96 



Cellular Structure and Activity 



trophotometry of spermatocytes from a grass- 

 hopper (as a cell in division) and salivary 

 gland chromosomes of Drosophila (as a rest- 

 ing cell). Unfortunately the quantitative de- 

 termination of different types of proteins in 

 chromosomes by this method is highly ques- 

 tionable. Until confirmed by other methods 

 and by analysis of individual chromosomes 

 through the mitotic cycle (instead of 

 comparing prophase or resting nuclei 

 with metaphase chromosomes), Caspersson's 

 scheme remains an interesting speculation. 

 A decrease in protein content of chromo- 

 somes from interphase to metaphase was also 

 suggested by Pollister ('51) on the basis of 

 comparison of Millon-stained interphase nu- 

 clei and metaphase chromosomes. So far our 

 information on the chemical composition of 

 chromosomes during mitosis is thus very 

 fragmentary. Certainly we are not justified 

 in generalizing from one type of cell. It has 

 long been known that metaphase chromo- 

 somes may vary in size in different tissues 

 of the same animal (cf. Geitler, '38; Biesele, 

 '46), under different physiological conditions 

 (tissue culture, e.g., Hance, '26) and in tu- 

 mors (Biesele, '47). The most striking exam- 

 ple is the decrease in the size of metaphase 

 chromosomes during cleavage of many forms 

 (Erdmann, '08). If we are correct in assum- 

 ing that the amount of DNA is constant, the 

 variations in volume must be due mainly to 

 proteins. The protein content of metaphase 

 chromosomes may thus be just as variable as 

 it was foimd to be in interphase chromo- 

 somes. A number of cytologists have inferred 

 chemical changes in mitotic chromosomes 

 from their visual appearance after staining 

 and have constructed ambitious theories on 

 such information (Darlington, '42; Serra, 

 '47). The recent quantitative cytochemical 

 studies have shown the dangers of overex- 

 tended deductive reasoning that has been so 

 common in cytology. To be productive, in- 

 ventive speculation needs reliable empirical 

 data as a basis. 



Chromosome Reproduction. The duplica- 

 tion of the chromosomes is perhaps the fun- 

 damental process of mitosis and one of the 

 most interesting problems of biology. In 

 what stage of the mitotic cycle does it take 

 place? At a time when the chromosome was 

 considered to be a single fiber that divided 

 longitudinally during mitosis it was appro- 

 priate to speak of the time of chromosome 

 splitting and this was often identified with 

 the time of gene reproduction. It appears now 

 that we must distinguish several processes 

 that may be quite independent: (1) Syn- 



thesis of the essential chemical components 

 of the chromosome; (2) duplication of 

 the submicroscopic elementary microfibrils. 

 These two processes together shall be called 

 "chromosome reproduction"; (3) subdivision 

 of the bundle of microfibrils into the units 

 that separate at anaphase, or that behave 

 independently with regard to x-ray breakage 

 or genetical exchange (crossing over) or be- 

 come resolved in the UV or light microscope. 

 We might call this splitting of the chromo- 

 some into chromatids, half- or quarter-chrom- 

 atids, etc. (cf. Fig. 13). While the first one 

 involves chemical synthesis and is presum- 

 ably irreversible, the second process is often 

 reversible. 



In the past the time of chromosome split- 

 ting has mainly been discussed. But this is 

 clearly of secondary interest and the funda- 

 mental process is chromosome reproduction. 

 With the discovery of DNA constancy in 

 chromosomes a study of this question became 

 at least partially possible. Chromosome re- 

 production must involve DNA synthesis and 

 the time in the mitotic cycle when this hap- 

 pens can be determined (for a review see 

 Swift, '53). Ultraviolet spectrophotometry 

 has been used (Caspersson, '39; Walker and 

 Yates, '52) but since this is not specific for 

 DNA the results are not decisive. The most 

 reliable information comes from absorption 

 measurements on Feulgen stained chromo- 

 somes. Extensive measurements on animal 

 and plant cells by Swift, Alfert, and Patau 

 and Swift have established that the increase 

 in DNA takes place in interphase, before 

 visible changes in the nucleus occur; earlier 

 measvirements by Ris ('47) must now be con- 

 sidered in error (for references see Swift, 

 '53). Unfortunately there are no good cri- 

 teria to subdivide interphase into the differ- 

 ent physiological phases which we know 

 must exist. 



In a diploid tissue with dividing cells 

 there are generally two classes of nuclei. One 

 contains twice as much DNA as the other, 

 in preparation for mitosis. The doubling in 

 DNA content, however, may also be asso- 

 ciated with endomitosis resulting in polv- 

 teny or polysomaty. The doubling of DNA in 

 interphase just before mitosis was demon- 

 strated in a very different wav by Price and 

 Laird ('50) in an analysis of liver regenera- 

 tion after hepatectomy. The DNA content 

 was determined chemically and from the 

 number of nuclei in the homogenate thp 

 amount per average nucleus was calculated. 

 In the first 12 to 24 hours the DNA per 

 average nucleus approximately doubled. Af- 



