394 AN INTRODUCTION TO MODERN GENETICS 



that in globular proteins, the same or very similar polypeptide chains 

 exist in a folded configuration, so that extension of a fibre constructed 

 of globular protein involves only the unfolding of the chains and can 

 proceed much farther before the fibre is ruptured. Duryee^ has shown 

 that the lampbrush chromosomes of amphibian oocytes (p. loi) can 

 be reversibly extended to about 3 J times their normal length, at least 

 under favourable conditions (in absence of calcium or other heavy 

 metallic ions). Salivary gland chromosomes easily stretch to at least 

 twice their length, and probably can be stretched farther when special 

 efforts are made to do so by microdissection methods. Thus even in 

 chromosomes in which the chromosome thread or chromonema is 

 apparently uncoiled, the thread itself has considerable elasticity, and 

 may perhaps be constructed of globular proteins in which the poly- 

 peptide chains are folded on a molecular scale. Much further study is 

 required, however, before this can be taken as more than a suggestion. 

 When we turn to consider the other main constituent of chromo- 

 somes, the nucleic acid, a series of facts emerge which are extremely 

 suggestive of an essential connection between nucleic acid and proteins , 

 but whose exact significance cannot yet be stated. Nucleic acid itself 

 easily forms fibres, and X-ray studies^ have shown that these consist 

 of a chain of phosphoric acid residues to the side of which are attached 

 a series of flat, plate-shaped groups each of which contains a purine 

 base attached to a sugar. The first remarkable fact is that the repeat 

 distance along the chain, i.e. the distance between neighbouring phos- 

 phoric acid residues, is almost exactly the same as the repeat distance 

 in a polypeptide chain; 0-336 for the nucleic acid, o-334m^ for the 

 pol5^eptide. The difference, which may not be significant, is at least 

 so small that it is easy to imagine that the polypeptide and nucleic acid 

 chains might unite parallel to one another to give protein-nucleate 

 chains. This can in fact actually be observed; Astbury^ has prepared 

 the nucleate of clupein, the protein isolated from fish sperm, and 

 shown that it is a fibrous material. Further confirmation comes from 

 a study of the double refraction. Protein fibres have a somewhat weak 

 double refraction which is positive in the direction of the fibre, while 

 nucleic acid, in which there are large flat plate-like groups sticking out 

 at right angles to the length of the fibres, has a much stronger double 

 refraction which is negative in the direction of the fibre axis. The 

 clupein-nucleate shows a double refraction negative in the fibre direc- 



^ Duryee 1938. 



2 Astbury and Bell 1938. ^ Cf. C. S. H. Symp. 1938. 



