28-6 THE YEAST CELL 



deficient, may transmit competent genes if well nourished but in- 

 competent genes if poorly nourished. 



Any environmental change alters the metabolic activity of the 

 cell. If the environmental change is within certain limits the cell 

 adapts to it and the alteration of metabolic activity is corrected by 

 an adaptive counteraction. Similarly if a nutrilite is presented which 

 the cell is unprepared to utilize, the cell adapts its metabolism to 

 the utilization of this material. The sensitivity of the gene to de- 

 grading influences suggests that it is easily affected by the environ- 

 ment and that the site at which both degrading and adapting substances 

 or influences finally produce their effects is the locus of the gene. 

 It seems probable tiiat the effect of a gene (in producing a specific 

 enzyme, for example) is the result of a long series of successive 

 reactions, rather than a single step reaction. We conceive of the 

 effect of the environment as also involving a series of steps before 

 the gene is ultimately affected. Therefore, a two-way path to and 

 from the gene is imagined in which effects can be produced by al- 

 terations occurring at any one of a number of points. 



The so-called environmental modifications (which are supposed- 

 ly temporary since they are not transmitted to sexual progeny) may 

 be the result of changes made at the locus itself; we have shown 

 that similar changes can be corrected in the sexual cycle. Permanent 

 modifications, which could be corrected by sexual reproduction, would 

 appear to be temporary if each new generation could only be produced 

 by sexual reproduction, as is the case in maize and Drosophila. 



UNEAR DIFFERENTIATION ALONG THE CHROMOSOME 



The discussion thus far has considered the chromosome as a 

 structure differentiated in cross -section with the central strand com- 

 prising (1) the locus (2) the chromogene which may be transferred 

 from one locus to another and which produces (3) the specific gene- 

 product. This structure comprises a single gene. I shall now pro- 

 pose that a second degree of complexity also exists and that single 

 genes, which are identified by their ability to produce a single gene- 

 product, are multipartite along the long axis of the chromosome as 

 well as in cross -section. What appears to be a Mendelian gene may 

 be a point on the chromosome at which a single step of the synthesis 

 of a gene -product (whose synthesis may require many steps) has 

 been deleted. The sjmthesis could proceed by a series of reactions 

 controlled by a series of catalysts placed in orderly sequence along 

 the chromosome, which pass the partially completed gene -product 

 from one catalyst to another in assembly-line fashion, until it is com- 

 pleted and leaves the chromosome. A Mendelian gene may corres- 

 pond to a point along this series at which a specific catalyst has been 

 lost or reduced in quantity. The chromosome is imagined to consist 



