Transposons: genes that can move

Contributed by: Julia Brose, Hannah Brown, Roger Ort @JARNBRAK, and Ethan Thibault @AGCThibault

 

Keywords

Agriculture, DNA, Evolution, Experimental, Field, Fundamental research, Genes, Genetics, Historical figure, Lab, Medicine, Molecular biology, North America, Observational, Plants, Technology, Woman

 

Slides

Note: click the gear symbol to see notes that accompany the presentation

View and download in google slides here

 

Resources

McClintock, B. (1950). “The origin and behavior of mutable loci in maize”. Proceedings of the National Academy of Sciences of the United States of America. 36 (6): 344–355.

McClintock, Barbara. “The stability of broken ends of chromosomes in Zea mays.” Genetics 26.2 (1941): 234. Figure comes from textbook “Introduction to Genetic Analysis” 9th edition.

“A Feeling for the Organism” by Evelyn Fox Keller

Pray, L. & Zhaurova, K. (2008) Barbara McClintock and the discovery of jumping genes (transposons). Nature Education 1(1):169

 

Notes

Slide 1: Researcher’s Background

Dr. Barbara McClintock was a geneticist and cytogeneticist at Cold Spring Harbor in New York where she studied chromosome replication in Zea mays (corn/maize). She theorized the existence of transposons and received the 1983 Nobel Prize in Physiology or Medicine for her discovery.

Biography in brief

Barbara McClintock was born in Hartford, Connecticut on June 16,1902 to a poor family who encouraged her, at first, to marry well. With her father’s eventual support, however, Barbara began studying agriculture at Cornell in 1919 at the young age of 17. Barbara McClintock grew up appreciating the study of science as her father was a physician. She received her B.Sc in botany in 1923 at Cornell University’s College of Agriculture. At this time women could not major in genetics and therefore her subsequent MS (1925) and PhD (1927) were both in botany. She remained at Cornell after completing her PhD and continued her work in cytology and genetics. She then received a fellowship from the National Research Council from 1931-1933 where she worked at the California Institute of Technology, the University of Missouri, and Cornell. She received a Guggenheim Fellowship and moved to Germany however left due to the rise of the Nazis. She return to the U.S. and attempted to obtain a faculty position at Cornell, only to find out that they would not hire a female professor. Eventually McClintock was hired by the University of Missouri where she continued her research on cytogenetics, however she eventually left because she did not think she would gain tenure. She accepted a one year position at Cold Spring Harbor which ended up turning into a full time position. She continued the remainder of her career there which included her Nobel Prize winning research on transposons or ‘jumping genes’. Barbara was also the first woman to become president of the Genetics Society of America. She became the third woman member of the National Academy of Sciences (1944). She also received the National Medal of Science in 1970. In 1983, she became first American woman to win an unshared Nobel Prize. Outside of her career, not much is known about McClintock other than she never married or had children. 

Is (or was) their research under-valued because of their identity?

Biographies of her life tell conflicting stories about whether or not being a woman in her field put her at a disadvantage or caused her to be discriminated against. Either way, she was a minority and accomplished incredible advances in her field that have become staples of molecular biology and genetics today.

Are there other scientists/research examples that this example can replace or be added to?

Botanist Harriet Creighton, a co-worker and collaborator

 

Slide 2: Research Overview

Take home message of study

What McClintock noticed was that the colorful mosaic patterns of corn changed immensely between generations, which could not be explained by genetic theory at the time. In attempting to explain this color instability, she discovered that some genes could “transpose” – move within the chromosome – and their location would affect gene transcription (and, therefore, the color of the kernel). Transposons are segments of DNA that are able to change their location in the genome. They are also referred to as jumping genes.

Study system

This bottom shows the primary study system of Dr. Barbara McClintock, Zea mays. Maize is an important agricultural crop being responsible for a large portion of today’s food source, both for direct consumer consumption as well as livestock feed. Maize contains a highly repetitive genome with a high number of transposons, making it the perfect model system for McClintock to study cytogenetics. The top images show corn with colorful kernels and a mosaic pattern. You can spot purple, colorless, and spotted kernels! The spotted kernels are due to transposons.

 

 

Slide 3: Key Research Points

Main figure

Here in the top row of the figure, you see a wild type version with the pigment gene (c) that is functioning normally and fully pigmenting the corn kernel. In the second row, the Ds element has ended up in the C gene and without the presence of Ac, cannot transpose out of the C gene. This leads to an entire corn kernel that lacks a functioning pigment gene and you get a colorless kernel. In the third row, the Ds element is present in the C gene, however, the Ac element is present and facilitates the transposition of the Ds element out of the C gene, but not constitutively. This leads to groups of cells either being pigmented or not, depending on if their developmental progenitor cell had the Ds element move or not move out of the C gene. The final row is very similar to the third row, however, the spotted kernel is a result of Ac element being in the C gene instead. The Ac element has the ability to cause its own transposition out of the C gene (but again, it doesn’t always happen) and this results in some developmental fields having all pigmented or all non pigmented cells.

Source: http://www.bio.miami.edu/dana/250/250SS13_17.html

Societal Relevance

Barbara McClintock changed how scientists think about genomes. Prior to her discovery, it was thought that the genome could not change in an organism and that rearrangement of genes could never happen. McClintock’s discovery was ahead of its time, and her attempts to publish and present her work in the 1930s-50s were met with skepticism. It was not until the 1970s, when other scientists began arriving at her same conclusions, that she was finally taken seriously and the implications of her work were felt in full. The understanding of how traits are passed down from generation to generation via chromosomes is vitally important to understanding agronomic traits as well as genetics medical conditions. Not only was her discovery in a major agricultural crop of Maize, therefore pushing forward our discoveries in agriculture, it was a basic genetic discovery that revolutionized the way we view genome structure. With this basic discovery of transposable elements, researchers have found them to be related to different human diseases (as well as in crops) and transposable elements have been genetically engineered to be used as genetic tools to study essentially all genes in the genome. These are called T-DNA lines where a modified transposable element is used to knockout the function of a gene and the model species Arabidopsis has a library of seed lines with a knockout of almost every gene in the genome. It has created a truly valuable tool for studying all of genetics.