Ill THE POISONING OF CHROMOSOMES 94 1 



An analogous situation has been found by Novick (1956) in the decreased 

 mutagenicity of free purines to bacteria under anaerobic conditions, which make 

 for higher concentrations of antimutagenic nucleosides in the bacteria. Moreover, 

 Gersh (cited by Novick, 1956) has observed that the breakage is decreased in the 

 presence of guanosine. 



However, some nucleosides do contribute to chromosome breakage. 2,6-Di- 

 amino-g-p-D-ribofuranosylpurine breaks chromosomes in mouse sarcoma 180 

 cultures (Biesele, Berger, Clarke and Weiss, 1952); and 9-[i-D-ribofuranosyl- 

 purine breaks chromosomes in onions (Kihlman, cited by Lofgren and Liining, 

 1953) and in sarcoma 180 cells (Biesele, Slautterback and Margolis, 1955). 



IV. POISONING OF THE SPINDLE 



There are several points in the formation and functioning of the mitotic spindle 

 at which antimetabolites like those under consideration in this chapter might inter- 

 vene. These points may be made evident in a short discussion of the composition 

 and behavior of the mitotic spindle as now understood. 



The fact that colchicine, the best-known spindle poison, has a nearly universal 

 influence on mitosis, indicates a fundamental uniformity in composition of the 

 spindle in diverse organisms susceptible to attack by this compound. The essential 

 part of the colchicine effect, according to Eigsti and Dustin (1955), is that it con- 

 verts the mitotic spindle from a polarized fibrous structure to an amorphous 

 hyaline globule (Gaulden and Carlson, 1951) no longer capable of supporting the 

 anaphasic movement of chromosomes. 



There are only two electrophoretically demonstrable oriented fibrous proteins 

 of the spindle (Schmidt, 1937; Swann, 1952a, 1952b; Inoue, 1952). This has been 

 shown by Mazia ( 1 956) and on the isolated mitotic apparatus of the sea urchin 

 egg by Mazia and Dan (1952). The two components do not differ in their amino 

 acid composition (Mazia, 1955), but do differ in that one is combined with ribo- 

 nucleic acid (Mazia, 1956). Other evidence for the occurrence of ribonucleic acid 

 on the spindle is also available. Kaufmann, McDonald and Gay (1951) found the 

 mitotic spindle of onion root tip cells to contain both ribonucleic acid and protein. 

 Stich (1951) noted an acumulation of RNA in the nuclear sap of the Cyclops egg 

 after the increase of polysaccharide in prophase. Perhaps synthesis of the spindle 

 protein occurs on a ribonucleic acid template, in which case antimetabolites of 

 nucleic acid as well as amino acid analogues, might be expected to disturb the 

 synthesis. 



Analogues of nucleic acid components might change the spindle in other ways, 

 in particular by interfering in nucleotide metabolism, if the hypothesis of Brachet 

 (1947) correctly names adenosine triphosphate as the source of energy for contrac- 

 tion of the mitotic spindle. The view was supported by Lettre's finding (1950) 

 that ATP antagonizes the effect of colchicine on the spindle. However, this find- 

 ing has met with criticism (Benitez, Murray and Chargaff, 1954). A manifold 

 participation of ATP in cell division is indicated by the experiments of Hoffmann- 

 Berling (1954a, 1954b, 1954c) with dividing cells extracted with cold aqueous- 

 glycerol. In these so-called cell models, adenosine triphosphate evokes elongation 



Literalure p. 947 



