Cell Structure and Protein Synthesis 463 



that definite mutational changes of single genes in lower and higher plants and 

 animals, including man, are associated with changes of enzymic pattern, con- 

 sisting in inactivation, disappearance, or, on the contrary, activation of indi- 

 vidual enzymes. It should be emphasized that the state of the enzymes which 

 catalyse various intermediate steps in the metabolism of different substances 

 (amino acids, vitamins, pigments, coenzymes etc.) depends on the stage of genes. 



Numerous studies have shown that transfer of the nuclear substance, deoxy- 

 ribonucleic acid, from one form of micro-organisms to another can result in 

 appearance in the recipient of the capacity to form new enzymes which were not 

 inherent in the recipient namely before, of enzymes characteristic of those 

 organisms from which the deoxyribonucleic acid was taken. Consequently, it is 

 the specific deoxyribonucleic acid, a characteristic nuclear substance, that can 

 induce synthesis of definite enzymes. 



It thus seems certain that the nature of enzymic properties of the cytoplasm 

 depends to a large extent on the nature of the cell nucleus. However, this is 

 not contradictory to the fact that the bulk of proteins are synthesized in the 

 cytoplasm rather than in the nucleus. Brächet et al. have shown that a considerable 

 quantity of proteins is formed in enucleated cell fragments of some unicellular 

 organisms. We may assume that once the cytoplasmic elements were formed, 

 they are capable of functioning relatively independently of the nucleus. This is 

 confirmed by a number of data, among which we may cite the following fact, 

 revealed in the course of our studies on the phenomenon of 'maternal effect' in 

 Drosophila. 



It is known that some lethal mutations cause in the homozygous state death 

 not only of the whole organism but also of individual cells, which can be revealed 

 with the aid of so-called somatic crossing-over. Such lethal mutations were 

 named 'cell lethals'. However, it was found that embryos developing in the eggs 

 of a normal mother, but homozygous 'cell lethals', develop normally during a 

 long period of time. This shows that hfe and growth of the embryonic cells 

 depend on the cytoplasm formed in the body of the maternal organism before 

 fertilization. After the cytoplasm has been formed in the presence of normal 

 nuclei, it functions relatively independently of the nucleus, and the abnormal 

 structure of the nuclear substance of the embryo itself is revealed only very 

 gradually, at late stages of embryonic development. 



These facts and many others show that the character of cytoplasm elements 

 is, to some degree, dependent on the nucleus. Apparently, the nucleus influences 

 in the first place the enzymic properties of the cj^oplasm. In this connection it 

 should be remembered that some coenzymes, the pyridine nucleotides, are 

 synthesized in the nuclei. 



The nature of the influence of the nucleus on cytoplasmic functions has not 

 yet been made clear. It has been suggested that deoxyribonucleic acid partici- 

 pates in the formation of specific ribonucleic acid in the nucleus ; the ribonucleic 

 acid then enters the cytoplasm and controls the specificity of proteins synthesized 

 in it. However, the synthesis of cytoplasmic ribonucleic acid in the nuclei has 

 not yet been finally proved. 



It seems probable that the nucleus is the site of synthesis of some of the 



