246 CELL HEREDITY 



ccntrifugt", as 70S. It consists of two basic units of unequal size and 

 shape, a 30S and a 50S particle; magnesium ions play an important role 

 in holding the 70S particle together. Sometimes two 70S particles aggre- 

 gate to form one unit which sediments at lOOS. 



Nucleoli are spherical structures found within the nucleus during 

 interphase; cells may have one or several of them. They break down at 

 metaphase, reappear at telophase, and their reorganization has been 

 shown to be under chromosomal control. In the electron microscope, the 

 nucleolus appears granular, composed of particles about 150 A in 

 diameter, and without any limiting membrane. A few chemical and 

 cytochemical studies have demonstrated that nucleoli are very dense, 

 probably quite dehydrated, consisting mainly of protein, the major com- 

 ponent of which in some organisms is an acidic protein rich in glutamic 

 acid, and 2 to 5 per cent RNA. Radioactive tracer experiments (dis- 

 cussed in Chapter 11) have shown that the nucleolus is a site of active 

 RNA svnthesis, and that the RNA produced there is later found in the 

 cytoplasm. There have been many speculations about the role of nucle- 

 olar RNA as intermediary between chromosomes and cytoplasm, but there 

 are few data. 



Kinetosomes: One class of particulate components of the cytoplasm 

 which have been observed to divide, and not to arise de novo, are the 

 basal bodies from which cilia and flagella develop, including the 

 blepharoplasts (also called centrosomes), kinetosomes, and the kineto- 

 plasts. Lwoff proposed that these are replicating units, on the basis of 

 his meticulous morphological studies of the complex cycle of differentia- 

 tion which these bodies undergo in ciliated protozoa. Instances have 

 been reported of irreversible loss of basal bodies following treatment 

 of the cells with the acridine dye, trypaflavin, and by dilution. 



Recentlv, a cytochemical analysis of the kinetoplast in particularly 

 favorable material — the parasitic flagellate, Trypanosoma cruzi — has 

 demonstrated the presence both of DNA and of its typical chromosomal 

 companion, a histone-type protein. The complete demonstration that 

 these bodies are replicating organelles has not yet been made, but the 

 chemical and morphological evidence suggests that eventually they will 

 be established as cytoplasmic organelles which contain genetic in- 

 formation. 



Mitochondria are membranous organelles structurally independent of 

 other cytoplasmic components. They contain the cytochrome enzymes of 

 terminal electron transport as well as enzymes of the citric acid cycle, 

 of fatty acid oxidation, and oxidative phosphorylation; they are the 

 principal energy source of the cell. 



As shown in Plate XIV, each mitochondrion consists of two sets of 



