90 J. IIKRBERT TAYLOR 



starved for thymine for 45 minutes and then returned to a complete 

 medium. DNA sanijilos folliM-ted before and for a short period after 

 restoring thymine had a nioh'cuhir \vci<;ht twice that of DNA isohited 

 from cells ahout one-half houi' after restoring thymine. They also found 

 an associated change in the kinetics of degradation by DNase which they 

 interpreted as a change from four-stranded DNA to two-stranded DNA. 

 Further information on changes in state of DNA is given in Chapters 

 III and IV. 



B. TYPES OF CHROMOSOMAL ABERRATIONS PRODUCED AT VARIOUS 

 STAGES OF THE CELL CYCLE 



Methods for detecting changes of chromosomes in relation to the 

 duplication cycle are quite limited. One of the most revealing techniques 

 is the study of changes produced by ionizing radiations. Soon after the 

 mutagenic effects of X-rays had been demonstrated, there were attempts 

 to determine the time of chromosome duplication by the type of aberra- 

 tions observed after irradiation at different stages of the nuclear cycle 

 in root cells (]\Iather and Stone, 1933). In early experiments most of the 

 aberrations observed were the type expected if the chromosomes were 

 broken before reproduction. Their conclusion was that chromosomes w^re 

 single during most or all of interphase and reproduction occurred late 

 in the division cycle. Their work was criticized by Huskins and Hunter 

 (1935) on the basis that the stage irradiated could not be determined 

 accurately, and that cells observed might not have been at the first 

 division after treatment. Huskins and Hunter and also Mather (1937) 

 improved on the design of the early experiments by selecting microspores 

 of higher plants as experimental material. After meiosis the haploid 

 microspores formed pass through a long interphase of several days to 2 

 or 3 weeks depending on the species and temperature of development. By 

 irradiating in this interphase and analyzing cells at the first microspore 

 division, tliey could be certain that the chromosomes were observed at 

 the first division after treatment. However, Huskins and Hunter (1935) 

 reported that the chromosomes of Trillium were already double when 

 irradiated. Matliei' (1937) fovuid tliat treatment at eaily intcM'phase 

 resulted in only chromosomal type aberrations, i.e., those produced 

 before production of the two chi'omatids. At later stages of interphase 

 single chromatids of a pair could be broken. Huskins and Hunter (1935) 

 contended, along with others, that the degree of subdivision of a 

 chromosome could not be revealed by irradiation because the passage 

 of an ionizing paiticle could sever several suhunits at once. Another 

 possibility is tiiat a chromosome broken befoi'c repi'oduction may i-emain 

 open and react with other broken ends after reproduction. 



