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CHAPTER 33 



A different application of microspectro- 

 photometry makes use of the fact that DNA 

 is highly absorbent of ultraviolet light of wave 

 lengths near 2600 A. When other substances, 

 which absorb ultraviolet of these wave 

 lengths, are removed, by enzymatic or other 

 treatments, the quantity of DNA can be 

 measured by its absorbence of these wave 

 lengths. As one test of the validity of the 



FIGURE 33-9. Polydeoxyribonucleotide. 



5'CH 



5'CH 



* Pyrimidine or purine base of 

 appropriate type (usually cyto- 

 sine, thymine, adenine or gua- 

 nine). 



absorbency, one can remove the DNA from 

 the chromosome by the use of enzymes, 

 deoxyribonucleases, or DNAases, which break 

 up the long DNA chains so that the pieces 

 can be washed out of the nuclei, leaving no 

 DNA in the chromosome. Such treatment 

 produces the expected loss of absorbency. 



Having digressed to study the chemical 

 content and quantitative measurement of 

 chromosomal DNA, let us list some results 

 which bear upon a possible association of 

 chromosomal DNA with the genetic material 

 in the nucleus: 



1. The amount of DNA increases during the 

 metabolic stage until it is approximately 

 double the amount present at the beginning 

 of this stage. Mitosis partitions the DNA 

 approximately equally among the two telo- 

 phasic nuclei. Accordingly, all diploid 

 nuclei of an individual have just about the 

 same DNA content when first formed 

 after mitosis. 



2. The amount of DNA in a haploid gamete 

 is approximately half that found in a 

 newly formed diploid metabolic nucleus 

 of the same individual. Fertilization, 

 which restores the diploid chromosome 

 condition, restores the DNA content 

 characteristic of the diploid cell. 



3. Cells which have extra sets of chromo- 

 somes, being therefore polyploid, have a 

 proportional increase in DNA content. 



4. Different cells in a tissue, like those in the 

 salivary gland of larval Drosophila, may 

 show different degrees of polyteny in their 

 chromosomes. The DNA content of these 

 different nuclei is found to be proportional 

 to the degree of polyteny. 



5. The capacity of different wave lengths of 

 ultraviolet light to induce mutations, in 

 fungi, corn, Drosophila, and other organ- 

 isms, is paralleled by the capacity of DNA 

 to absorb these wave lengths. In other 

 words, the mutational efficiency of ultra- 

 violet light parallels the absorption curve 

 of ultraviolet by DNA. 



