pletely known. The most constant chemical fraction appears to be DNA. 

 This finding has played a major role in establishing the now widely held 

 concept that DNA is the most significant part of the chromosome geneti- 

 cally. The protein fraction of the chromosome is variable, both in 

 amount and kind, but in most cases it can be divided into basic and 

 nonbasic protein types. In addition to DNA, the chromosome can be 

 demonstrated to contain some RNA as well as lipid substances. 



Heterochromotin 



The chromosome is obviously a very complex organelle and no doubt 

 serves a number of functions other than the purely genetic in the ac- 

 cepted sense. Despite this complexity, with very few exceptions the 

 chromosomes are remarkably similar in all kinds of cells, tissues, and 

 organisms. As a result, cytologists and cytogenetisists have been forced 

 to look for different "kinds" of chromatin which may be revealed either 

 naturally or under experimental conditions in terms of morphological 

 differentiation. Two major kinds have been recognized generally, namely, 

 euchromatin and heterochromotin. This distinction is by no means a 

 happy one, for no single definition of either can be considered all- 

 embracing. From the genetic point of view, euchromatin is considered 

 to be that part of the chromosome which can be shown to contain genetic 

 material by classical methods of genetic analysis, while heterochromatin 

 represents segments of, or whole, chromosomes which are relatively 

 inert. Cytologists have attempted to distinguish these two types on a 

 morphological basis, usually by looking for differential staining. As has 

 already been noted, the chromosomes in interphase frequently show 

 segments which are deeply staining and markedly condensed. During 

 division, however, this distinction frequently disappears. Cytological 

 recognition of a distinction between heterochromatin and euchromatin 

 depends exclusively on the two kinds being out of phase with respect to 

 staining reaction at different stages of the mitotic cycle. Such discordance 

 is not always apparent and even when the distinction can be made fre- 

 quently there is no certainty that the cytological and genetic distinctions 

 are the same. Probably a reasonable definition of heterochromatin would 

 be that segment of a chromosome which can be deleted without produc- 

 ing any obvious phenotypic change. In many chromosomes, segments 

 near or adjacent to the kinetochore are covered by this definition and 

 are also frequently heteropycnotic in the purely cytological sense. For 

 example, in the X-chromosome of Drosophila melanogaster, all of the 

 short arm and about one-third of the proximal part of the long arm are 

 heterochromatic by any definition, as is almost all of the Y-chromosome 



90 / CHAPTER 4 



