204 ESSENTIALS OF BIOLOGY 



Typically they are single or double rods that may appear to be 

 made up of single or double rows of granules, and such a structure 

 is assumed even when it cannot always be observed. When we 

 trace a cell throughout a series of activities the chromosomes may 

 appear to lose their identity, ceasing to " take the stain " (and 

 thus ceasing to be the chemical individual called chromatin). 

 But since the typical numbers and approximate shapes of the 

 chromosomes may reappear in further phases of the activity of 

 the cell it has been customary to assume their continued existence, 

 even though their substance appears to undergo chemical decom- 

 position. Thus a certain morphology, additional to that which 

 can really be observed, is assumed of the chromosomes. 



We return to these matters in later chapters and it is important 

 now to treat of 



73 fl. The Chemistry of " Chromatin." Chemical tests 

 cannot yet be applied to the chromatin of a single cell, but some 

 animal tissues such as the ripe testis of a fish, or the thymus gland 

 of a mammal, are made up of very small cells with relatively 

 large nuclei. In mass, then, these tissues consist predominantly 

 of chromatin and the latter substance can be isolated from a large 

 quantity of the tissue and so can be examined in a. nearly pure 

 condition. 



Chemically, *' chromatin " is nuclein, which itself is a salt of 

 nucleic acid. What the cytologists see when they examine 

 " chromatin " is the nucleic acid combined with some basic 

 substance which is used as a '' fixative." In the natural state, 

 in the nucleus, the nucleic acid is combined with the (or a) 

 protein called protamine. Protamine is one of the simplest of 

 proteins and it is known to be composed of a chain of tripeptides, 

 thus : 



I'arginine^ ^arginine^ ("arginine' 



etc. — . valine ^ serine r proline 



(arginine) (arginine) (arginine 



These groups within the brackets are tripeptides and there may 



be about half a dozen of them in the molecule of a protamine. 



The substances, arginine, etc., are amino-acids and arginine is, 



NH H NH2 



II I I 



NHo - C - N - CH2 - CH2 - CH2 - C - C - OH. 



I II 

 H O 



