By Sarah Rios & Alexandra Glushenko

Barbara McClintock "The Origin And Behavior Of Mutable Loci In Maize"

American Scientist Barbara McClintock was born in Hartford Connecticut in 1902. After attending high school, she became a biology major at Cornell University. She received a bachelor of science in 1923, and a master’s degree two years later, and, she earned a Ph. D in1927 after specializing in genetics, zoology, and cytology. In 1983 she earned the nobel prize for physiology for her work in 'mobile genetic elements'. 


Barbara McClintock first hypothesized that during cellular division, some cells lost genetic material but others gained what the others had lost. Second  hypothesizes was that the mechanism behind unstable genes in fruit flies and maize is the same.

Procedure/ Methods 
To prove her hypothesis she used phenomenon studied in Drosophila (a species of fly), which she found occurring in corn. Mutable loci occurred in the progeny coming from self-pollination of about 450 plants, where the short arm chromosome 9 was subjected to drastic structural modification. Some of the plants had other modifications. The experiment had two major mutations : 
1. Activator-requiring class- those that require a separate activator factor for instability to be expressed.
2. Autonomous class- others that are autonomous with respect to the factor that controls the onset of mutability.
 After chromosomes was breaking, transposable elements was moving along the chromosome and was inserting  themselves into the chromosome at a different location. Important part of this experiment was the use of dissociation (D), that illustrated how new mutable loci could arise.

While monitoring chromosome 9, McClintock noted a common break on the Ds -or dissociation- locus. Plant that was carrying chromosomes 9 had the dominant C allele. It appeared that when Ds would occur in new please, C locus would disappear from that place forever, while C locus would restore again. The restoration of the action was due to the removal of the Ds chromatin.Previous studies determine that the C locus will give rise to a c phenotype. Moreover, after more research she realized the Ds could change position within chromosome 9.Shifts in the Ds locus also required a secondary element at a separate locus that could initiate its own shift -known as the Ac locus-. Breaks in Ds region of the chromosome could also be activated by Ac elements at different sites or even on different chromosomes. 

After breeding several generations of maize, McClintock discovered the presence of 'transposable elements' within the genetic information. At the time, it was believed genes remained in a single locus on a chromosome instead of moving to separate locations. But she believed that  the mechanism underlying the phenomenon of variegation is basically the same in all organisms. Her work was an innovative discovery in the field, although it took several decades before it was widely accepted.Eventually, her work led to a nobel prize in 1983. Today, scientist's knowledge about transposable elements is now applied to disease, since they have the potential to disturb normal gene function.  

What We Learned About Genetics

McClintock's paper taught us that genes do not always stay in one place, and that certain pieces of genetic information can switch places on the chromosome. Also she noted that chromosomes can physically break and reattach in cells as an organism develops.