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Barbara McClintock was a pioneering American scientist and one of the world's most distinguished cytogeneticists. McClintock received her PhD in botany from Cornell University in 1927, where she was a leader in the development of maize cytogenetics; the field remained the focus of her research for the rest of her career. Her work was groundbreaking: she developed the technique to visualize maize chromosomes and used microscopic analysis to demonstrate many fundamental genetic concepts, including genetic recombination by crossing-over during meiosis—a mechanism by which chromosomes exchange information. She produced the first genetic map for maize, linking regions of the chromosome with physical traits, and she demonstrated the role of the telomere and centromere, regions of the chromosome that are important in the conservation of genetic information. During the 1940s and 1950s, McClintock discovered transposition and using this system showed how genes are responsible for turning on or off physical characteristics. Awards and recognition of her contributions to the field followed, including the Nobel Prize in Physiology or Medicine awarded to her in 1983 for the discovery of genetic transposition; she was the first and only woman to receive an unshared Nobel Prize in that category.