Once upon a time, geneticists thought of genes like beads on a string. In 1950, Barbara McClintock published a paper that would refute and update this view – an accomplishment that wouldn’t be recognized until her Nobel Prize, thirty five years later.
She started by studying colorful generations of maize and ended up with the discovery of transposons, affectionately referred to as jumping genes. Rather than remaining in their place as beads on a string, they can jump in and out of place, inserting themselves into new regions of the genome.
How did she discover this? McClintock took a look at chromosome structures (she could tell whether they had been broken or not), and she could check the phenotype of the kernels in her maize plants. She could determine the location of a gene in the genome. But DNA sequencing technology wouldn’t be available until the 1970’s. The methylation that silences genes (an observation she made in other research) wouldn’t be discovered till 1978. How was McClintock able to make these prescient discoveries?
The Colors of Maize
Maize is a colorful type of corn, with purple, yellow, and speckled kernels found throughout the cob. McClintock wanted to know where these color patterns came from.
The gene for a purple kernel is dominant, and denoted as gene C. Yellow kernels are recessive, using gene c. That explains that. But what about the speckled kernels? What gene controlled that?
McClintock noticed that sometimes, there was a break in the chromosome, left by a missing gene she called Ds. (Ds is for dissociation, by the way. It’s a gene that disassociates with a single locus, or location in the genome). But this Ds gene wasn’t gone forever. It showed up again elsewhere in the genome. When Ds popped up inside the C gene, the kernel wasn’t purple anymore.
You may recall the experiments of Gregor Mendel, who played with pea plants in his monastery and founded the field of genetics. To pin down traits like purple (P) or yellow (p) flowers, and tall (T) or short (t) plant height, he used self-pollinated lines of pea plants. In this way, he could identify the exact genes the pea plants had. Draw out your Punnett squares: self-pollinate a yellow flowered pea plant, and its offspring will only have yellow flowers. A purple-flowered pea plant could have the genes Pp or PP; even if the recessive yellow gene was present, only purple flowers would be seen. It’s the dominant one, after all. If you want to determine the genetics of this pea plant (is it a heterozygous Pp, or a homozygous PP?), you can again self-pollinate it. A self-pollinated Pp pea plant will result in about 75% of the offspring with purple flowers, but 25% will inherit pp and have yellow flowers. If the parent is PP and self-pollinates, however, all of its offspring will have purple flowers – they can only inherit P.
McClintock also used self-pollinated corn plants to really hone down on the genetics of her maize. She knew that cc produced a yellow kernel, but Cc and CC produced purple kernels. But when working with her homozygous purple-kernelled maize, she sometimes saw yellow kernels! When that happened, it was because Ds had jumped right into the C gene. This silenced the gene, yielding yellow kernels. They weren’t yellow because of the c; they were yellow because of this jumping Ds gene.
Now, McClintock noted that Ds worked under the role of a mastermind. A separate gene, Ac (for activator) controlled whether or not Ds jumped into the purple kernel C gene. It could also tell Ds to jump out! If this happened during the kernel’s development, the kernel would experience a time when C was silenced, but also when C was expressed. That gave it a dappling of purple and yellow within the same kernel. A jumping gene during development explained the speckled maize pattern.
Transposons can play multiple roles within the genomes of living things, maize or otherwise. Sometimes they can be destructive, and are silenced in a genome. In the case of maize, they play a harmless role in pigmentation. When transposons jump in and out of a locus, they aren’t always terribly careful about their cut sites. That means that sometimes, they take a gene, or part of a gene, with them on the jump! This shuffling of genetic information adds to the complexity of genetics and genomics, assisting in the piecing together of the ancestry of many species. Finally, transposons have been used by biotechnologists as yet another tool for inserting new information into DNA.
McClintock was awarded the Nobel Prize in Physiology or Medicine in 1983. “It might seem unfair,” she said, “to reward a person for having so much pleasure over the years, asking the maize plant to solve specific problems and then watching its responses.” Well, McClintock my feel so, but I doubt the rest of us are any less impressed with her love of science and the discoveries she made in it.
McClintock, B. (1950). The origin and behavior of mutable loci in maize. PNAS, 36, 344–355.
Fedoroff, N. V. (2012). McClintock’s challenge in the 21st century. PNAS.
Pray, L. (2008). Transposons: The jumping genes. Nature Education 1(1), 204.
Ravindran, S. (2012). Barbara McClintock and the discovery of jumping genes. PNAS.