Cellular Differentiation 73 



Fig. 37. The 



DNA accordl 

 and Crick. 



structure ot 

 ig to Watson 



geneticist that perk up most sharply. For the possibihty arises that the 

 genetic endowment of the progeny has been altered. In actuality, cellular 

 differentiation can be explained in two ways from the geneticists' view- 

 point. To understand the explanations, it is necessary to review the mod- 

 em concept of heredity. 



The genetic endowment of a cell is simply a collection of macro- 

 molecules that contains a set of instructions for making the remainder of 

 the cellular constituents. The instructions are provided in the form of a 

 code. The separate letters of the code are represented by the small build- 

 ing blocks of which the macromolecules are composed. The order in which 

 the building blocks are strung together determines the meaning of the 

 built-in message. A strand of deoxyribose nucleic acid ( DNA ) exemplifies 

 macromolecules of this type ( Fig. 37 ) . Its backbone is a chain of alternat- 

 ing sugar (deoxyribose) and phosphate molecules. Attached to each 

 sugar is one of four molecules: Adenine (A), Guanine (G), Thymine 



(T), or Cytosine (C). Thus the se- 

 quence, ATTCG . . . , would have 

 a meaning different from TATCG. 

 ... It is these sequences, biologists 

 think, that must constitute the mes- 

 sage. The nature of the code is one 

 of the most exciting problems in 

 present-day biology, and we all 

 await with impatience the Rosetta 

 stone that will unravel the genetic 

 language. 



Most of the genetic informa- 

 tion is in the nucleus, borne by the 

 chromosomes in the form of genes 

 or, if you will, separate sentences of 

 the message. But it is equally clear 

 that the cytoplasm also contains 

 macromolecules bearing informa- 

 tion not present in the nucleus ( see 

 Chapter 2). 



How is the genetic informa- 

 tion employed in making the rest of 

 the cellular constituents? The pres- 

 ent view, supported by strong but 

 not conclusive evidence, is that the 

 separate genetic sentences built into 

 genes or extra-nuclear elements de- 

 termine the order of the amino acids 



