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CHAPTER 33 



FIGURE 33-7. Deoxyribonucleotides. 



Purine 



or 



Pyrimidine 



Deoxyriboside 3'-monophosphate 



Deoxyriboside 5'-monophosphate 



Most of the DNA analyzed does not occur 

 in single deoxyribonucleotide units, but is 

 found to be composed of polydeoxyribonucle- 

 otides, chains in which the individual deoxyri- 

 bonucleotides comprise the links. The way 

 these links are joined can be understood by 

 examining the two deoxyriboside 5'-mono- 

 phosphates at the right of Figure 33-8. These 

 two compounds can become linked together 

 if the topmost O of the bottom compound 

 replaces the OH at position 3' of the sugar 

 in the top compound. This reaction would 

 be the equivalent of adding a phosphate to 

 position 3' of the pentose in the top com- 

 pound. The occurrence of such a reaction 

 has already been mentioned, and is illustrated 

 in the two molecules at the left of Figure 33-8. 

 Since deoxyriboside 5'-monophosphates are 

 capable of joining to each other by means of 

 a phosphate linkage at 3', single unbranched 

 chains of polydeoxyribonucleotides of great 

 length are produced. Figure 33-9 shows a 

 portion of such a chain. Note here that the 

 polydeoxyribonucleotide is a hnear, un- 

 branched, molecule, whose backbone is made 

 up of sugar-phosphate linkages, and whose 

 linearity is independent of the particular bases 

 present at any point. This means that the 

 structure of the chain is uninfluenced by the 



sequences of bases, which are therefore in 

 indeterminate, or unspecified, array. Notice, 

 moreover, that this polymer (a molecule com- 

 posed of a number of identical units) of 

 deoxyribonucleotides does not read the same 

 in both directions. In the direction indicated 

 by the arrows the sugar linkages to phosphates 

 are 3'5', 3'5', etc., while in the opposite 

 direction they read 5'3', 5'3', etc. Because of 

 this, the polymerized DNA molecule is said 

 to be polarized. 



There are two main methods of determining 

 the amount of DNA present in the nucleus. 

 One method is histochemical and employs 

 whole tissues for the chemical extraction and 

 measurement of DNA. In such work, one 

 may perform the chemical analyses using 

 masses of nuclei from which the surrounding 

 protoplasm has been largely removed by 

 special treatment. As a result of such 

 studies, one can determine the average 

 amount of DNA per nucleus. 



The second main method is a cytochemical 

 one, by which the DNA content of individual 

 nuclei, or chromosomes, or of chromosomal 

 parts, is determined. This procedure makes 

 use of the fact that DNA is the only substance 

 in the cell which is stained when certain pro- 

 cedures are followed. The Feidgen-Rossen- 



