The Jumping Geneticist

By Toby Lanser


Mice, rats, flies, and the microscopic worm, C. elegans, are commonly thought of as the quintessential model organisms in science. Mice and rats offer their mammalian heritage to those wanting to relate their research closest to humans, flies offer their easily accessible and manipulatable genome to geneticists, and C. elegans offer their overall simplicity and high rates of reproduction to those looking for general manageability. From dizzying complexity to beautiful simplicity, researchers have honed in on ideal organisms to break down what makes life possible. But these are all animals, and we all know that there exists much more than animals in the diverse world of life.


It’s common in the fields of microbiology and genetics to use model organisms far simpler than our rodent cousins to study genes. Since DNA exist in all living creatures, anything from the most basic to highly complex mechanisms involving DNA can be studied in a microorganism. The size of the genome and number of genes has much less to do with the complexity of an organism than the organization of these genes. For example, the water flea has nearly 1.5 times as many protein-coding genes as humans, but the latter organism painted the Mona Lisa, created the atomic bomb, and put itself on the moon. So what can plants, fungi, and bacteria tell us about ourselves? According to ethnobotanist turned geneticist, Barbara McClintock, quite a bit.

Artwork by Reese Green

McClintock was born in 1902 in Connecticut to two British immigrants. At the age of eight, she moved to Brooklyn, New York, where she completed high school. She grew interested in botany in high school, and was accepted into Cornell University’s College of Agriculture. Before entering university, she was almost prevented from enrolling due to her mother’s fear that a college education would make her “unmarriageable.” Luckily, for both McClintock and the scientific community, her father interfered and she enrolled in 1919.


At Cornell, McClintock studied botany, and received a BS in 1923. Briefly debating what to do after graduation, she spoke with C. B. Hutchison, a famous geneticist whose course at Harvard was the scaffold for McClintock’s botany curriculum at Cornell. Looking back, McClintock noted that after this conversation she knew that her future lay in genetics. A certain roadblock prevented her from “officially” pursuing genetics, however. At Cornell, where she decided to enroll for graduate school, women were not allowed to pursue a degree in genetics. Rather, they could receive a degree in botany and concentrate in genetics, which is exactly what McClintock did. She graduated with a masters in 1925 and a PhD in 1927.


In graduate school and during her postdoc years, McClintock took a keen interest in cytogenetics, the study of how the chromosomes of a cell relate to the cell’s behavior. Her model organisms were different from the traditional biologist, however. Since as a botanist her interest mainly lay in plants, McClintock chose the cereal grain, maize (better known as corn), as her ideal specimen. Using maize as her organism of interest, McClintock was able to visualize the individual chromosomes of the maize cells under a basic light microscope. Her cytogenetic studies revealed many valuable results, including the link between chromosomal crossing over during meiosis and the recombination of genetic traits, revealing the mechanisms behind the process that provides life with its immense diversity. Through this, she was able to observe how the recombination of chromosomes correlated with new traits in the offspring, a key discovery in genetic diversity.


McClintock springboarded off her discoveries, hopping around the United States and Europe and landing a position at Cold Spring Harbor Laboratory in Long Island. At Cold Spring Harbor, McClintock made her most astonishing discovery, the movement of individual genes along a chromosome. These genes, which she named transposons or “jumping genes,” contained two new dominant and interacting genetic loci, locations on a chromosome, named Dissociation (Ds) and Activator (Ac). The Ds loci causes breakage in the chromosome, and when the Ac loci is present it has adverse effects on the neighboring genes in the chromosome. This discovery launched a series of investigations into the interactions of the chromosome and the genetic expression of the cell.


McClintock’s discoveries in maize genetics had far reaching implications in the field of genetics, embryology, and cytogenetics, and in 1983 these discoveries lead her to be the first female to win a solo Nobel Prize in any field. She also received the National Medal of Science in 1970, the Thomas Hunt Morgan Medal in 1981, and the Louisa Gross Horwitz Prize in 1982. Throughout her life, McClintock overcame immense social challenges and to this date, she still remains the only woman to have received an unshared Nobel Prize in Physiology or Medicine. She died in 1992, having never married or having children, proving her mother right in at least one sense.


References:


“The Nobel Prize in Physiology or Medicine 1983.” NobelPrize.org, www.nobelprize.org/prizes/medicine/1983/mcclintock/biographical/.


“The Embryo Project Encyclopedia.” Barbara McClintock (1902-1992) | The Embryo Project Encyclopedia, embryo.asu.edu/pages/barbara-mcclintock-1902-1992.


“The Barbara McClintock Papers: Biographical Information.” U.S. National Library of Medicine, National Institutes of Health, profiles.nlm.nih.gov/ps/retrieve/Narrative/LL/p-nid/45.

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