MAMMALL\N CFXL GROWTH IN TISSUE CULTURE 191 



efficiency as high as one chromosome break per 20 roentgens per cell. 

 The incidence of such breaks drops steadily as the time between ir- 

 radiation and fixation is increased. 



On the basis of these observations, it appears likely that the re- 

 versible lag of mitosis represents an inhibition due to chromosome 

 breakage which prevents the normal coiling of the chromosomes during 

 the stage just preceding the onset of mitosis. Cells in which the coiling 

 process has been virtually completed can go on into mitosis despite the 

 presence of these breaks, which then become visible when the mitotic 

 figures are examined. Those cells that are a little further removed from 

 mitosis are prevented by the chromosome breaks from attaining the 

 requisite degree of coiling needed to initiate mitosis. Hence these cells 

 experience a lag until metabolic processes enable the breaks to become 

 resealed. In cells which receive a relatively small number of breaks, 

 the healing processes usually proceed so as to permit reproduction to 

 occur, and the eflFect of the radiation on reproduction is temporary un- 

 less a lethal gene mutation has accompanied the chromosomal damage. 

 When many breaks occur, the result is abnormal chromosome recombi- 

 nations, loss of chromosomal fragments, and other abnormalities, which 

 render a large proportion of the cells incompetent to form a colony. 

 This view offers a unitary picture of the nature of radiation damage 

 which may explain both the reversible and the irreversible effects on 

 cell reproduction. 



By means of tritiated thymidine, the major time relationships of the 

 cell life cycle have been elucidated (Painter and Drew, 1959). The 

 times required for the various phases of the cell life cycle {i.e., mitosis, 

 postmitotic resting phase, DNA synthesis, and the premitotic resting 

 phase) have been elucidated for the hyperploid HeLa cell and for 

 normal human cells (Yamada and Puck, 1961). These methodologies 

 permit comparison of the phases of the growth cycle for cells in differ- 

 ent states of differentiation and should make possible intimate study of 

 molecular dynamics of the differentiation process, as well as of the 

 effects of specific drugs, hormones, and other agents on the metabo- 

 lisms of normal and malignant cells. We have found by this technique 

 that small, non-lethal doses of X-irradiation in the S3-Hela cell produce 

 only a reversible block in G2, the period immediately preceding mi- 

 tosis. Similar irradiation of the normal human cell, however, produces 

 a block not only in G2, but also in Gl, the period preceding the syn- 

 thesis of DNA. 



Advances in the understanding of cell nutrition have been carried 

 out in many laboratories. The elucidation of a medium containing only 

 two purified macromolecular fractions, which permits the quantitative 

 growth of single cells into large colonies, has opened up many pos- 

 sibilities for biochemical studies. One of these macromolecular frac- 



