144 THE BIOSYNTHESIS OF PROTEINS 



them are maternally inherited; this was well established for the mutant 

 'poky' and for a few others designated by the symbol mi (for maternally 

 inherited). The character again does not segregate as a mendelian marker. 

 In these mutants, the respiratory system shows various abnormalities in the 

 cytochromes system (Mitchell et aL, 1953; Tissieres et al., 1953; Tissieres 

 and Mitchell, 1954). 



In Neurospora as well as in yeast, the formation of several enzymes of the 

 respiratory chain thus depends on some hereditary non-mendelian factor. 

 In Neurospora, just as in yeast, it looks as if an extrachromosomal element 

 were required, as if this object could ensure its own continuity through 

 generations of cells, and as if it were in certain cases able to replace the 

 corresponding factor of the wild type (Pittenger, 1956). 



To complete the picture, it must be emphasized that although the 

 synthesis of the respiratory enzymes in yeast and mould depends on the 

 presence of an extrachromosomal factor, it also depends nevertheless on 

 typical mendelian genes, just like any other enzyme synthesis. This last 

 point is established by the existence of still another type of respiration 

 deficient mutants, in which the character segregates in a perfectly mende- 

 lian manner. The mutation, this time, concerns the synthesis of a single 

 protein (Chen et ah, 1950; Mitchell et al., 1953). For instance, the mende- 

 lian mutants C-115 and C-117 of Neurospora lack cytochrome-^ and cyto- 

 chrome-6 respectively. The synthesis of these proteins, like that of any 

 other enzyme, is therefore controlled by regular mendelian genes; the 

 extrachromosomal factor is a further requirement for the formation of the 

 respiratory enzymes, which has not been observed for the common 

 enzymes. 



Another group of enzymes which depend on cytoplasmic hereditary 

 factors are those of the chloroplast in green plants. Certain green flagellates 

 possess one single chloroplast per cell; this chloroplast normally divides 

 before cell division and each daughter cell receives one chloroplast. 

 Occasionally, the plast division lags behind cell division, and the daughter 

 cells separate at a time when the division of the chloroplast is not com- 

 pleted; under such circumstances, one of the daughter cells is deprived of 

 chloroplast. The striking fact is that cells which have thus lost the chloro- 

 plast, as a result of a mechanical accident, never recover the capacity of 

 producing a new chloroplast. The progeny of such cells is for ever devoid 

 of photosynthesis apparatus (Lwofi^, 1949). 



Because of these facts and of observations made on mosaics in higher 

 plants, chloroplasts are commonly described as self-duplicating organelles. 

 This is a mere expression of direct microscopical observation. 



It is possible to influence by various means the relative rate of division of 

 the cell and of the chloroplasts in protozoans like Euglena, by changing the 

 medium or the temperature or by growing the cells in the dark. The num- 



