Extranuclear Genes and Nuclear Genes 



413 



have been part of the normal gene content of 

 a cell? 



This question is particularly pertinent when 

 viruses are considered. From what has been 

 learned in previous Chapters, we can no 

 longer retain any preconceived notions either 

 that viruses were always foreign infective 

 agents, or that all presently known viruses 

 are of this nature. Present-day virulent 

 phages seem to be acting as foreign organisms 

 when they lyse their bacterial hosts. But, 

 the lytic capacity of phage depends upon its 

 genotype and that of its host, so that under 

 some genotypic conditions lysis is relatively 

 rare. The decision as to normality or 

 abnormality of present-day viruses is even 

 more difficult when temperate phages are 

 considered, for not only are they less lytic 

 yet still capable of transduction, but the very 

 genes characterizing their prophages seem 

 to be associated with a part of a normal 

 chromosome. As we learn more about 

 viruses, particularly phage, our understanding 

 of what is now genetically normal, and what 

 is foreign, is undergoing radical revision.^ 

 With the future increase in our knowledge of 

 the genetics of viruses and their "hosts," we 

 will also be in a better position to postulate 

 how they originated. 



Let us continue our search for other 

 extranuclear genes, restricting our attention 

 now to cytoplasmic components which seem 

 to be normal constituents of present-day 

 cells, even though we shall make no decision 

 as to their normality when they, or their 

 precursors, first arose. 



Many plant cells contain cytoplasmic 

 bodies called plastids, which when green 

 because of the presence of chlorophyll are 

 called chloroplasts, and when white are called 

 leucoplasts. In the absence of sunlight, 

 chloroplasts lose their pigment and become 

 leucoplasts, the process being reversed when 

 the plastids are again exposed to sunlight. 



2 See A. Campbell (1961). 



Chromosomal genes are known in corn 

 whose mutants affect the production of 

 chlorophyll in plastids by interfering with the 

 sequence of reactions leading to chlorophyll 

 production. For example, such a nuclear 

 gene may prevent plastids from producing any 

 chlorophyll at all; thus there is a type of 

 leucoplast which is incapable of becoming 

 green for this reason. If a seedling possesses 

 the appropriate nuclear genotype, it will be 

 nongreen; it will grow until it exhausts the 

 food supply in the seed, then die because in 

 the absence of chlorophyll there is no photo- 

 synthesis and no sugar manufactured. Such 

 nuclear genes act as lethals when they produce 

 albino seedlings. 



Corn plants are also known whose leaves 

 are mosaic, having stripes of green and white 

 (Figure 45-2), the white parts having only 

 leucoplasts incapable of becoming green. ^ 

 The white portions of the leaf survive be- 

 cause they receive nourishment from the 

 green parts. What is the basis for this 

 mosaicism? Is it due to nuclear genes 

 causing different paths of differentiation in 

 different portions of the leaf? 



It is observed that the striping occurs not 

 only within the leaves but also elsewhere, and 

 seems to extend even into the reproductive 

 organs, so that pollen and eggs can be ob- 

 tained, derived both from green and from 

 white parts. This may permit us to determine 

 the answer to the question last posed. For, 

 if the striping is due to a nuclear gene acting 

 upon differentiation, such a gene should be 

 transmissible through the male or female 

 gamete, without relation to the whiteness or 

 greenness of the tissue giving rise to the 

 reproductive structures. 



What is done is to obtain an ear of corn 

 derived from an ovary which was likely to be 

 mosaic, having originated partly from green 

 and partly from white tissue. The kernels in 

 such an ear are the Fi, and are grown in rows 



^ The following account is based primarily upon 

 work of M. M. Rhoades. 



