Cell Division 



95 



phase of meiosis (see Manton, '50). Whether 

 such changes take place generally during 

 mitosis is not known. 



What causes the spiralization of chromo- 

 somes? So far we have no satisfactory ex- 

 planation. Much more needs to be known 

 about the chemical changes during coiling 

 and the submicroscopic structure of the 

 chromosome before we can begin to under- 

 stand the mechanism of coiling. 



During prophase the chromosomes are said 

 to become surrounded by a membrane (pel- 

 licle) and a matrix in which the coiled 

 chromonemata are embedded or which sur- 

 round each individual chromonema (cf. Kauf- 

 mann, '48), but there is no good evidence 

 for this contention. 



Chemical Changes in the Chromosome. In 

 recent years much progress has been made 

 in our knowledge of chromosome chemistry 

 since it has become possible to isolate nuclei 

 and chromosomes for chemical analysis and 

 with the development of appropriate cyto- 

 chemical techniques. (For a review see Dav- 

 idson, '50; Mirsky, '51; Mazia, '52.) Chromo- 

 somes consist mainly of nucleoprotein. The 

 bulk of the nucleic acid is of the desoxypen- 

 tose type (DNA), which normally occurs 

 only in chromosomes. Variable amounts of 

 pentose nucleic acid (PNA) have been de- 

 tected cytochemically and in isolated chrom- 

 osomes. The basic proteins of chromosomes, 

 the histones and protamines of certain sperm 

 nuclei, have been known for a long time. 

 The presence of more complex, non-histone 

 type proteins was first demonstrated by the 

 Stedmans ('43) and by Mirsky and Ris ('47). 

 These four groups of compounds are the 

 major components of chromosomes. Lipids 

 and carbohydrates may be present in nu- 

 clei, but have not been localized in the 

 chromosomes themselves. Of inorganic con- 

 stituents besides the phosphorus of nucleic 

 acids, magnesium and especially calcium 

 have been found in chromosomes (Scott, '43). 

 Quantitative determinations on isolated nu- 

 clei and cytochemical studies have revealed 

 the remarkable fact that the amount of DNA 

 is constant per chromosome set (Boivin, 

 Vendrely and Vendrely; Mirsky and Ris, 

 Swift; for references see Swift, '53). The ab- 

 solute amount of DNA in a diploid nucleus 

 is characteristic for an animal and varies 

 considerably from one taxonomic group to 

 another (Mirsky and Ris, '51). In contrast to 

 the DNA, the amounts of non-histone protein 

 and PNA seem to vary a great deal with 

 physiological conditions and from one tissvie 

 to another (Mirsky and Ris, '49). 



Changes in the composition of chromo- 

 somes during the mitotic cycle have been 

 studied so far only cytochemically. Quanti- 

 tative determinations by histospectropho- 

 tometry have been hampered by the uneven 

 and changing distribution of the absorbing 

 material. The recently developed methods of 

 Ornstein ('52) and of Patau ('52), however, 

 have overcome this difficulty and accurate 

 determinations of DNA (with the Feulgen 

 reaction) are now possible through the mi- 

 totic cycle. Thus Patau and Swift ('53) have 

 demonstrated that the amount of DNA in 

 the chromosomes does not change from earli- 

 est prophase up to metaphase. If the quan- 

 tity of DNA per chromosome is constant, 

 any increase in this substance would have to 

 be associated with the reproduction of chro- 

 mosomes. How cytochemical determination of 

 DNA can be used to determine the time of 

 chromosome duplication will be shown later. 

 Changes in PNA content of chromosomes 

 during mitosis were reported by Schultz ('41), 

 Brachet ('42), Kaufmann ('48), Turchini 

 ('49), Battaglia and Omodeo ('49), and 

 Jacobson and Webb ('52). These investigators 

 fovmd marked changes in the staining of 

 chromosomes with certain basic dyes before 

 and after digestion with ribonuclease, espe- 

 cially in metaphase and anaphase, and con- 

 cluded that there was an increase in PNA 

 in chromosomes in late prophase followed 

 by a decrease in telophase. According to 

 Jacobson and Webb ('52) the loss of ribonu- 

 cleoprotein from anaphase chromosomes is 

 accompanied by an increase of this material 

 in the spindle area between the separating 

 chromosomes, suggesting that chromosomes 

 give off ribonucleoprotein during anaphase 

 movement. The findings are supported by 

 the photographs of chick fibroblasts in ana- 

 phase taken by ultraviolet light (at 2650 A), 

 which show increased absorption at this 

 wave length between the separating chromo- 

 somes (Davies, '52). The "chromatin elimina- 

 tion" at anaphase of the first meiotic division 

 in eggs of Lepidoptera (Seller, '14) and some 

 other insects (Cooper, '39) involves shedding 

 of nucleoprotein material from chromosomes 

 in discrete bodies that remain in the equa- 

 torial plane of the spindle and later disin- 

 tegrate (Ris and Kleinfeld, '52). It remains 

 to be seen whether the loss of ribonucleopro- 

 tein from anaphase chromosomes is a gen- 

 eral phenomenon and what physiological 

 significance it may have. 



Changes in the proteins of chromosomes 

 during mitosis were first described by Cas- 

 persson ('40) on the basis of ultraviolet spec- 



