50 Inside the Living Cell 



The small microsomal granules are much smaller. They are about 

 200 A (1/100/^), i.e. the diameter is only about one per cent of the 

 average length of the mitochondrion. They are of great interest 

 because they have been found to be the main site of protein synthesis 

 in the cell. They are therefore the central point of the cell's opera- 

 tions — a sort of inner sanctum where the essential components of the 

 cell are formed. 



PROTEIN SYNTHESIS 



We have now come to the heart of the matter and we must consider 

 how proteins are made. As proteins are perhaps the most essential 

 and typical of all the constituents of living cells, this is clearly one 

 of the basic questions about life, which has attracted an enormous 

 amount of attention in recent years. 



As we have seen, the proteins are highly complex chains of amino 

 acids arranged in a particular order. There are about twenty separate 

 amino acids and we know that there must be in the cell a mechanism 

 which reproduces exacdy and without error the specific order of the 

 amino acids in every natural protein. The number of possible ways of 

 arranging say a chain of one hundred units chosen from twenty 

 different kinds is enormous. The cell selects the amino acids and 

 places them in the correct order with great ease and speed. In many 

 bacteria a new generation is produced in thirty minutes or even less 

 time. In this period, the full complement of proteins for a new cell 

 must be synthesized. It is evident that the protein synthesizing mecha- 

 nism works with great speed and efficiency. What is its nature? 



In the first instance we need to know something of the nature of 

 microsomes and other cytoplasmic constituents. It has been found 

 that the cytoplasmic constituents, as well as the nucleus, contain a 

 nucleic acid component known as ribonucleic acid (rna), which is 

 somewhat similar to the dna of the nucleus, but differs from it in 

 using a different sugar and also one of the component bases is 

 different. 



Nucleic acids of this type are probably present in most of the 

 cytoplasmic constituents, and occur to a marked extent in the 

 microsomes; but they are present in a highly concentrated form in the 

 sub-microsomes, which were mentioned above. As the initial site of 

 protein synthesis appeared to be the microsomes and it was suspected 

 that RNA was implicated, it was natural to consider the sub-micro- 

 somes as a possible site of the synthesizing mechanism. It has long 

 been thought that the synthesis of a protein with amino acids in a 

 particular order requires a 'template' which will act as a guide or 



