10:3/ Cellular Events Produced by Ionizing Radiations 189 



population are set at one tenth those of radiation workers, on the 

 assumption that genetic changes as well as somatic ones would be 

 undetectable. 



By 1960, radiation doses to the entire population included about 

 4 rem in thirty years from background, about 5 rem in thirty years from 

 medical and dental sources, and about 0.3 rem from 1946-1959 from 

 fall-out. The last-mentioned number will continue to increase. A 

 further discussion of fall-out is included in Section 6. 



3. Mitosis and Meiosis 



Many of the abnormal cellular events resulting from ionization become 

 apparent as a result of cellular division. To appreciate the significance 

 of these alterations, it is necessary to be acquainted with the normal 

 mechanisms of cell division. In most cells, this occurs in a series of 

 characteristic steps called mitosis. This is modified in the formation of 

 the cells of sexual reproduction (the sperm and egg cells) into a homol- 

 ogous series of steps referred to as meiosis. The major exceptions to the 

 more or less universal nature of mitosis and meiosis are the bacteria 

 which do not possess a clearly defined nucleus and divide in a less 

 organized fashion. 



Figure 2 illustrates the process of mitosis. The chromosomes within 

 the cell nucleus are believed to carry most of the genetic information of 

 the cell, controlling its form, metabolism, and function. However, as 

 shown in Figure 2a, the chromosomes do not exist as such in the nucleus 

 during most of the cell life. Rather, during the period between divisions 

 they appear broken up into heavily staining birefringent granules called 

 chromatin material. This portion of the cell life between divisions is 

 called interphase. 



As the cell prepares to divide, the chromatin material is organized 

 into long filaments which pull together to form chromosomes. These 

 are double filaments at this point in mitosis, which is called prophase. 

 Simultaneously, a spindle starts to form, the nuclear membrane starts 

 to dissolve, and the nucleoli disappear. This stage is shown in Figure 2b. 



In the next stage, metaphase, the chromosomes attach at a specific 

 point, the centromere (also called kinetochore), to the spindle, and line 

 up at the center of the cell. As shown in Figure 2c, the nuclear mem- 

 brane is completely gone. 



The chromosomes then each pull apart into two separate fibers and 

 follow the spindles to the cell centers. In the absence of a spindle (which 



