272 ORIGIN OF STRUCTURES AND FUNCTIONS 



SO to speak, they are ' assembled ' into a single structural 

 aggregate which works up the energy-rich compounds 

 required for the synthesis of proteins. The synthesis itself is, 

 however, carried out in another structural aggregate, the 

 microsome or, as Brachet^" calls it, with reference to its high 

 content of ribonucleoproteins, a * ribonucleoprotein granule '. 

 A comparison of the evidence concerning the synthesis of 

 peptide bonds in the protoplasmic structures of animal and 

 vegetable cells has revealed some differences in these pro- 

 cesses. Thus, according to N. Sisakyan,^^^"" the incorporation 

 of labelled glycine takes place considerably faster in those 

 fractions of a homogenate of tobacco leaves which contain 

 mitochondria than in those which contain plastids. Never- 

 theless it is only in the plastids that one can observe an 

 increase in the amount of protein nitrogen derived from 

 mixtures of amino acids. 



The idea that RNA plays an important part in the syn- 

 thesis of proteins arose quite a long while ago on the basis 

 of biological observations and quantitative experiments which 

 showed a close correlation between the rate of synthesis of 

 proteins and the amount of nucleic acids in organs, tissues 

 and the organelles of cells. In growing and secreting organs, 

 i.e. those in which proteins are being synthesised fastest, 

 the amount of ribonucleic acid is found to be greatest. On 

 the other hand the parts of adult organisms which only grow 

 slowly or have stopped growing altogether only contain a 

 relatively small amount of RNA even in cases where the 

 organ is biologically extremely active and carries out a great 

 deal of work in the organism (e.g. the heart or kidneys).^" 



In parallel with this, cytological studies have sho^vn that 

 the synthesis of proteins proceeds with special intensity in 

 just those parts of the cell which are richest in RNA. In 

 particular, in the nucleus the synthesis of proteins is con- 

 centrated only in the heterochromatic nucleoli while in the 

 cytoplasm it is concentrated in the mitochondria, which are 

 exceptionally rich in ribonucleic acid.^''* 



The association between the intensity of protein formation 

 and the concentration of ribonucleic acid has been confirmed, 

 not only by cytochemical means but also by the use of more 

 accurate methods,^" in particular by the use of labelled 



