COMPAHATIVE PHYSIOLOGY AND BIOCHEMISTRY 



Fish sperm comprised the experimental material for the first studies on 

 the chemical constitution of the chromosomes, but subsequent studies on 

 such diverse materials as yeast and mammalian liver cells have all led to 

 similar conclusions. It appears that, throughout the entire living world, 

 the chromosomes consist of basic proteins in combination with nucleic 

 acid. Histone and protamine, which are the simplest types of proteins, 

 predominate, but globulin and some incompletely identified proteins are 

 also present. It may be that these more complex proteins contribute to 

 the diversity of the hereditary material in greater proportion than their 

 quantity would indicate. The nucleic acids are also rather uniform. The 

 basic unit of structure, or nucleotide, consists of phosphoric acid, a pen- 

 tose (a sugar based upon a five-carbon chain), and a purine or pyrimidine 

 base. These units are highly polymerized ( like molecules joined together 

 to form much larger molecules ) , to form long, double, spiral chains joined 

 to each other by weak bonds between the bases ( see Figure 87, p. 228 ) . 

 Nucleic acids differ from one another chiefly in the sequence of base pairs 

 which join together the nucleotide chains, and much evidence now indi- 

 cates that the specificity of the gene (the unit of inheritance: see Chapter 

 13 ) depends upon this trait also. In view of the great diversity of organ- 

 isms, all of which owe their attributes to the chromosomes which they 

 possess, it is astonishing that the chromosomes themselves should have 

 so uniform a constitution. But this is most readily understood in the same 

 light as is the general unity of protoplasm. 



Enzymes gnd Hormones. Again, closely similar or identical enzymes 

 and hormones are likely to be common to large groups of animals. This is 

 especially true of some of the digestive enzymes. Trypsin, the protein- 

 splitting enzyme, is found in many groups of animals ranging from the 

 Protozoa to the Mammalia. Amylase, the starch-splitting enzyme, is found 

 from sponges to man. The thyroid hormone is found in all vertebrates, and 

 it has been proved to be interchangeable among them. It is well known 

 that beef thyroid is used successfully in the treatment of human thyroid 

 deficiencies. This hormone is also essential for the metamorphosis of 

 frogs. If a frog's thyroid gland is removed surgically, the frog will not 

 metamorphose. Yet feeding of mammalian thyroid tissue to such a frog 

 will correct the deficiency and permit metamorphosis. Even more striking 

 is the case of the melanophore-expanding hormone of amphibians. This 

 pituitary hormone causes the pigmented cells of the skin to expand, thus 

 darkening the color of the animal. It has no known efiFect in mammals, yet 

 an extract of mammalian pituitary glands is just as effective in amphibians 

 as is their own hormone. Thus the melanophore-expanding hormone of 

 mammals might be regarded as a "vestigial" hormone, the presence of 

 which is understandable only on the basis of descent from an ancestor to 

 which the hormone was useful. 



Yet vertebrate hormones are variable. It has long been known that the 

 pituitary growth hormone is not interchangeable among mammals. Beef 

 hormone, for example, is not effective in treating deficiencies of this sub- 

 stance in children. Analysis of growth hormones, isolated and purified 

 from various mammals, has revealed small but functionally significant dif- 



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