and contain a smaller variety of amino acids than other types of proteins 

 such as the globulins and albumins. 



The basic proteins of the nucleus vary with the cell type and under 

 different physiological conditions. For example, in the salmon the nuclei 

 of both somatic and developing germ cells contain histones, while those 

 of the mature sperm cell in the same animal contain only protamine. 

 Whether this change is a conversion or a replacement process remains 

 to be determined. 



The electrochemical properties of DNA are such that it would be 

 expected to complex with both acid and basic proteins. Since both types 

 of proteins occur in the nucleus, it is somewhat surprising to find that in 

 some cases the ratio of DNA to basic protein approaches unity. There 

 is also fairly good correspondence between replication of basic protein 

 and DNA (Bloch and Godman, 1955). 



Acid, or at least tryptophane-containing proteins, have been demon- 

 strated in the nucleus, especially by Stedman and Stedman (1947) and 

 Mirsky and Ris (1947). The latter workers digested chromosomes iso- 

 lated from interphase nuclei and found that about 90 per cent of the 

 mass was accounted for by DNA and histone, with the remaining 10 per 

 cent persisting as a coiled nucleoprotein thread made up largely of a 

 tryptophane-containing protein, 2 to 3 per cent DNA and 12 to 14 per 

 cent RNA. It has been suggested that this thread constitutes the "back- 

 bone" or structural framework of the intact chromosome. The amount 

 of protein contained in these residual threads apparently differs greatly 

 between nuclei in different tissues of the same animal. For example, in 

 calf thymus chromosomes it constitutes some 8 per cent, while in calf 

 liver chromosomes the amount is as high as 39 per cent. Use of suitable 

 radioactive isotopes has indicated that the turnover of nuclear protein 

 is greatest in nondividing, metabolically active cells. This turnover seems 

 to be lower in the histones than in other proteins of the nucleus (AUfrey, 

 etal, 1955). 



In addition to the proteins mentioned above, the nucleus also contains 

 globulin protein which, in certain cases such as calf liver nuclei, may 

 be present to the extent of some 40 per cent (Dounce, 1955). The 

 nuclear location and functional significance of this protein have not been 

 determined. 



Lipids 



The nucleus contains significant amounts of lipid and in some cases 

 may account for as much as 40 per cent of the dry weight of the nucleus. 

 Phospholipids, in particular, have been shown to be present in the nucle- 



110 / CHAPTER 4 



