Chapter 40 



REGULATION OF GENE 

 ACTION-ITS MOLECULAR BASIS 

 IN HIGHER ORGANISMS 



N' 



one of the cases discussed in 

 Chapters 37 through 39 have 

 yet been analyzed far enough 

 to specify the biochemical elements of the 

 control mechanisms involved in the regula- 

 tion of gene activity. In this chapter, addi- 

 tional examples of gene action regulation in 

 higher organisms are explored in an effort 

 to gain some insight into its molecular basis. 



Dipteran Polynemic Chromosomes ' 



Evidence obtained from radioautography 

 strongly suggests that the ordinary chromo- 

 some of higher organisms is polynemic or 

 polytenic; that is, it contains more than one 

 double helix of DNA per chromatid.-' Inter- 

 phase chromosomes of larval Diptera which 

 are highly polynemic may be considered 

 merely more extreme examples of a normal 

 tendency toward polynemy. 



At various times during the growth and 

 differentiation of a dipteran cell containing 

 highly polynemic chromosomes, different 

 cross-bands "puff out" (Figure 40-1 ) and 

 later "unpuff" in a regular sequence. Al- 

 though the sequences vary, they are char- 

 acteristic of different larval tissues. Puffing 

 may be interpreted as a local unwinding of 

 the chromosome and its DNA. In Drosoph- 

 ila and the midge Chironomus, a puff region 



1 See W. Beermann (1962). W. Beermann and U. 

 Clever ( 1964), and H. J. Becker ( 1964). 

 -See W. J. Peacock (1963). 

 492 



synthesizes more RNA than an equivalent 

 Qonpuff region; in the salivary gland cells 

 of Rhyncosciara and Glyptotendipes larvae. 

 the amount of DNA synthesized in puffed 

 regions is greater than in equivalent non- 

 pulTcd regions. 



In certain species of Chironomus, the cyto- 

 plasm of cells in one lobe of the larval sali- 

 vary gland contains (protein secretion?) 

 granules due to a gene located near one end 

 of chromosome IV. Such granules are ab- 

 sent in other species. In cells forming gran- 

 ules, a puff is also found near the tip of 

 chromosome IV but none is found in non- 

 granule containing cells not even those of 

 the same gland. Moreover, larvae produced 

 by an interspecific mating of granule formers 

 and nonformers have some granules and are 

 cytogenetically hybrid; in other words, one 

 homolog does and the other does not have 

 this puff. 



Injecting larvae with the pupation hor- 

 mone ecdysone causes specific bands to puff 

 and others to unpuff. It has also been shown 

 that the RNA synthesized in a puff does not 

 have A = U or C = G (in terms of quan- 

 tity) and is probably messenger RNA. We 

 may conclude, therefore, that puffing (un- 

 winding DNA) is directly associated with 

 gene activity. In a dipteran salivary cell, 

 only about 20 per cent of the bands ever 

 seem to puff, which indicates that not all 

 genes are functional in every nucleus. 



Conserved vs. Nonconserved DNA 



What is the nature and fate of DNA syn- 

 thesized in "excess"? The following evi- 

 dence indicates that DNA can leave the 

 nucleus because it is in excess or for other 

 reasons: 



1. In certain organisms (for example, the 

 fungus gnat Sciara) some chromo- 

 somes are regularly eliminated from 

 the nuclei of certain cells. 



2. In Drosophila, DNA is extruded from 

 the nuclei of nurse cells in the ovary. 



