STRUCTURAL UNITS IN CELLULAR PHYSIOLOGY 



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cleoprotein and non-nucleoprotein parts, 

 respectively. Now consider a short ele- 

 ment of chromosome containing equal 

 amounts of nucleoprotein and non-nucleo- 

 protein sections (Fig. 4) ; it will attract a 

 similar section at any distance, but at short 

 range the energy of interaction will be per- 

 ceptibly different when like and unlike 

 parts come together. In the first case it 

 would be represented by - (a^ + h^) f (r) ; 

 in the second case, by -2ab/f{r). Now if 

 a and & are both positive, the energy in the 

 first case will always be absolutely greater 

 than in the second ; in other words, the de- 

 gree of binding will be closer if like fits 

 with like or vice versa. The principle is 

 the same physically as that accounting for 

 the immiscibility of oil and water. Water 

 molecules attract water molecules, and 

 they also attract oil molecules even' more 

 than oil molecules attract each other, but 

 the greater attraction between water mole- 

 cules for each other forces the oil out. 

 Granted this mechanism, the like-to-like 

 approximation of chromosomes becomes 

 the position of lowest energy, but since it 

 is only one of innumerable arrangements, 

 it would be so improbable as not to occur 

 if there were not a further mechanism. 

 The chromosomes as a whole are in move- 

 ment in the cell fluid, and if two like chro- 

 mosomes happen to touch at some point 

 where they have no homologous parts, we 

 must assume that the energy of interaction 

 is not sufficient for them to remain long in 

 this position; if, however, the parts are 

 homologous, the moment they touch all the 

 other parts up and down from the point of 

 contact will also be homologous, the process 

 of approximation will spread, and it will be 

 no longer possible for the thermal motion to 

 separate them (Fig. 4, C, D). This may be 

 called the zipper hypothesis of chromosome 

 pairing. There may indeed be a whole 

 series of zipper actions in the pairing of 

 chromosomes. Two ends with any nucleo- 

 protein will tend to line themselves up at 

 some distance apart. Later these will find 

 the appropriate places to enter into close 



union (Fig. 4, E, F). This explanation 

 will account not only for normal pairing 

 of chromosomes, but for abnormalities, 

 loops, inversions, etc., where the pairing 

 process is interrupted. It fails, however, 

 to give a satisfactory reason why pairing 

 is limited to two identical chromosomes, 

 such as occurs in polyploids. Possibly 

 further interactions occur after the con- 

 jugation of the two chromosomes which 

 prevent the process recurring. But spec- 

 ulation as to its nature would seem un- 

 warranted at this stage. 



The fundamental question of the molec- 

 ular structure of the chromosomes must 

 necessarily wait for its full elucidation on 

 the knowledge of protein structure. How- 

 ever, its marked similarity of chemical 

 composition to the viruses of the tobacco- 

 mosaic type suggests that it is also a long- 

 moleculed nucleoprotein with internal 

 crystalline structure. This would account 

 for its growth by addition of similar parts 

 of an identical nature at each point. But 

 how and why such an arrangement usually 

 becomes unstable when it has doubled its 

 original size, but does not always become 

 so, as in the salivary gland chromosomes, 

 is not easy to see. It probably depends on 

 the repulsive element of the long-distance 

 forces, but no system even remotely analo- 

 gous to it has yet been studied. 



The suggestions in this paper are put 

 forward with great reserve, on the basis of 

 what is at present quite insufficient evi- 

 dence. It is not my intention to claim that 

 they are the explanations of these facts, 

 but rather to show that we have in the new 

 physicochemical knowledge a way of pro- 

 viding general explanations for biological 

 phenomena. A closer study of biological 

 material in the light of the newer physical 

 knowledge may give rise to other and more 

 valid and verifiable hypotheses. My main 

 object will have been achieved if I have 

 shown that these methods have some con- 

 tribution to make and that it would be un- 

 wise to ignore them in the attempt to 

 understand cellular mechanisms. 



