THE MITOTIC CYCLE * 



Structure and composition 



Several lines of research have given information on the structure and 

 composition of the nuclear membrane. It has been shown in several 

 instances that it bears a positive charge (Dahlgren;^^^ Churney and 

 Klein/^^) and that the isoelectric point of its proteins is far to the 

 alkahne side of the physiological jf?H (Churney^^^). The reverse is true 

 in both instances for chromatin. The nuclear membrane must therefore 

 in part consist of a protein-anion complex, with a high proportion of 

 basic amino acids. An observation on cultivated chick and mammalian 

 cells in mitosis may be related to this. It is striking that at the end of 

 prophase, the nucleoli and the nuclear membrane suddenly disappear, 

 always at exactly the same moment. The nucleoli are known to contain 

 basic proteins (p 39) ; probably some event within the nucleus, per- 

 haps the liberation of an enzyme, causes similar material in both 

 structures to be dissolved at the same time. 



Churney^^^ has found that unlike the cell membrane, a torn nuclear 

 membrane is not 'repaired' in the presence of calcium because a calcium- 

 proteinate cannot be formed on the acid side of an isoelectric point. It 

 is thus improbable that the ash which is seen at the site of the nuclear 

 membrane in cells submitted to micro-incineration (Scott^^^) can mean 

 that the living structure has a high mineral content. The location of 

 cations within the cell is all too readily disturbed by fixation, and it is 

 probable that the positive reaction for potassium in the nuclear mem- 

 brane found by Bureau^^^ is also an artefact. 



Some experiments of Luyet and Ernst^®^ have shown that the 

 nuclear membrane of the cells of the onion root has marked plastic 

 properties. These authors centrifuged whole roots at a force of 30,000 x g 

 for periods of from 10 minutes to 2 hours, and found that the nuclei 

 moved to one wall of the cells, against which they were flattened, while 

 their opposite surfaces were drawn out into bottlenecks. These changes 

 must have involved a considerable increase in nuclear surface. The fine 

 structure of the nuclear membrane has been investigated both by the 

 polarizing and the electron microscope. The sign of its birefringence is 

 that of fibrous protein; Baud^'" was able to observe this by precipitat- 

 ing on the nuclear membrane antimony sulphide in the same orienta- 

 tion. Callan^'i found that the membrane of germinal vesicles of 

 Triton became strongly birefringent when dilated. Callan and 

 TomlinI'2 have pubhshed an elegant study of the oocyte nuclear 

 membrane of Triturus and Xenopus by electron micrography. It consists 

 of an outer porous layer, and an inner one, apparently continuous, 

 which is the thinner of the two. The porous layer apparently contains 

 some lipoidal material. It would be of very great interest to compare 

 the structure of this membrane with those of normal somatic nuclei, 



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