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Look closely at a cob of multicoloured maize and you may see kernels with spots and streaks. Most people see decoration. Barbara McClintock saw a record of genes switching on and off as the kernel grew, and from it she worked out that some pieces of the genome can move. This post explains what she found, how she read it from kernels, and why it took so long to be accepted.
Who was Barbara McClintock?
According to the Cold Spring Harbor Laboratory archives, she was born in Hartford, Connecticut, on 16 June 1902 and took her BS (1923), MA (1925) and PhD (1927) at Cornell. The Nobel facts page notes that her family had little money and thought marriage more important than research, but with her father's support she began studying at Cornell's College of Agriculture in 1919.
Early on she was already a leading cytogeneticist, someone who links genes to what can be seen in chromosomes under a microscope. With Harriet Creighton she published a 1931 paper in PNAS showing that genetic crossing over is accompanied by a physical exchange between chromosomes. In 1941 she joined the Carnegie Institution's department at Cold Spring Harbor, New York, where she stayed. She was elected president of the Genetics Society of America in 1945. She died on 2 September 1992.
What did she discover?
Her Nobel lecture describes an experiment that began in the summer of 1944 with about 450 maize plants, each grown from a zygote that had received from both parents a chromosome with a newly broken end. Among the offspring she saw unusual patterns of mutation.
The 1983 Nobel press release explains what she worked out. On chromosome 9 she found an element that caused the chromosome to break, which she named Dissociation, or Ds. Ds could move along the chromosome, and where it landed it could switch off neighbouring genes. If it moved away again, the gene could work again. Ds moved only when a second element, Activator or Ac, was present. She called these "controlling elements". The Nobel Prize was awarded "for her discovery of mobile genetic elements".
The CSHL archive notes that she confirmed and extended her observations for six years before publishing, in The origin and behavior of mutable loci in maize in PNAS in 1950.
A worked example: reading timing from spot size
The press release explains that the number and size of coloured spots tell you how often a change happened and when during development. Here is the logic in a simplified model where every cell divides in step.
A kernel's coloured tissue grows from one cell by repeated division: 1, 2, 4, 8, ... cells.
Suppose a gene for colour is switched off by Ds, but in one cell Ds jumps away and colour returns. All descendants of that cell are coloured.
If the jump happens when there are 8 cells, that one cell's descendants make up 1/8 of the final tissue: a large streak.
If it happens when there are 1,024 cells, its descendants are 1/1,024 of the tissue: a tiny dot.
So big sectors mean early jumps, small dots mean late ones, and the count of dots tells you how often jumps happen.
The press release adds that different forms of Ac signalled early or late in development, which could be read from the size of the spots. Real kernels do not divide perfectly in step, so this is only a model, but it captures the reasoning the press release describes.
Why did it take so long to be accepted?
Her first public presentation was at the 1951 Cold Spring Harbor Symposium. The CSHL archive says she expected acceptance but met "silence and derision", and that scientists respected her data while doubting her conclusions. The Nobel press release suggests reasons: much of her work appeared in her institution's annual reports and maize newsletters, and she was far ahead of other fields of genetics.
The picture changed in the mid-1960s, when, the press release says, mobile genetic elements were found in bacteria and shown to help spread antibiotic resistance. They were later found in insects, animals and humans. She received the National Medal of Science in 1970, CSHL records, and the 1983 Nobel Prize, which she did not have to share.
What can a student take from her way of working?
Question the accepted explanation. The press release notes that spotted kernels were already known and were thought to reflect fragile chromosome regions. She asked what really made the spots.
Connect two kinds of evidence. She matched visible traits with chromosome changes under the microscope. Linking a result to its mechanism is what makes a finding convincing.
Check for years before you publish. Six years of confirming her observations gave her data that others respected even when they doubted her conclusions.
Do not wait for applause. She kept working through years of doubt.
For the genetics behind her work, our posts on Mendel's pea experiments and the molecular basis of inheritance cover crossing over, linkage and gene expression. If a term like transposon or linkage map keeps slipping, the Fermi Concept Library has every concept in biology with its own lesson.
Find a picture of a spotted maize cob, pick one large streak and one small dot, and explain which change happened earlier.
