Cell Differentiation : A Problem in Selective Gene 

 Activation Through Self-Produced Micro-Environ- 

 mental Differences of Carbon Dioxide Tension 



\V. F. LooMis 



The Loomis Laboratory, 

 Greenwich, Conn., U.S.A. 



It is an interesting fact that the subject matter of most of this sym- 

 posium, i.e. DNA, RNA, ribosomes, mitochondria, etc., concerns the 

 Uving cell as it was present on this earth a billion years ago, before 

 Darwinian evolution even started. In those dark ages, before there were 

 metazoa of any kind, the primary inventions of protein and nucleic acid 



Fig. I. Relative sizes of a sulphur-bottomed whale- 

 and the African elephant "Jumbo" (from Lull). 



-the largest living animal- 



synthesis were combined to etfect the miracle of replication. Once this had 

 been achieved, a second series of inventions could begin, inventions by 

 which replication could lead to differentiation and larger and larger multi- 

 cellular organisms arise. InterceWuhr chemistry in other words is needed 

 to explain how thirty quadrillion cells of about a hundred different types 

 co-operate to make a sulphur-bottomed whale (Fig. i). Embryologically, 

 of course, this vast number of cells is derived by clonal growth from a 

 single fertilized ovum. 



Basically, the problem is one of selective gene activation. Since the 

 nucleus of the fertilized egg contains all the genetic information needed to 

 make each of the final differentiated cells present in the adult body, it is 

 clear that only part of this information is used in any one cell. Take, for 

 example, the insulin-secreting cells of the Islets of Langerhans. Sanger 



