JJ^Genes: Nature and 

 Mode of Action 



THREE GENERAL conclusions as to what 

 genes are and how they act are now widely 

 accepted: (I) each gene is part of a specific 

 DNA compound; (2) genie effects upon de- 

 velopment are mediated mainly by enzymes; 

 and (3) each gene is responsible for guiding 

 the synthesis of some particular one of the 

 protein catalysts of the organism. These im- 

 portant conclusions have been reached within 

 the past five years, although the origins go 

 back almost a full century. In 1869 Friedrich 

 Miescher first extracted a type of material, 

 which he called nuclei n, from fish sperm and 

 from the nuclei of thymus gland tissue. Since 

 then the trail of evidence that finally has led 

 to the identifying of DNA as the code bearer 

 in heredity has been long and complex. Only 

 a few of the principal clues can be given in 

 any short account, however. 



IDENTIFICATION OF DEOXYRIBONUCLEIC 

 ACID (DNA) AS THE GENIC MATERIAL 



It was first realized in 1914, as a result of 

 a staining technique devised by Robert Feul- 



gen, that deoxyribonucleic acid is the one 

 kind of substance found almost exclusively 

 in nuclei and that a large proportion of the 

 chromosomal material consists of DNA. But 

 it was almost 1950 before quantitative meas- 

 urements — by A. E. Mirsky and Hans Ris of 

 the Rockefeller Institute and by A. Bowin 

 and C. Vendrely of the University of Stras- 

 bourg — showed that the DNA content of the 

 nuclei in each organism is of a definite quan- 

 tity in each species. Moreover, the haploid 

 nuclei of a species contain exactly half as 

 much DNA as the diploid nuclei; and if 

 tetraploid nuclei occur, as they do occasion- 

 ally in some tissues (for example, liver), 

 these polyploid nuclei contain quadruple the 

 haploid amount of DNA. 



The foregoing observations indicated that 

 there must be a very close relationship be- 

 tween DNA and the genie material. Then, 

 later, a more direct demonstration that DNA 

 determines hereditary transmission came 

 from the bacterial transformation experi- 

 ments of F. Griffith in England and of 

 O. T. Avery, C. M. MacLeod" M. McCarty, 



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