NONCHROMOSOMAL GENES 247 



membranes, a smooth continuous outer coat, and a convoluted inner 

 membrane, the folds of which consist of platelike double membranes 

 called cristae. The number of cristae per unit length varies with the 

 physiological state of the cell and probably is an adaptive, not a heredi- 

 tary, property. The outer membrane is probably concerned with perme- 

 ability control; the inner membrane, with the cristae, contains most if not 

 all of the insoluble enzymes of respiration. 



When mitochondria are treated with a suitable solvent, the structure is 

 disrupted and small empty vesicles of assorted sizes are obtained. They 

 contain the respiratory enzymes of the electron transport chain, and have 

 been called electron transport particles, ETP. Most other enzyme activi- 

 ties of the mitochondria are lost in the deoxycholate treatment. 



The ETP contains an array of insoluble enzymes which catalyze the 

 oxidation of carbohydrate derivatives to produce energy utilized to drive 

 other metabolic systems. The over-all oxidation of carbohydrate to COg 

 and H2O is accomplished in the cell by a large number of successive 

 reactions such that energy is released in small discrete packets at many 

 steps. The terminal stages of this process, involving the transport of 

 electrons from a carbohydrate donor to molecular oxygen, is accom- 

 plished by a chain of enzymes, like a bucket brigade. The enzyme com- 

 ponents of this chain include at least two different flavoproteins and 

 four different cytochromes. The function of electron transport suggests 

 the need for precise spatial orientation of these enzymes, and recent 

 evidence based upon studies of fragmented ETP supports this view. 



Mitochondria have been observed to divide by transverse splitting, and 

 it is a reasonable inference that they arise solely from pre-existing 

 mitochondria; but there is no definitive evidence on this point. The 

 presence in the membranes of many insoluble enzymes requiring a high 

 degree of orientation certainly suggests that the enzymes are formed 

 in situ. Evidence has been obtained for the formation of cytochrome 

 c in isolated mitochondria (Chapter 11). One may speculate whether 

 mitochondrial membranes are replicated in the sense of copying of the 

 array of macromolecules on a supertemplate, e.g., a pre-existing bit 

 of mitochondrial membrane. Since mitochondria vary considerably in 

 size and cristae content in cells of the same genotype, depending upon 

 physiological conditions, it is unlikely that the mitochondrion as a whole 

 is the relevant unit in replication. 



If mitochondria do replicate, are nucleic acids involved somewhere 

 in the process? No nucleic acids have been detected in mitochondria 

 by cytological or cytochemical techniques, but chemical analysis of care- 

 fully isolated mitochondria indicates that there is probably a small 

 amount of RNA, approximately 1 per cent of the protein content. It is 



