Chemical Nature of Genes 



261 



in the cell which stains under certain condi- 

 tions. The Feulgen technique stains DNA 

 purple (see p. 8), whereas the methyl green 

 method stains DNA green. When properly 

 applied, not only are these stains specific 

 for DNA, but the amount of stain retained 

 is directly proportional to the amount 

 of DNA present. A given amount of dye 

 bound by DNA will make a quantitative 

 change in the amount of light of differ- 

 ent wavelengths it transmits. These meas- 

 urements can then be used to calculate the 

 amount of DNA present. For example: a 

 stained nucleus is placed under the micro- 

 scope; different appropriate wavelengths in 

 the visible spectrum are sent through the 

 nucleus, and a series of photographs is taken; 

 its DNA content is measured by density 

 changes of the nucleus. From the italicized 

 portions of words comes the name of this 

 procedure, microspectrophotometry . 



A different application of microspectro- 

 photometry utilizes another property of the 

 purines and pyrimidines in DNA. These 

 bases absorb ultraviolet light of wavelengths 

 near 2600 A {Angstrom units). When 

 other substances absorbing ultraviolet of 

 these wavelengths are removed by enzymatic 

 or other treatments, the quantity of DNA 

 can be measured by its absorbence. As one 

 test of the validity of the absorbency, one 

 can remove the DNA from the chromosome 

 by the use of enzymes — deoxyribonucleases , 

 DNA uses, DNAses, or DNases. These or- 

 ganic catalysts break the long DNA chains 

 into short pieces which then can be washed 

 out of the chromosomes and the nucleus. 

 Such treatment produces the expected loss 

 of absorbency. 



DNA as Genetic Material 



Having described the chemical nature and 

 quantitative measurement of chromosomal 

 DNA, we are in a position to consider some 

 results bearing upon the relationship be- 



tween chromosomal DNA and the genetic 

 material of the nucleus: 



1 . The quantity of DNA increases during 

 the metabolic stage until it is exactly 

 double (within the limits of experimental 

 error) the amount present at the begin- 

 ning of this stage. Mitosis apparently 

 distributes equal amounts of DNA to the 

 two telophase nuclei. Therefore, when 

 first formed, all the diploid nuclei of an 

 individual have approximately the same 

 DNA content. 



2. The amount of DNA in a haploid gamete 

 is roughly half that found in a newly 

 formed diploid metabolic nucleus of the 

 same individual. Fertilization, which re- 

 stores the diploid chromosome condition, 

 also restores the DNA content character- 

 istic of the diploid cell. 



3. Polyploid cells increase proportionally in 

 DNA content. 



4. Different cells of a tissue such as the 

 salivary gland of larval Drosophila show 

 different amounts of polynemy in their 

 chromosomes. Since the DNA content 

 of these different nuclei is found to be 

 proportional to their volume, it is as- 

 sumed to be a direct reflection of the 

 degree of polynemy. 



5. The capacity of different wavelengths of 

 ultraviolet light to induce mutations in 

 fungi, corn, Drosophila, and other organ- 

 isms is paralleled by the capacity of DNA 

 to absorb these wavelengths. 



6. By tagging or labeling atoms (those that 

 are radioactive or have an abnormal 

 weight), it is found that many cellular 

 components are being replaced continu- 

 ously during metabolism. Despite this 

 "atomic turnover," however, the total 

 amount of cellular material does not in- 

 crease. DNA is unusual because it shows 

 little, if any, turnover; in other words, 

 DNA maintains its integrity at the molec- 

 ular level. 



