244 The Nature of Biological Diversity 



The construction manual (to continvie the analogy) is "read" by 

 the individual cells, which in all but the earliest embryonic stages 

 are numbered in the millions. From a fairly early stage in embryonic 

 development we can separate these cells into a series of cytological 

 types — muscle, nerve, epithelium, and so on. What begins as a unity 

 rapidly becomes a multiplicity of vastly different forms and functions. 



All these cells, as far as we know, continue to possess a full set of 

 genes, corresponding to the construction manual for the organism as 

 a whole. And yet, during the course of embryonic development, the 

 construction "crew" resolves itself into clones of diverse cell types, 

 which (in the manner of bricklayers, plumbers, and electricians) 

 make use of only their own specific assignments in the construction 

 manual. 



The central dogma 



In the interpretation of these happenings we come face-to-face with 

 a series of concepts which everyone will recognize as a central dogma 

 of present-day biology. At the risk of recapitulating what has gone 

 before, let me briefly review this theory. 



The hereditary information of the genes is coded in the nucleotide 

 sequence of DNA, presumably in the form of a series of three-letter 

 words made up of the letters A, T, G, and C. At any one time, much 

 and perhaps most of this genetic information is not at the disposal of 

 the synthetic centers of the cell. In one way or another a certain 

 fraction of the DNA is repressed and therefore unexpressed. Under 

 appropriate conditions, specific genes or gene combinations are de- 

 repressed. At these particularly activated sites, the coded information 

 of the DNA is transferred to newly synthesized RNA in the form of a 

 series of three-letter words made up of the letters A, U, G, and C. This 

 so-called "messenger RNA" then passes to the cytoplasm. It enters 

 the ribosomes and each of its three-letter words is used to code the 

 precise position of one of the 20 kinds of amino acids in newly syn- 

 thesized protein. Once the amino acids have been properly positioned 

 in the peptide chain, the molecules of protein automatically fold into 

 their distinctive so-called "tertiary structure." The protein thereby 

 attains its characteristic enzymatic activity, as dictated by its amino 

 acid sequence and overall shape. And since most, if not all, of the 

 reactions of the cell are catalyzed by enzymes, it is alleged that all 

 happenings in the cell are a sequel to the synthesis of specific en- 

 zymes. 



