CELLULAR TOPOCHEMISTR Y 279 



microsomes. Succinic dehydrogenase, fumarase, and the oxido-reduction 

 systems for a-ketoglutarate and oxaloacetate are also found only in the 

 mitochondria, which contain in addition the enzymes of the fatty acid cycle. 



Their enzymatic equipment shows that the mitochondria are the ex- 

 clusive sites of aerobic oxidation and oxidative phosphorylation. 



In addition to these heavy mitochondria, the exclusive carriers of 

 cytochrome oxidase, de Duve distinguishes "light mitochondria" or 

 "lysosomes", which contain alkaline phosphatase, ribonuclease, desoxy- 

 ribonuclease, "cathepsin" (i.e. the complete assembly of intracellular 

 peptidases), and most of the ^-glucuronidase. In these intact particles, 

 these hydrolases do not have access to their substrates when the latter 

 are situated in the medium around the particles. 



C. The Nucleus 



The cellular nucleus, during interphase, consists principally of a nuclear 

 sap or caryolymph, a nucleolus and certain chromatin filaments or chro- 

 monemata which condense into chromosomes during mitosis. 



In the nucleus we find a whole series of compounds. The nuclear sap 

 contains proteins not belonging to the histone or protamine families. In 

 addition, we find in the nucleus either protamines or histones, depending 

 on the cell, and also lipides (around 10%). 



The nucleus contains practically all the DNA in the cell. For a given 

 species, the amount of DNA per nucleus of diploid cells in interphase is 

 constant. Although DNA in each case is a complex mixture of different 

 nucleotides, its composition is the same for all the different cells of the 

 organism, for they are all derived from the same fertilized &gg. It differs 

 from one species to another. Two insect viruses parasitic on the same 

 organism, for example, will have different structures of their DNA. A cell of 

 E. colt and its bacteriophage have DNA of differing structures. 



The fact that a spermatozoid contains only half the amount of DNA 

 present in normal cells clearly identifies DNA with the chromatin filaments 

 which also contain protamines and histones. 



However, the nucleus also always contains some RNA located in the 

 nucleolus (which does not contain DNx\) and in the chromatin filaments. 

 The ratio RNA/DNA varies from one species to another and, in the same 

 species, from one organ to another. Unlike DNA whose composition is 

 characteristic of all the cells of a species, the composition of RNA varies 

 in the same cell from one point to another. Thus the RNA of the nucleus, 

 the microsomes, and the mitochondria are all of different composition. 

 Physiological conditions can also modify the composition of the various 

 types of RNA in an organism, although they have no influence on the 

 composition of the DNA. 



