74 Cellular Differentiation 



in corresponding protein molecules. ( A protein of molecular weight 10,000 

 contains about 75 amino acids linked together in chains. If a chain were 

 composed of a series, alanine-glycine-tryptophane-alanine-vahne, it would 

 have unique properties directly attributable to the particular order of the 

 amino acids.) In other words, the gene controlling the hydrolysis of the 

 sugar, lactose, determines the order of amino acids in a particular protein. 

 The unique amino acid sequence permits this protein to act as an enzyme, 

 to combine specifically with lactose and to split it. Other genetic deter- 

 minants guide the synthesis of still other enzymes. Ultimately, the re- 

 mainder of the constituents and products of a cell arise through the action 

 of these enzymes. 



To recapitulate, then, the genetic material of a cell has two jobs to 

 perform : 



1. Each coded macromolecule must guide the synthesis of an exact 

 copy so that when the cell divides each daughter will have a complete set 

 of genetic instructions. 



2. Each coded macromolecule must guide the synthesis of a specific 

 enzyme or enzymes. 



Genetic information provides the cell with the potential for making 

 a variety of cell constituents and for performing many functions. How- 

 ever, the fact that a cell is genetically equipped to carry out a particular 

 activity does not guarantee that it will do so, for the external environment 

 in which it lives also tells the cell what it can and cannot do. Three ex- 

 amples are provided to show what we mean by this statement: 



a. Euglena is a protozoan-Hke organism that is capable of photo- 

 synthesis and contains about 10 chloroplasts per cell. When grown in the 

 dark, where it generates energy not by photosynthesis but by oxidation of 

 organic nutrients, the chloroplasts disappear. The instructions for making 

 chloroplasts are still intact, but whether or not the cell does make them 

 is decided by the light; when Euglena is once again exposed to light, 

 chloroplasts are again formed. 



b. The bacterium, Escherichia coli, can synthesize the amino acid, 

 arginine, from simple starting materials via a series of stepwise reactions. 

 It therefore does not require an external supply of arginine for growth. 

 When, however, arginine is supplied to the cell in high concentration, it 

 inhibits the formation of an enzyme, Ornithine transcarbamylase, which 

 catalyzes a step in the synthesis of arginine. The absence of the enzyme 

 causes the cell to stop making arginine, and it uses only the external 

 supply. Thus E. coli is told by its environment not to synthesize its own 

 arginine although it retains the genetic instructions that will enable it to 



