200 SUBCELLULAR PARTICLES 



the living nucleus are DNA-dependent. The question which now arises concerns 

 the significance and interpretation of DNA substitution experiments. 



It has been seen that it is possible to remove the greater part (up to 76%) of 

 the DNA of isolated thymus nuclei and to substitute other negatively charged 

 molecules (such as polyadenylic acid) for the missing nucleic acid. This substitu- 

 tion, if properly carried out, yields a nucleus which again shows a capacity for 

 ATP synthesis, amino acid incorporation into protein, and adenosine uptake into 

 RNA. In tests of amino acid or adenosine uptake the substitution of the DNA by 

 other polyelectrolytes is effective only in the case of polyanions. Added polycations, 

 such as polylysine and protamine, do not restore function to nuclei depleted of 

 their DNA. On the contrary, a polylysine supplement will actually inhibit amino 

 acid uptake in nuclei which retain their DNA. Much of this uptake represents 

 incorporation into the proteins of the chromosome (6), and the dependence of 

 uptake on charge suggests that electrical charge is one of the main variables 

 involved in chromosome activity. In line with Dr. McLaren's suggestions, it may 

 be that a zone of decreasing pH, associated with negative surface charge, plays 

 a role in regulating the biochemical activity of the chromosome. 



One aspect of the DNA substitution experiments which is important to their 

 interpretation concerns the role of the residual DNA which remains in the nuclei 

 even after prolonged DNAase treatment. Much of this residue has been depoly- 

 merized and is soluble in acid, but 5-15 per cent of the original DNA may remain 

 insoluble in acid. Because this polynucleotide residue remains one cannot conclude, 

 on the basis of substitution experiments, that any polynucleotide is as effective as 

 DNA in stabilizing the structure of the chromosome or in mediating chromosome 

 function. Yet it is surprising that, given an organized nucleus, more than 75 per 

 cent of the DNA can be replaced by comparatively simple molecules such as poly- 

 adenylic acid, and that 'hybrid' nuclei, so constituted, are still capable of carrying 

 out at least three fundamental nuclear activities. 



DEPENDENCE OF ACTIVITY OF ISOLATED NUCLEUS ON ITS CHEMICAL ENVIRONMENT 



In addition to its value in studying the function and synthesis of the nucleic 

 acids, the isolated thymus nucleus has considerable utility as a test system for 

 studying the effects of a changing chemical environment on nuclear function. 

 Apart from testing the effects of inhibitors of protein or nucleic acid synthesis, 

 (which will not be discussed here), the nuclear response to changes in the sus- 

 pension medium indicates some interesting requirements for its biochemical ac- 

 tivity. 



One of the most important variables in controlling nuclear syntheses is the 

 osmotic concentration of the medium. It was soon found that nuclei exposed to 

 high sucrose concentrations lost their ability to incorporate amino acids into 

 protein. A more detailed picture of this dependence of uptake upon sucrose con- 



