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Barbara McClinktock, Transposition “jumping genes”
Group “Jumping Genes”
Alicia, Edeline, Yves, Brittinie, Gaelle
Molloy College
DR. Cutter
Bio 127

Jumping genes are transposable elements that move from one location to another on a genome. McClintock showed that chromosome swapped genetic information by crossing over when sex cells are formed. This experiment was designed to reveal the genic composition of the short arm of chromosome 9. A rare phenomenon occurred in maize and has resulted in high frequencies in the cultures. Heterochromatic materials of the chromosomes have a role to play, which is in inducing and controlling the type of variegation and its time and frequency of occurrence. The maize cultures were found to have a large number of mutable loci. Each locus such as Wx, C and Ds are each responsible for a particular trait. The transfer of a knob or bump on the chromosome during the formation of sex cells has occurred and the rings on the chromosome could express different genes. The dissociation of the chromosome of the short arm of chromosome 9 cause purple spotted kernels. The dissociation loci act as an activator and inserts and disable the color gene. This results in white or yellow kernels. The mechanism underlying the phenomenon of variegation is basically the same in all organisms. This experiment shows the different effects of what could happen in mitosis or meiosis. A mutation could occur at any time. In the maize, it happens to be chromosome 9 that carries the mutation. The traits on the chromosome are expressed for their function on the chromosome. This experiment reorganizes the genome to see what happens in mutations for this plant. Mutations produced by transposition are a source of variation to drive the process of evolution.

McClintock conducted her experiment with several maize genes using a method now known as deletion mapping. Mclintock made use of two genetic marker that were on chromosome 9 of the maize (Nanjundiah, 1996). One of the genes affected seed coat color. C was for colored and c was for colorless and the second was for the composition of food reserve Wx for starchy and wx was for starchy (Nanjundiah, 1996). C and Wx are dominant which exert their effects in a single dose. Allele’s c and wx are recessive and exerted their traits in double doses (Nanjundiah, 1996). A plant that contained the chromosomal combination wx C/Wx c, with a knob on the chromosome that contained the C, was crossed with a plant that contained no knobs, two copies of c and one each of Wx and wx (Nanjundiah, 1996). Many different types of progeny were produced. Some possessed the colored characters with starchy, this was because of the exchange of that part of the chromosome containing the knob (Nanjundiah, 1996). In the created progeny, the presence of the gene that was located closest to the knob was because of the knob itself, this observation compelled McClintock to think of genes as characters on chromosomes (Nanjundiah, 1996). In this experiment she discovered several facts about chromosomes and their genes but it wasn’t the only experiment she conducted. In further experiment she began to study gene transposition. She crossed a male parent that was composed of CC Ds Ds Ac Ac with cc Ds Ds Ac Ac. Ac is an activator and Ds is a Dissociator are two unstable genetic elements discovered by McClintock (Nanjundiah, 1996) . Half of the resulting progeny showed absence of Ac, and Ds is stable. In the second half a chromosomal break occurred and C was lost because of Ds, resulting in some colourless patches (Nanjundiah, 1996). But in one kernel Ds wandered into C and turned it into an unstable colorless variant which made numerous colored patches. She observed that broken ends of maize chromosomes behaved in abnormal ways and that repairing the breaks required genetic activity (Nanjundiah, 1996). Genetic instability didn’t pose an issue but... Show More

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