938 ANTIMETABOLITES AS MITOTIC POISONS I3 



similar effects; thus extracts of dried bulb scales cause "sticky" bridges in onion 

 root mitoses in anaphase (Resende, 1951). The toxic materials causing chromo- 

 somal agglutination may be nucleic acids or their degradation products, for Woll 

 (1953) has found that ribonucleic acid clumps chromosomes in root tip mitoses. 

 Adenine causes "sticky" anaphase bridges in mouse tissue cultures at 1.5 mM 

 concentration (Biesele, Berger, Clarke and Weiss, 1952), and the 8-ethers and 

 8-thioethers of caffeine produce much chromosomal "stickiness" in onion root 

 tips (Kihlman, 1952). 



Understanding of chromosomal breakage presents several difficulties. One of 

 these lies in the extension of theoretical concepts of an event at the molecular 

 level in a poorly understood structure to the complex organization of the chro- 

 mosome. The individual mitotic chromosome is moderately polytene, the unit 

 microfibrils being perhaps 100-200 A in diameter (Ambrose, Cuckow and Gopal- 

 Ayengar, 1955). Prophase chromosomes in staminate hair cells of Tradescantia are 

 each made up of as many as 64 small strands of some 125 A diameter (Kaufmann 

 and De, 1956). Unit microfibrils about 200 A wide have been reported in chromo- 

 somes of various organisms by Ris (1956), who suggested that the microfibril 

 consists of a core of DNA surrounded by a protein sheath. Breakage in such multi- 

 microfibrillar chromosomes would appear to require multiple ruptures across 

 the entire structure. There is the possibility of only partial breakage across the 

 chromatid, however, such as the subchromatid errors observed by Kihlman (1955a) 

 in onion root tip cells treated with 8-ethoxycaffeine during prophase. An error or 

 interruption in synthesis in a chromosomal subunit might not be immediately 

 evident, but it might result in the deferred appearance of breaks some cell gener- 

 ations later, as McLeish (1954) observed with maleic hydrazide and as Ambrose, 

 Cuckow and Gopal-Ayengar (1955) suggested, might occur with carcinogens. 



{a) Amino acids 



The necessity of a balanced supply of amino acids for normal growth has been 

 discussed already, as well as the interrelationships of protein synthesis and nucleic 

 acid synthesis. It is accordingly to be expected that chromosomal damage shoud 

 result from deficiencies or excesses of given amino acids or from the presence of 

 inhibitory analogues. Chromosomal damage has been reported in chick tissues 

 cultured in media deficient in lysine (Kieler, 1953b), glutamic acid (Kieler, 

 1953c), and tryptophan (Kieler, 1954a). Excess of aspartic stimulates mitosis and 

 causes displacement and breakage of chromosomes (Kieler, 1953c). Chromosomal 

 bridging and fragmentation become more frequent in cultures of mouse sarcoma 

 180 treated with the three isomeric fluorophenylalanines (Biesele and Jacquez, 

 1954). Azaserine, which Hemerly and Demerec (1955 )found to be one of two 

 substances with outstanding mutagenic effect on bacteria tested in a series of 28 

 agents of cancer chemotherapy, is not highly active in increasing the frequency of 

 chromosomal aberrations in mouse tissue cultures (Biesele, 1958a). However, it 

 greatly potentiates the action of 6-mercaptopurine in this respect (Biesele, 1958b). 

 This same effect is also more marked when adenine is added to the gammahydra- 

 zide of L-glutamic acid (Biesele, 1958b). 



