Transposons
What are transposons?
The genetic studies of Zea Maize by Barbara McClintock and also by Marcus Rhoades, revealed that there are jumping elements in the genome. They have been detected through the abnormalities they produce in the activities of the genes near their new position to which they move. If a transposon has integrated into a gene involved in corn pigmentation, the resulting corn reveals a mosaic phenotype. This is seen in the picture below. Transposon-induced mutations are therefore said to be unstable and the resulting variegated phenotypes can be taken as evidence for the presence of transposons within the genome of this organism.
The discovery of transposons
The
chromosomal basis of heredity was already well established by the time McClintock
began her
graduate training in the Botany Department at Cornell
University. Her experiments laid the groundwork
for a serie of cytogenetic discoveries by the Cornell maize genetics
group between 1929 and 1935. McClintock developed
a method for using broken chromosomes to generate new mutations. Among
the
progeny of plants that had received a broken chromosome from each parent,
she observed unstable mutations at an unexpectedly high frequency, as well
as a
unique mutation that defined a regular site of chromosome breakage. These
observations so intrigued her that she began an intensive investigation of
the chromosome-breaking
locus. Within several years she had learned enough to reach the conclusion,
published in 1948, that the chromosome-breaking locus did something
unknown for any genetic locus: it moved from one chromosomal location to
another, a phenomenon she called transposition.
The study of transposable genetic elements and transposition became the central theme of her genetic experiments from the mid 1940s until the end of her active research career. This was incredulous at the time, DNA was believed to be stable and invariable. These jumping elements were isolated from the bacterium Escherichia coli in the late 1960's and were further defined as specific, small fragments of DNA which were given the name transposons. The scientific interest in transposons increased during the 1970's, when it appeared that they assisted in the transfer of bacterial resistance to antibiotics. Furthermore, it soon became evident that they caused most of the spontaneous mutations occurring in laboratory populations of more sophisticated organisms, such as yeast and the fruit fly. We now know that transposons are ubiquitous and may comprise up to 20% of an organism's genome.
How can transposons move through the genome?
Several mechanisms of transposition are found in procaryotes as well as eucaryotes. In the E. coli bacteria there are replicative and conservative methodes of transposition. In the replicative way, the new copy of the transposable element appears at a new site. The original element stays at the old location. This is a duplication. In the conservative way there does not exists a copy of the original element. The element moves from one site in the chromosome to another. This means the transposable element is jumping around in the genome of the host. By these movements throught the genome of the host, transposons can generate a large amount of deletions and inversions in the genome. A special kind of transposable elements are retroviruses. Retroviruses are viruses that can integrate there genome in the genome of the host, by copying RNA into DNA by the enzyme reverse transcriptase. It can stay there for a long time, untill the integrated genome of the virus gets some kind of signal, that makes it copying itself out of the host genome.