Cell Division 



97 



ter that mitoses appeared and the average 

 amount of DNA per nucleus returned to the 

 value of normal liver. 



Another interesting property of DNA sug- 

 gested an independent appx'oach to the prob- 

 lem. Experiments with radioactive phos- 

 phorus (P^-) had shov^^n an extremely low 

 turnover of the phosphate of DNA in tis- 

 sues with no or few dividing cells (e.g., 

 Hevesy, '48). Apparently P^- is incorporated 

 into DNA only when this is synthesized dur- 

 ing chromosome reproduction. Howard and 

 Pelc ('51a) using an ingenious radioauto- 

 graphic technique of high resolution studied 

 the uptake of P^- into the DNA of chromo- 

 somes during mitosis in the root tips of Vicia. 

 Their results are in complete agreement with 

 the cytochemical and chemical studies. P^- 

 is incorporated into chromosomes only during 

 interphase before mitosis, not during mitosis 

 proper or in interphase of differentiated non- 

 dividing cells. 



In disagreement with the work discussed 

 so far is the view held by Lison and Pasteels 

 (see Swift, '53), who made absorption meas- 

 urements on Feulgen-stained nuclei of the 

 rat, chicken and sea urchin embryo. They 

 claim that DNA doubling takes place in 

 telophase immediately after formation of the 

 nuclear membrane. This may be true in 

 some rapidly dividing cells, but the evidence 

 so far is against this being the general situa- 

 tion. In the sea urchin, for instance, Mc- 

 Master (see Swift, '53) found that DNA 

 doubling takes place early in interphase 

 during the first cleavage divisions. Later on 

 the DNA synthesis occurs during interphase 

 at rates that vary among different blasto- 

 meres. The DNA content of interphases in 

 different cells thus differs but is intermediate 

 between the telophase and metaphase value. 



With regard to the synthesis of other com- 

 ponents of chromosomes we are much less 

 fortunate. Is there a specific protein in 

 chromosomes that is synthesized only at 

 the time of chromosome reproduction? Is it 

 made at the same time as DNA? From ex- 

 periments of Howard and Pelc ('51b) on 

 incorporation of radioactive sulphur (S^^) 

 into chromosomal proteins it looks as if S^^ 

 is taken up into proteins mainly by dividing 

 nuclei and at about the same time as P^^ 

 But no general conclusions can be drawn 

 from these preliminary results. 



The most interesting aspect of mitosis is, 

 no doubt, the question of how the cell pro- 

 duces an exact copy of the infinitely com- 

 plex and specific organization of the chromo- 

 somal fiber. This involves not only the syn- 



thesis of specific proteins and nucleic acids, 

 but also the weaving together of the proper 

 components into the right patterns. The syn- 

 thesis of nucleic acids and proteins is today 

 intensively investigated by the biochemist, 

 who may soon give us a better understanding 

 of what goes on during this stage in the 

 life of the chromosome. Models of chromo- 

 some duplication were suggested by Fried- 

 rich-Freksa ('40), Bernal ('40) and Delbriick 

 ('41). The precursors of the new DNA gen- 

 erally appear to be small molecules. How- 

 ever, in the frog's egg Zeuthen and Hoff- 

 J0rgensen ('52) found that the cytoplasm 

 contains considerable amounts of desoxyribo- 

 sides. Apparently they are stored in the 

 egg cytoplasm to be used during the rapid 

 chromosome synthesis of early cleavage. The 

 total content in desoxyribosides of the egg 

 remains constant until the late gastrula 

 stage. 



The Chromosomes in the Interphase Nucleus. 

 During telophase the chromosomes swell into 

 optically homogeneous vesicles (cf. Lewis, 

 '47). If these are widely spaced on the 

 spindle each chromosome vesicle may form 

 its own nuclear membrane and as a result a 

 large number of small nuclei appear (karyo- 

 meres). This is common in cleavage divi- 

 sions (Fig. lOH). Karyomeres may contain 

 one or several chromosomes. In general, how- 

 ever, the chromosomes are so close together 

 at telophase that a single nucleus is formed. 

 The nuclear membrane originates at the 

 interphase between chromosome and cyto- 

 plasm and has a complicated structure 

 (Monne, '42; Schmidt, '39). In the electron 

 microscope two layers can be distinguished 

 (Callan and Tomlin, '50; Bairati and Leh- 

 mann, '52; Harris and James, '52). One is 

 mainly lipoid in nature and contains regu- 

 lar perforations. The other has a uniform 

 structure and consists apparently of lamellae 

 of fibrous proteins. The porous layer may 

 be on the outside (amphibian oocyte, Callan 

 and Tomlin, '50) or inside (Amoeba, Harris 

 and James, '52). 



The changes taking place in the chromo- 

 somes are imperfectly understood. In part a 

 despiralization of the chromonemata takes 

 place. Probably there is also a change in 

 chemical composition, especially an increase 

 in chromosomal proteins. Furthermore, a 

 change in the physical state of chromosomes 

 can be demonstrated. Chromosomes are or- 

 ganized nucleoprotein gels which can swell 

 and contract. In the living interphase nu- 

 cleus they are in the extended state and usu- 

 ally fill the nucleus completely so that no 



