8 THE CELL AS A PHYSICO-CHEMICAL UNIT 



contain several genes. According to the other theory, the 

 normal chromosome contains one or more polypeptide 

 chains extending the whole length of the chromosome. 

 The chromomeres, however, are believed to be not 

 additional material, but regions in which the main poly- 

 peptide chain is much folded upon itself. As in the first 

 theory, each chromomere is believed to consist of one 

 or more genes. 



As Stedman & Stedman and Mirsky have shown, 

 chromosomes consist of at least two types of protein, 

 the basic proteins known as histones and acidic proteins 

 which Stedman calls chromosomin, and also two types 

 of nucleic acid, one of which contains a pentose and the 

 other a desoxypentose. The information about the phy- 

 sical structure of chromosomes has been very largely 

 derived from studies of exceptionally large chromo- 

 somes, such as those found in certain plant cells, and 

 the giant salivary chromosomes of the Diptera. Which- 

 ever of the theories of chromosome structure may be 

 correct, it is certain that the total amount of matter in 

 the chromomeres is greater than that in the parts of the 

 chromosomes lying between the chromomeres. Cyto- 

 chemical studies have shown that by far the greatest 

 part of the purine and pyrimidine, the desoxy sugar, 

 tyrosine, histidine, tryptophane, and alkaline phospha- 

 tase are present in the chromomeres. The interbands are 

 relatively lacking in these substances. Since genes are 

 known to be located in the chromomeres, it seems pro- 



