 

 composed of phosphates joined at 5' oinucleo- 



