 McClintock 's breakthrough publications , and support from her colleagues , led to her being awarded several postdoctoral fellowships from the National Research Council . This funding allowed her to continue to study genetics at Cornell , the University of Missouri , and the California Institute of Technology , where she worked with E. G. Anderson . During the summers of 1931 and 1932 , she worked at Missouri with geneticist Lewis Stadler , who introduced her to the use of X @-@ rays as a mutagen . Exposure to X @-@ rays can increase the rate of mutation above the natural background level , making it a powerful research tool for genetics . Through her work with X @-@ ray @-@ mutagenized maize , she identified ring chromosomes , which form when the ends of a single chromosome fuse together after radiation damage . From this evidence , McClintock hypothesized that there must be a structure on the chromosome tip that would normally ensure stability . She showed that the loss of ring @-@ chromosomes at meiosis caused variegation in maize foliage in generations subsequent to irradiation resulting from chromosomal deletion . During this period , she demonstrated the presence of the nucleolus organizer region on a region on maize chromosome 6 , which is required for the assembly of the nucleolus . In 1933 , she established that cells can be damaged when nonhomologous recombination occurs . During this same period , McClintock hypothesized that the tips of chromosomes are protected by telomeres . 
