NONCHROMOSOMAL GENES 269 



orange, induce mutations to nonchromosomal petites, loss of the F factor 

 in E. coli, and in one study, loss of the kinetoplast in a trypanosome. 

 Streptomycin induces loss of chlorophyll-forming ability in many plants. 



No critical evidence has been found as yet to support the super- 

 template hypothesis, but considerations of the highly organized state of 

 chloroplast and mitochondrial membranes and the low content (if any) 

 of RNA, as well as the observed origin of these organelles by splitting 

 from pre-existing ones, have led to the prediction that a novel mechanism 

 of replication may be involved. In kinetosomes and other basal bodies, 

 the presence of nucleoprotein indicates that these bodies may represent a 

 sort of extranuclear chromosome. 



Infectious particles such as kappa, which are not well integrated with 

 the cell and can multiply at an asynchronous rate, represent one extreme 

 of a group of viruslike particles which are infectious and nonmapable. 

 In examples like the determinants of CO2 sensitivity and sex ratio in 

 Drusophila, neither their relation to normal cell constituents nor their 

 indispensability is known. Lysogenic phages and F factors in E. coli 

 have both chromosomal and nonchromosomal states, but no particles 

 with this property have been identified in other systems. Because of 

 their infectivity, this class of determinants can be most readily identified, 

 but eyen here little information is yet available about the chemical 

 constitution, mode of replication, and cell localization of most of the 

 systems of this type. 



Much more difficult methodological problems are involved in distin- 

 guishing between steady-state systems and those determined by replicat- 

 ing nonchromosomal genes. Catcheside has referred to the "conditional 

 stability of the steady state,' which we can contrast with the "un- 

 conditional stability of a gene. How can we sort them out experi- 

 mentally? The physical or chemical characterization of the determinant 

 is required, but has not been achieved in any of these systems as yet. 

 In our view, such systems as those determining nonchromosomal male 

 sterility, streptomycin resistance, and cytochrome abnormalities probably 

 represent examples of '"genes without chromosomes rather than steady- 

 state systems, because of their ""unconditional stability regardless of 

 environment, state of expression, phenotypic interaction with other 

 genes, or cellular reorganization following fertilization and meiosis. 



Two systems have been described which may illustrate the inheritance 

 of a steady state. One involves antigenicity in Paramecium, and the 

 other, maintenance of a particular rate of enzyme formation in E. coli. 

 Their properties differ sharply from those discussed in this chapter; they 

 will be considered in Chapters 11 and 13. 



