BIOSYNTHESIS 265 



they must be obtained from outside the cell. If the medium is rich in amino 

 acids, they accumulate in the cell during the period of growth and the 

 intracellular pool is increased. However, if protein synthesis is rapid and 

 normal, the amino acids do not accumulate. 



2. The template 



A large number of experimental facts and a series of indirect experiments 

 have led Brachet and Caspersson, independently of each other, to the 

 conclusion that nucleic acids are involved in protein synthesis. The con- 

 centration of RNA in the cell is approximately proportional to the growth of 

 Bac. lactic aerogenes, thus leading Caldwell to consider RNA as being the 

 template itself. Jeener has shown for his part that during experimental 

 modifications of the volum.es of the nucleus and the cytoplasm of Thermo- 

 bacteriiim acidophilus protein synthesis was quantitatively related to the 

 level of RNA. Finally, direct experiments have shown that in various cells 

 or fragments of cells, treatment with ribonuclease suppresses protein 

 synthesis. 



HbA Val— His— Leu— Thr— Pro— G/«— Glu— Lys 



HbS Val— His— Leu— Thr— Pro— Ffl/— Glu— Lys 



HbC Val— His— Leu— Thr— Pro— Lj'^— Glu— Lys 



HbG Val— His— Leu— Thr— Pro— Glu— G/j— Lys 



Fig. 80. (Perutz). Sequence of amino acid residues in a small segment of OHe of the 



polypeptide chain of haemoglobin. 



These are the facts which point to RNA being the template on which 

 the synthesis of proteins takes place. Another confirmation has been 

 brought by experiments showing that the isolated nucleic acid of tobacco 

 mosaic virus can introduce the disease into a leaf cell as well as the whole 

 nucleoprotein of the virus. 



3. The synthetic process 



It is now a well established fact that the system of activation of the dif- 

 ferent amino acids in the presence of the specific enz}'Tne for each of them, 

 is the same in all cases of protein synthesis, and that the characteristic 

 structure of the protein synthetized is due to the structure of the RNA 

 template. This template is located in the cytoplasm. It is probably syn- 

 thetized in the nucleus where the coded message for the synthesis of a 

 particular protein is transferred from a section (or gene) of a long DNA 

 macromolecule to a corresponding section of RNA. Each amino acid is 

 brought to the template by a specific carrier. Crick has suggested that this 

 carrier is a short length of RNA chain with a specific sequence of bases 

 coding for the particular amino acid. It has been shown that each specific 



