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Retrospect

The height of knowledge

63 years ago, Barbara McClintock discovered on making a study of corn that the genome is not stable

ReproductionBarbara working in her laboratory in 1947Reproduction

In May of this year, the release of the first version of the Aedes aegypti mosquito genome surprised geneticists by its size: 1.4 billion base pairs, the chemical units that make up the genetic code. The surprise was connected with human DNA, as almost half of the entire Aedes sequencing (47%) is comprised of transposition elements (or TE), which are DNA parts that can change their position within a genome, hence biologists’ informal name for them: jumping genes. A TE may contain one or more genes and genetic varieties produced through mutation. Today, these genomic details are studied in depth and considered valuable to understand each animal or vegetable species better. However, when TEs were first discovered in 1944 by Barbara McClintock, an American biologist, the scientific world practically overlooked the fact.

Barbara (1902-1992) became interested in genetics before she turned 20, while still in high school. “Only 21 years had gone by since the rediscovery of Mendel’s hereditary principles”, she mentioned in a short autobiographic article. “Genetic experiments, driven by those principles, grew quickly from 1900 to 1921”. At Cornell University, where she began studying in 1922, she specialized in the genetics of corn. She was the first person to draw up its  genetic map and to show the importance in  cell division of telomers – the end portion of a chromosome. While working at the Cold Spring Harbor laboratory in 1944, she discovered TEs, which she called controlling elements at first.

This is how she described the main part of her observations: “Believing that I was witnessing a basic genetic phenomenon, all my attention was turned toward determining what one cell had gained and the other cell lost as a consequence. It turned out that these were transposition elements that could regulate the expression of genes precisely. That is why I called them ‘controlling elements’.” She realized that two of the transposition elements could switch places in the corn chromosomes. Changing position in the chromosome changed certain pigments’ synthesis, which is why certain cobs had kernels of various colors. Later, she also managed to explain that TEs only moved when the cells were under some form of stress, such as reproduction.

ReproductionNobel Prize awarded 39 years after her discovery of transposition elementsReproduction

Her work changed the notion that the genome was a static thing. However, ten years would go by before her work was recognized. Barbara was only awarded the Nobel Prize for Medicine or Physiology in 1983, at the age of 81 and 39 years after her chief discovery.

“Barbara had the courage to say that the genome is not stable and this didn’t go down well at the time”, says biologist Marie-Anne Van Sluys, from the Genomics and Transposition Elements Laboratory of the Botany Department at the University of São Paulo. She was, moreover, a woman going against other contemporary researchers who were all male and insisted that genes always stayed put.

Her work continues to bear fruit. Until a few years ago, TEs were regarded as part of non-coding junk DNA. However, the very idea of junk DNA is now under review. “There are results showing that TEs may have a function and not only in regulating other genes; there is evidence that some have been ‘domesticated’ and co-opted into playing a ‘normal function'”, explains Marie-Anne.

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