490 ANIMAL BIOCHEMISTRY 



of nucleic acid must actually be synthesized at the time the protein 

 appears. Be that as it may, the consensus of opinion holds that both 

 ribose and deoxyribonucleic acids are important factors. Since these 

 macromolecules are strong acids, they undoubtedly occur in cells as 

 anions, probably in the form of nucleoproteins. 



During cell division (reproduction at the cellular level), the heredi- 

 tary information is thought to reside in deoxyribonucleoproteins. 

 Somehow this coded information induces the formation of specific 

 protein material which in turn catalyzes the synthesis of deoxynucleo- 

 proteins like the originals. The two sets are then apportioned to 

 each new daughter cell. The deoxyribonucleoprotein is postulated to 

 function through the mediation of ribonucleoproteins which serve as 

 intermediate patterns between the deoxyribonucleoprotein and the 

 catalytic formation of the duplicate molecules. 



In a similar way, the deoxyribonucleoproteins must exercise genetic 

 control over the formation of all the new proteins necessary for the 

 new cell. Theories have been advanced to account for the transfer of 

 information from deoxyribonucleoprotein to ribonucleoprotein but 

 are purely speculative up to now. There is some evidence suggesting 

 a mechanism for the next step, control of protein structure by the 

 ribonucleoprotein. The theory supposes the formation of activated 

 intermediates of the nucleotide-amino acid type of page 488. From 

 such compounds the amino acids are transferred to linkages on the 

 ribonucleoprotein itself. When the required amino acids are available 

 to fill the specific sites of the pattern ribonucleoprotein molecule, 

 specific protein is formed. 



Though this arrangement is the best so far devised, there are still 

 some difficulties to be resolved. Ribonucleic acid contains only four 

 different nucleotide components. Since at least twenty amino acids 

 occur in most proteins, direct interaction governed only by the in- 

 dividual nucleotide components cannot account for the structural 

 specificity of proteins. Furthermore, nucleic acids are believed to be 

 rather rigid cylindrical molecules. If protein forms around the outside 

 of this cylinder in direct contact with the pattern, spontaneous re- 

 moval of the layer of finished protein becomes difficult. Then, too, 

 once removed, the polypeptide chain must be specifically coiled to 

 form a protein molecule with the required groups on the outside. 

 This last difficulty becomes major for the globular proteins which 

 occur in solution as spheres or ellipsoidal particles. Although all 

 these problems of protein synthesis have been examined and sugges- 

 tions made, no proposals are generally accepted and entirely free 

 from serious objection. 



