Barbara McClintock revolutionized the field of plant genetics, receiving the Nobel Prize in Physiology in 1983 for discovering “mobile genetic elements”. The science of genetics, to which McClintock made groundbreaking contributions, both experimental and conceptual, has come to dominate all of the biological sciences; from molecular biology, through cell and developmental biology, to medicine and agriculture.
McClintock discovered genetic transposition and used it to demonstrate that genes are responsible for turning physical characteristics on and off. She developed theories to explain the suppression and expression of genetic information from one generation of plants to the next.
Barbara McClintock began her scientific career at Cornell University, where she pioneered the study of cytogenetics, a new field in the 1930s, using maize as a model organism. The marriage of cytology—the branch of biology concerned with the structure and function of plant and animal cells—and genetics became official in 1931, when McClintock and a graduate student Harriet Creighton provided the first experimental proof that genes were physically positioned on chromosomes by describing the crossing-over phenomenon and genetic recombination.
Although Thomas Hunt Morgan was the first person to suggest the link between genetic traits and the exchange of genetic material by chromosomes, 20 years elapsed before his ideas were scientifically proven, largely due to limitations in cytological and experimental techniques. McClintock’s own innovative cytogenetic techniques allowed her to confirm Morgan’s ideas, and these techniques are numbered among her greatest contributions to science.
To better grasp this idea of McClintock’s model, consider every maize kernel as a single individual, originating as an ovule that has undergone double fertilization. During double fertilization, one sperm fuses with the egg cell’s nucleus, producing a diploid zygote that will develop into the next generation. Meanwhile, the other sperm fuses with the two polar nuclei to form a triploid endosperm, which forms an outer protein layer known as the aleurone layer. As a result, the colored—or colorless, as the case may be—tissue that makes up the aleurone layer of the kernel is triploid, not diploid.
McClintock worked with what is known as the Activator/Dissociation (Ac/Ds) system in maize, which she discovered by conducting standard genetic breeding experiments with an unusual phenotype. Through these experiments, McClintock recognized that breakage occurred at specific sites on maize chromosomes; the first transposable element she discovered was a site of chromosome breakage, aptly named “Dissociation” (Ds). Although McClintock eventually found that some transportable elements can “jump” autonomously, she initially noted that the movements of Ds are regulated by an autonomous element called “Activator” (Ac), which can also promote its own transposition.
Beyond her discovery of transportable elements and her revolutionary cytogenetic research techniques, Barbara McClintock was also the first scientist to correctly speculate on the basic concept of epigenetics, or heritable changes in gene expression that are not caused by changes to DNA sequences. Mainly, she recognized that genes can be expressed and silenced during mitosis in genetically identical cells. McClintock proposed this theory before the molecular structure of DNA and more than 40 years before the concept of epigenetics was formally studied, thereby further cementing her reputation as an innovator in her field.
Barbara McClintock’s discovery of transposable elements changed the way scientists think about genetic patterns of inheritance. Although not widely accepted at the time of its discovery, McClintock's observation of the behavior of kernel color alleles was revolutionary in its proposition that genomic replication does not always follow a consistent pattern. As a result of both autonomous and activator-controlled transposition at different stages of seed development, the genes of maize kernels are capable of producing a variety of coloration patterns.
Over time, McClintock’s work with transportable elements became widely accepted, and McClintock eventually earned a Nobel Prize for her discoveries in this area. Today, McClintock is also recognized for her groundbreaking cytogenetic techniques, as well as her early speculations on the concept of epigenetics. Thanks to these and other valuable contributions to the field, Barbara McClintock is rightly viewed as one of the pioneering figures in modern genetics.
References
www.nobelprize.org
www.nature.com
www.britannica.com