180 CELL HEREDITY 



about 9 per cent of a nonhistone, tryptophane-containing protein which 

 remains insoluble after extracting much of the DNA and histone, and 

 which they called residual protein. They considered it the backbone 

 protein to which the DNA and histone attach. Other work, especially 

 with enzymatic hydrolysis, supports the view that neither the DNA nor 

 the histone are essential constituents of the backbone, because the re- 

 moval of either one does not disintegrate the structure. However, some 

 investigators view the chromosome as a complex with no single constitu- 

 ent comprising the framework for the others. 



Chromosomes also contain RNA, but its content per nucleus has been 

 found to vary greatly in cells of different tissues and from one organism 

 to another. Values which have been reported include both chromosomal 

 and nonchromosomal RNAs, because as yet no satisfactory techniques 

 have been developed for isolation of the various nuclear components. 

 Information about chromosomal RNA has come primarily from cyto- 

 chemical studies, which have demonstrated cyclical fluctuations in the 

 amounts of RNA bound to the chromosome. There is no definite evi- 

 dence whether any RNA is a permanent chromosomal constituent. (See 

 Chapter 11 for further discussion.) 



Generally, chromosomes are visible only in the division stages of 

 mitosis and meiosis. During interphase the nucleus appears optically 

 homogeneous. However, the presence of DNA in these nuclei has been 

 demonstrated by UV microscopy, by staining, and by chemical extraction. 

 Why cannot the chromosomes of interphase nuclei readily be seen with the 

 light microscope? As chromosomes become visible in early prophase, 

 they appear as long, thin fibers; metaphase chromosomes are short, thick 

 rods. The transition is achieved by coiling. In large meiotic chromo- 

 somes, such as those of the plant Tradescantia, one can see two orders 

 of gyres, and there the mechanism of chromosome condensation by 

 coiling is apparent. It is tempting to assume that submicroscopic 

 coiling occurs too, and that the visible chromosome may be simply the 

 condensed product of many ranks of coils, sufficient to bring the chromo- 

 some from a diameter of less than 100 A to that of a metaphase chromo- 

 some 1000 times thicker. 



ELECTRON MICROSCOPE STUDIES OF CHROMOSOMES 



Studies with the electron microscope are just beginning to fill the gap 

 between light microscope observations and physical-chemical studies of 

 the molecular composition of the chromosome. Observations are not 

 yet adequate to provide an understanding of chromosome structure. 



