94 



Cellular Structure and Activity 



Table 2. Illustrations of Living Cells in 

 Mitosis 



MATERIAL AUTHOR 



Amblystoma heart tissue Wang, '52 



culture 

 Grasshopper sperma- Belar, '29; Michel, '50 



tocytes 

 Frog fibroblast tissue Hughes and Preston, '49 



culture 

 Chick embryo osteoblast Hughes and Swann, '48 



tissue culture 

 Mouse spleen tissue Fell and Hughes, '49 



culture 



into the daughter cells. Cytokinesis divides 

 the cytoplasm more or less equally around 

 the daughter nuclei. Cytoplasmic components 

 are generally segregated at random bvit may 

 become differentially distributed in certain 

 cells. 



KARYOKINESIS 



THE CHROMOSOME CYCLE 



One of the most striking aspects of mitosis 

 is the cyclical change in the chromosomes. 



Fig. 



Half chromatid 



Chromosome 



13. Diagram of the subunits in a completely 

 uncoiled chromosome. 



This involves chromosome reproduction, 

 changes in structure (spiralization cycle) 

 and chemical and physiological changes. 



Structural Changes in Chromosomes. It is 

 generally agreed today that chromosomes 

 consist of a bundle of threads that undergo 

 a cycle of coiling and uncoiling. The exact 

 number of units (chromonemata) has been 

 much debated. There is good evidence that 



in many cases the daughter chromosomes 

 at anaphase contain at least two and some- 

 times even fom- microscopically visible units. 

 Recent studies with the electron microscope 

 in this laboratory, however, have shown that 

 the elementary longitudinal unit is svibmi- 

 croscopic and in several animals and a plant 

 was found to be of the same thickness (about 

 500 A). The number of longitudinal subunits 

 in a chromosome therefore must be deter- 

 mined with the electron microscope. We have 

 found evidence that the number of these 

 elementary^ microfibrils is not the same in 

 comparable chromosomes of different organ- 

 isms (Ris and Kleinfeld, '52). The micro- 

 fibrils and the larger units they compose may 

 be either closely appressed or separate from 

 each other laterally. This fact and the submi- 

 croscopic size of the structiu-al unit are 

 probably responsible for most disagreements 

 and uncertainties of chromosome structure. 

 The unit of reproduction in the chromo- 

 some must therefore be sought in the ele- 

 mentary microfibril. Since chromosomes con- 

 tain more than one microfibril it is, however, 

 not the unit of division during mitosis. This 

 unit is the chromatid, that itself may be 

 visibly separated into two or more bundles 

 of microfibrils (half-chromatids and quarter- 

 chromosomes) (Fig. 13). 



In early prophase chromosomes show a 

 large number of small gyres. These increase 

 in width throvigh a process of "despiraliza- 

 tion." While the nvimber of gyres decreases, 

 through this process of "gyre elimination" 

 the chromosomes become shorter and thicker 

 and gain their typical metaphase form (Spar- 

 row, Swanson, Ris; for references see Man- 

 ton, '50). 



In prophase of meiosis the chromosomes 

 usually condense more than in somatic cells. 

 This is connected with the formation of a 

 double spiral (minor and major coil). The 

 number of gyres in a metaphase chromo- 

 some seems to be remarkably constant for 

 one type of cell under similar conditions, 

 but may vary considerably in different types 

 of cells (somatic mitosis, first and second 

 division of meiosis, for instance). In telo- 

 phase the process of gyre elimination con- 

 tinues so that only a general waviness re- 

 mains in the interphase nucleus (relic 

 spiral). 



A few workers have attempted to calcu- 

 late the length of the chromonemata during 

 the coiling cycle. In the fern Osmunda (Man- 

 ton) and in Trillium (Sparrow, Wilson and 

 Huskins) considerable variation in the length 

 of the chromonema was found during pro- 



