At a time when molecular biologists struggled to devise a stable biochemical model for genes, Barbara McClintock discovered genetic transposition, or genes that are able to change position within the genome. However, she did not receive recognition for her work until late in her career.
McClintock was a cytogeneticist, best known for her discovery in 1948 of transposons, mobile genetic elements that do not necessarily stay fixed on the chromosome, but she had made many fundamental discoveries before that. She invented a method for visualizing the chromosomes of maize and used it to visualize the crossing over of chromosomes during meiosis for the first time. She was the first to describe ring chromosomes formed after X-ray irradiation, and demonstrated the necessity for the nucleolus organizer on maize chromosome six. Her work on X-ray generated mutations prepared her for the unusual properties of the transposons. These she called 'controlling elements', because she believed that they were important for gene regulation.
McClintock gave a famous seminar on transposons at a Cold Spring Harbor Symposium in 1951, which was not at all well-received. Her mostly male colleagues were dubious if not outright hostile to her ideas. She ceased publishing on transposons in 1953, when she realized "acutely, the extent of disinterest and lack of confidence in the conclusions" she was drawing.
McClintock is often made the poster woman for gender discrimination in the sciences, but this characterization is really not fair. It is true that early in her career she complained of her career prospects while an assistant professor at the University of Missouri. But in 1944, at the age of 44, McClintock was made a member of the National Academy of Sciences, only the third woman to be so honored. A year later, she was elected the first woman president of the Genetics Society of America.
Not until 1983 did she receive a belated Nobel Prize for her work on transposons. But the delay in recognition was less about her gender, than the fact that she was way ahead of the molecular biologists of her day. They were having enough trouble coming up with a biochemical model for a genome that was stable from generation to generation without having to accommodate genetic elements that clearly were not. "One must wait for the right time for conceptual change," wrote McClintock.
Today, it is clear that much of what was once considered 'junk DNA' in the human genome is composed of transposons, some virally-related, which may represent fast-forward mechanisms for evolutionary change. For instance, the P elements of Drosophila were not present in the laboratory strains isolated in 1905 but are now ubiquitous in wild type flies. The mariner-like elements have managed an impressive horizontal spread through hundreds of species of insects, vertebrates and even plants. The positive aspects of such genetic parasitism are hard to ascertain, but the suspicion is that there must be some; otherwise it would not be so prevalent.