The Emergence of Order in Developing Systems

Our major consideration in this symposium will be the emergence of order during cellular differentiation and growth. The concept "emerging order" implies an organized, genetically complex process taking place over a reasonably extended stretch of time. In contrast, the restatement of linear genetic information in the form of threedimensional protein structure results from a rapid and spontaneous interaction of amino acid side chains with each other, with the completed polypeptide backbone, and with the environment, without the necessity for additional genetic information (Anfinsen, 1967; Epstein et al., 1963). The achievement of this unique geometry might be visualized as a rather helter-skelter process. An almost infinite number of sets of interactions are possible as an extended polypeptide chain coils upon itself (Fig. 1). If the process of folding involved even a small fraction of this number of conformational states, the specific folding of the chain could clearly require considerable time. It is probable that the rapidity of folding is made possible through the formation of one or more "nucleation sites" by side chain interactions that would predispose, during subsequent interactions, to the tertiary structural characteristics of the native structure. The only obvious driving force during this approach to native conformation is the selection of progressively more stable conformations with ultimate fixation of geometry in the form possessing the most favorable free energy of conformation, the native protein. Thus, unlike the complex predetermined pattern of successive changes occurring during differentiation, the cell must rely, in its first steps of development, on a relatively random process but involving explicit information—the amino acid sequence of a polypeptide chain.