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CHAPTER 19 



tides. These molecules are actually chains 

 in which the individual deoxyribotides com- 

 prise the links. The way these links are 

 joined can be understood by examining the 

 two separate deoxyriboside 5'-monophos- 

 phates shown at the right of Figure 19-8. 

 I hese two compounds can become linked 

 if the topmost () of the lower compound 

 replaces the OH at position 3' of the sugar 



* Pyrimidine or purine base of 

 appropriate type (usually cyto- 

 sine. thymine, adenine or gua- 

 nine). 



figure 19-9. Polydeoxyribotide. 



in the upper compound. (The same reac- 

 tion occurs when a phosphate is added to 

 position 3' of a deoxyriboside to produce a 

 deoxyriboside 3'-monophosphate as illus- 

 trated in the two molecules shown at the 

 left of Figure 19—8.) Since deoxyriboside 

 ^'-monophosphates are capable of joining 

 together by means of phosphate linkage at 

 3', polydeoxyribotide chains of great length 

 are produced. Figure 19-9 shows a portion 

 of such a chain. Note that the polydeoxy- 

 ribotide is a linear — that is, unbranched — 

 molecule, whose backbone consists of sugar- 

 phosphate linkages and whose linearity is in- 

 dependent of the particular bases present at 

 any point. This independence means that 

 the structure of the chain is uninfluenced by 

 the sequences of bases which can be in any 

 array. Notice, moreover, that this polymer 

 (a molecule composed of a number of iden- 

 tical units) of deoxyribotides does not read 

 the same in both directions. As indicated 

 by the arrows, the sugar linkages to phos- 

 phate read 3'5', 3'5', and so on; whereas in 

 the opposite direction they read 5'3', 5'3', 

 et cetera. Because of this difference, the 

 polymerized DNA molecule is said to be 

 polarized. 



Measuring DNA Quantity 



Two main methods are commonly used in 

 determining the amount of DNA present in 

 the nucleus: the histochemical and the cyto- 

 chemical. The histochemical method em- 

 ploys whole tissues for the chemical extrac- 

 tion and measurement of DNA. Sometimes 

 chemical analysis is made of masses of 

 nuclei, from which most of the adhering 

 cytoplasm has been removed by special treat- 

 ment, to determine the average amount of 

 DNA per nucleus. In the second, cytochem- 

 ical, approach the DNA content of single 

 nuclei, chromosomes, or chromosomal parts 

 is determined. This method is based upon 

 the finding that DNA is the only substance 



