GENETIC CONTROL OF CELL INTEGRATION 



317 



cvtologists to ansv\er this question have been severely limited by the in- 

 visibihty of the chromosomes, except during mitosis and meiosis, under 

 the Hght microscope. As yet only the polytene chromosomes in certain 

 tissues of some insect larvae, which remain visible throughout larval 

 development, have provided material for a detailed analysis of intra- 

 nuclear syntheses of DNA, RNA, and protein along the chromosomes in 

 interphase. These chromosomes are magnificent with respect to size, 

 morphological detail, correlation with genetic analysis (in Drosophila), 

 and accessibility for short-term labeling experiments. Their principal 

 shortcoming is that they occur in larval tissues which degenerate with 

 the onset of pupation. The close correlation of salivary gland chromo- 

 some cytology with genetic events in Drosophila has provided a firm basis 

 for viewing the salivary-gland chromosome as a trustworthy mirror, re- 

 flecting such changes as chromosomal rearrangements which have oc- 

 curred. In using these chromosomes to study macromolecular synthesis, 

 however, the investigator makes a new inference, namely, that in late 

 larval development, these chromosomes are still actively integrated in the 

 metabolism of the organism, and reflect its synthetic capacities. 



5 6 7 



Time (hours) 



FIGURE 1 1 .6. Incorporation of adenine-C '^ into the RNA of cell fractions of develop- 

 ing Drosophila larvae. Determinations of total RNA by photometric methods and of 

 new RNA synthesis by grain counts of autoradiographs of C'* incorporated into RNA, 

 made possible an estimate of relative specific activity of radioactive RNA in various cell 

 fractions in the salivary gland of Drosophila replefa larvae. The resulting data demon- 

 strates that uptake into the nucleolus precedes and exceeds that into the chromosomes, 

 which are well separated in this favorable material. The label appears in the cyto- 

 plasm considerably later (from Taylor and Woods, 1959, in: Hayashi, Subcellular 

 Particles, Ronald Press, p. 172). 



