92 J. nKHMKin' tavi.ok 



from that in wliich chroinosonici^ act as single units in the formation of 

 aberrations to the state in which they react as two units, is correhited 

 in time with DNA replication. The change either occurs progressively 

 during synthesis or rathci- ahiuptly preceding syntlicsis. The earlier 

 discrepancies reported by lluskins and Hunter (1935) and by Mather 

 (1937) are very likely the result of variations in the biological material. 

 The time of synthesis of DNA has not been reported foi- tlic two genera 

 used, Trillixim and Allium. However, in Tradescantid thi' change and 

 synthesis both occur in late interphase, while in Tiilbaghin both DNA 

 synthesis and the change in state occur in very caiiy interphase 

 (Taylor, 1958c). 



In meiosis where chroniosonie ivpi-oduction has often been supposed 

 to occur in the long prophase of the first meiotic division, the change in 

 state from single to double occurs in interphase (Mitra, 1958; Sauerland, 

 1956). Likewise DNA rei)lication and the synthesis of basic proteins of 

 the nucleus are correlated with the change (Taylor, 1959c; De, 1961). 

 The synthesis of basic proteins (histones) of chromosomes is apparently 

 correlated with DNA replication in other cell types (Alfert, 1955; Bloch 

 and Godraan, 1955; Gall, 1959). In all of these studies the correlation 

 of changes in response to ii'radiation and the synthesis of DNA and 

 proteins lack the precision in timing for specific chromosomes or regions 

 of chromosomes that would be desirable. Further discussion of a change 

 in state of DNA in relation to its replication cycle will l)e found in 

 Chapter IV and the last section of this chapter. 



Another type of aberration induced })y radiation at limited stages of 

 the cell cycle are half-chromatid exchanges. They can be produced only 

 in mitotic prophase (Sax, 1957) and in the stages after pachytene in 

 meiosis (Mitra, 1958; Grouse, 1954). Although these connections are 

 strong enough to lead to breakage of chromatids as they stretch in 

 anaphase, the bridges do not persist through the subsequent interphase 

 and usually do not become chromatid aberrations at the next division 

 (Ostergren and Wakonig, 1954). After anaphase the chromatids revert 

 to a state in which they act as if composed of single axial elements in 

 breakage and reunion rather than doul)le as in the pi'evious jirophase. 

 However, there is some evidence that they can become double to radia- 

 tion breakage before DNA synthesis begins. See the last section of this 

 chajjter for a model that could b('lia\'e in this fashion. 



C. HEALING AND REUNION OF BROKEN rHROMO.'^OMES 



The synthesis of the major components of chromosomes is restricted 

 to one-half or less of the cell cycle in many cells of higher organisms. 

 Yet, the repair or reunion of broken chi'omosomes can occur at other 



