 During her time at Missouri , McClintock expanded her research on the effect of X @-@ rays on maize cytogenetics . McClintock observed the breakage and fusion of chromosomes in irradiated maize cells . She was also able to show that , in some plants , spontaneous chromosome breakage occurred in the cells of the endosperm . Over the course of mitosis , she observed that the ends of broken chromatids were rejoined after the chromosome replication . In the anaphase of mitosis , the broken chromosomes formed a chromatid bridge , which was broken when the chromatids moved towards the cell poles . The broken ends were rejoined in the interphase of the next mitosis , and the cycle was repeated , causing massive mutation , which she could detect as variegation in the endosperm . This breakage – rejoining – bridge cycle was a key cytogenetic discovery for several reasons . First , it showed that the rejoining of chromosomes was not a random event , and second , it demonstrated a source of large @-@ scale mutation . For this reason , it remains an area of interest in cancer research today . 
