HYPOTHESIS: Genes Can Move
The prevailing opinion among most geneticists in Barbara McClintock's time was that genes were lined up on chromosomes in an unchanging way, much like beads on a string. However, observations from her experiments with maize (Zea mays) told McClintock otherwise.
As a graduate student at Cornell, McClintock developed new staining techniques and discovered that each of the 10 maize chromosomes could be distinguished under the microscope. She noted that some changes in the appearance of the corn kernels and plants were coupled with changes in the shape of one or more of the maize chromosomes. She also noted that kernels that had been exposed to X rays germinated and grew into seemingly normal plants but that the shapes of some of their chromosomes had changed. She proposed that the maize genome had a dynamic chromosome repair system that allowed for growth even after major chromosome damage that X rays had caused. Geneticists at the time assumed that a mutated gene was dead and could not be reactivated. McClintock's findings, however, challenged this belief.
METHODS: Analyze Maize Chromosomes
McClintock wanted to examine more closely the results of growing maize that contained broken chromosomes. So during the winter of 1944-1945, she planted self-pollinated kernels that were the product of many generations of inbreeding and self-fertilization. She hoped to track chromosome repairs by observing changes in chromosome shapes.
RESULTS: Unexpected Changes
When the plants germinated, McClintock was astounded at the results. The leaves had odd patches that lacked the normal green coloration. These patches occurred regularly along the leaf blades. She compared chromosomes of these plants with those of the parent plants under the microscope and concluded that parts of the offspring plants' chromosomes had changed position.
CONCLUSION: Genes Have the Ability to "Jump"
The changes that McClintock saw in these plants and their chromosomes led her to conclude that genes are not stable within the chromosome but can move to a new place on a chromosome or to a new chromosome entirely. McClintock called these movable genes controlling elements. They were later called transposons.
McClintock observed two kinds of transposons: dissociators and activators. Dissociators could jump to a new chromosomal location when signaled by activators. The dissociators would then cause changes in nearby genes on the chromosome and in the color of the kernels and leaves in the maize. McClintock verified her conclusions through repeated experiments.
McClintock summarized her findings in 1951 at a Cold Spring Harbor symposium, but her work was not well received. After many years, recognition of the value of her work grew and she was awarded the Nobel Prize in medicine or physiology in 1983.
The colorful maize plant Zea mays was harvested at Cornell University during the late 1920s and early 1930s to study its genetics. The varied colors of its kernels worked as a multicolored spreadsheet of genetic data.
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