. Eredità di cella. Citogenetica. Ereditarietà IN CELLULE SOMATICHE 381 Figura 12.12. Formazione di /3-galattosidasi in E. co/i in risposta a differenti concen- trotions di induttore (da Cohn, in: McElroy e vetro (eds.), la base chimica di sviluppare- ment, Baltimora Johns Hopkins Press, p. 458).. 50 100 aumento della massa batterica (brocca x ml^), in una moda autocatalitica. A bassi livelli di induttore, ogni cella ha una piccola probabilità costante di formando una molecola di permease; una volta che si esegue il processo di induzione va in modo esponenziale per il completamento. Il processo di induzione enzimatica è sotto queste c

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. Cell heredity. Cytogenetics. HEREDITY IN SOMATIC CELLS 381 FIGURE 12.12. Formation of /3-galactosidase in E. co/i in response to different concen- trotions of inducer (from Cohn, in: McElroy and Glass (eds.), The Chemical Basis of Develop- ment, Baltimore, The Johns Hopkins Press, p. 458).. 50 100 Increase in Bacterial Mass (jug x ml~^) on, in an autocatalytic fashion. At low levels of inducer, every cell has a small constant probability of forming a molecule of permease; once it does, the process of induction goes exponentially to completion. The process of enzyme induction is under these conditions an all-or-none process resembling mutation, and is transmitted cytoplasmically to the daughter cells as long as the inducer is present in the environment. This is a good example of an autocatalytic mechanism determining a hereditary difference between cells and independent of any change in chromosomal genes or extrachromosomal particles other than the enzymes themselves. It seems likely that mechanisms involving reversible expression of phenotype are the best candidates for the origin of the orderly variation in cell type that leads to normal development. It is conceivable that almost all cells respond to a stimulus from the environment through a nonchromosomal mechanism. Although the possibility must be left open, it seems less likely that cells could behave in such a uniform way through chromosomal mechanisms which may rather give rise to path- ological processes, initiated nonuniformly in single cells. It should not be assumed, however, that those types of variation that are random in their origin need give rise to a disorderly result. The necessary order can be achieved by the cellular environment; population change may be. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Sager, Ruth; Ryan, Fran

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• bookcentury1900
• bookcollectionbiodiv
• bookpublishernewyorkwiley