826 J. Bonner 



cells are different because they possess different kinds of enzymes. They 

 have different enzymatic complements. They have, as it were, different 

 enzymatic spectra. One type of cell will have lots of enzyme A and 

 little of enzyme B, while of a second type, the converse will be true. 

 And as a matter of fact, I believe that most students of biology now 

 begin to be pretty well convinced that differentiation with respect to 

 enzymatic composition is not only an accompaniment of the differen- 

 tiation process but is very probably the cause of differentiation itself. 



How do cells of common descent and of common genetic consti- 

 tution get different enzymatic complements? It is a tenet of modern 

 biolog)', of course, that each enzyme in the cell is produced under the 

 control of a specific gene, a gene which sits in the nucleus and says 

 to the cell, "Please make me some of this kind of enzyme." For each 

 enzyme there is a responsible gene. This is the central dogma of our 

 time. We now know a little bit about how enzyme molecules are made. 

 This is information which has accrued to cell biologists in very re- 

 cent years. We know that enzyme molecules are made of amino acids 

 and that the specificity of an enzyme molecule, the fact that it is this 

 enzyme instead of some other enzyme, depends in a great measure 

 upon the sequence of amino acids in the peptide chain of which the 

 molecule is made. We might say that an enzyme is a message written 

 in a 20-letter alphabet — the 20-letter alphabet of the 20 amino acids 

 which occur in protein molecules. And we know, too, that enzyme 

 molecules are synthesized in the cell, or even outside the cell, by spe- 

 cial little objects which sit in the cytoplasm and do this task — objects 

 which we in the \\'est call microsomes and which, in the East, people 

 call microsomal nucleoprotein particles. And the evidence is pretty 

 good that each one of these kinds of particles carries the information 

 required to make one — and only one — kind of enzyme molecule. 



The newer cell biology tells us, too, that microsomal particles con- 

 tain their information in the form of nucleic acid — information writ- 

 ten in language of ribonucleic acid, RNA. This information is some- 

 how transmitted to and used in the assemblage of the amino acids in 

 the specific sequence of a specific enzyme. The microsomal particles 

 in turn are made within the nucleus, and we don't know how this 

 happens. W^e know only that microsomal particles are made in the 

 nucleus, that they are made in the nucleus only in the presence of de- 

 oxyribonucleic acid, the DNA of the genetic material. The implica- 

 tion is iliat the message contained in the DNA of the nucleus is some- 

 how printed off in the language of RNA, and then extruded into the 

 cytoplasm in the form of microsomes which are responsible for the as- 

 semblage of enzyme molecides. 



