Concept 32 Some DNA can jump.

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Barbara McClintock presents her work with maize.

Hello, I'm Barbara McClintock. Ever since graduate school I had been interested in corn. I tried to correlate chromosome behavior with the results of breeding experiments in corn (maize). I spent most of my scientific career focusing on the short arm of chromosome 9, where, as a graduate student, I observed a characteristic bump or "knob." In the '30s, Harriet Creighton and I followed the transfer of this knob between two chromosomes during formation of sex cells. This chromosome "mixing" exactly paralleled the mixing of visible traits in the offspring. This was the first direct demonstration of the physical basis of genetic "crossing over," which had been discovered two decades earlier by Thomas Hunt Morgan’s group. Crossing over in corn works the same way it does in Morgan's fruit flies. Harriet and I tracked the inheritance of the knob and four traits on chromosome 9. Crossover events are more frequent between genes that are further apart, like C and Yg... ... and less frequent between genes that are closer together like C and Sh. Using crossover frequencies, we figured out the order of the genes on the chromosome and the relative distance between the genes. When I looked at a cell, it was like I was part of the system. The more I looked, the bigger the chromosomes got, and I was even able to see the integral parts of the chromosomes. I felt like I was right there with them. I was able to identify chromosomal abnormalities. I could see chromosome fragments left over from broken chromosomes. I also figured out that "ring" chromosomes are actually chromosomal fragments whose ends fused to form a ring structure. I tracked the inheritance of these rings through mitosis and found that they can be lost. So, different populations of corn cells can express different genes depending on when they keep or lose the ring chromosome. I suspected that some kind of chromosome breaking — or dissociation — on the short arm of chromosome 9 caused purple-spotted kernels. I tracked the source of instability to a locus I called Dissociator (Ds), which was under the control of a second locus called Activator (Ac) on the long arm. Let me explain. Although you surely are familiar with white or yellow corn, certain types of corn naturally produce dark purple or blue kernels! So called "blue corn" tortillas are made from such kernels. The Colored (C) gene on the short arm of chromosome 9 controls the production of one of the purple pigments. The Colored gene is passed on as cells divide to produce the kernel. However, the purple color only develops in the "skin" of the kernel. I reasoned that colorless kernels resulted when a copy of the Ds element inserts and disables the Colored gene. Then, all of the cells in the developing kernel inherit a nonfunctional gene, which cannot produce the purple pigment. This results in a white or yellow kernel. Now imagine what happens if Ds is destabilized in the presence of Ac. Kernel development begins with the Colored gene disabled by an inserted Ds element. However, at some point a transposition event occurs in a single cell. Under control of Ac, Ds "jumps" out of the Colored gene and reinserts elsewhere on Chromosome 9. This results in a functional Colored gene, which is passed on to a large group (or clone) of daughter cells. A second Ds transposition occurs later in development, resulting in a smaller group of daughter cells with a restored Colored gene. So, the number and size of spots indicates the frequency and timing of transposition events during kernel development. Early transpositions produce fewer and larger pigmented spots, while frequent events later in development produce a finely speckled pattern. I believed that transposition had a more important function than merely to turn color genes on and off. In 1983, at my Nobel Prize lecture, I emphasized that transposition can provide a means to rapidly reorganize the genome in response to environmental stress. In this sense, mutations produced by transposition are a source of variation to drive the process of evolution.

factoid Did you know ?

The McClintocks had picked out the name "Eleanor" for their 3rd daughter. They changed this to "Barbara" because "Barbara" seemed more masculine, and more in tune with her independent spirit.


How and why would a "transposing" DNA have evolved?