184 BO THORELL 



reduced in size as compared with the original isolated chromosomes. They 

 consist of 80 % high-molecular-weight protein containing inter alia 1 .36 % 

 tryptophan and resembling the "chromosomin" isolated from nuclei by 

 Stedman and Stedman.'^ This may perhaps represent the structural back- 

 bone of the chromosomes which is of importance for the cytogenetical 

 phenomenon of breakage induced, e.g., by radiation. ^^ The concept of a 

 structural function is consistent with the trypsin digestion experiments 

 mentioned later in Section II.2.6. The PNA content of the residual chromo- 

 somes varies between 7.5 to 14 %. Some DNA is also present (1.5 to 2.6 %). 

 Compared with the whole isolated chromosomes, the nucleic acid content 

 of the "residual chromosomes" is thus relatively small in amount and 

 mainly PNA in type. 



It should be noted that the proportions of the various constituents 

 analyzed as mentioned above seem to vary in isolated chromosomes from 

 different tissues. A larger proportion (40 to 50%) of chromosomes from 

 liver consists of "residual chromosomes" as compared with chromosomes 

 from erythrocyte nuclei (5%). Consequently liver chromosomes contain 

 more PNA (12%) than thymus (3%) or trout sperm (0.15%) chromo- 

 somes. Probably this reflects the different functional states of the respec- 

 tive nuclear material. Changes in the DNA content of isolated chromosomes 

 from different stages during experimental carcinogenesis have been re- 

 ported.'^ 



It is generally believed that all of the highly polymerized DNA of the 

 cell is contained in the chromosomes and that each complete set of chromo- 

 somes contains a constant amount of DNA (see Chapter 19). On the other 

 hand, if the analytical values for PNA from isolated chromosomes are 

 compared with those obtained from isolated nuclei, '° it will be seen that in 

 liver nuclei there must be some extrachromosomal PNA provided that no 

 PNA is lost during the preparation procedure. A part of this is probably 

 bound to the nucleolus, the morphology of which varies considerably with 

 different functional states. The physiological importance of this observa- 

 tion is still however obscure. The nucleic acids of the nucleolus are discussed 

 in Chapter 18. 



Summarizing, it may be said that the analysis of isolated chromosomes 

 has given a picture of the main quantitative composition of these struc- 

 tures, but the informations obtained must be regarded with caution owing 

 to the general pitfalls associated with the isolation in bulk of a particular 

 cellular structure. 



'* E. Stedman and E. Stedman, Nature 152, 267 (1943) ; Cold Spring Harbor Symposia 



Quant. Biol. 12, 224 (1947). 

 " A. H. Sparrow, M. J. Moses, and R. J. DuBow, Exptl. Cell. Research Suppl. 2, 245 



(1952). 

 '^ A. R. Gopal-Ayengar and E. V. Cowdry, Cancer Research 7, 1 (1947). 



