CHROMOMERES AND PROTEINS 



Chromotneres and Proteins 



There are, as we saw in the first chapter, two types of nucleus 

 which reveal their chromosome structure while they are still active 

 in producing proteins. They are the polytene and the pachytene 

 nuclei. In both of these, chromomeres appear. The explanation of 

 these chromomeres, advanced by Caspersson, is that the nucleic acid 

 is attached to the active genes whose products stretch the normally 

 folded thread between them. Their gradual emergence from a 

 homogeneous thread at the beginning of the prophase of meiosis 

 confirms this view (Fig, 36). 



Thus chromomeres are genes making themselves visible as 

 working units. This is not so in the diffuse resting nucleus where 

 nucleic acid is not attached and where proteins are pushed straight 

 outwards to give the appearance sometimes described as a lamp- 

 brush chromosome. In the polytene nuclei these products of gene 

 (or chromomere) activity can be observed. Heterochromatin, 

 which is active in these nuclei, produces simpler proteins of a histone 

 type, while euchromatin produces larger, more complex and 

 therefore, we must suppose, more specific proteins of a globulin 

 type. The chromomeres of heterochromatin are accordingly smaller 

 and less exactly paired than those of euchromatin. 



There are chromosomes which contain little but heterochromatin, 

 for example the Y chromosome in Drosophila, and supernumerary 

 chromosomes in maize and other plants. These appear to have little 

 specific effect on the organism. When extra ones are added, however, 

 other things being equal, the size of the nucleolus and the quantity 

 of nucleic acid in the cytoplasm are said to be increased. Such an 

 increase within the organism can be shown to go with increased 

 protein production. For example, ribose nucleic acid, which is 

 absent from actively fermenting yeast in the absence of a source of 

 nitrogen, immediately appears when nitrogen is made available and 

 proteins begin to be produced. 



Heterochromatin, ribose nucleotides, and protein production are 

 thus closely bound up together. Ribose nucleotides are abundant, 

 not only in embryonic and cancerous tissue, but also in the egg 

 cells of plants and animals where protein production is not 

 immediately taking place. But here, as Brachet has shown, it is 



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