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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.
In the process, they may
- cause mutations
- increase (or decrease) the amount of DNA in the genome.
These mobile segments of DNA are sometimes called "jumping genes".
There are three distinct types:
- Class II Transposons consisting only of DNA that moves directly from place to place.
- Class III Transposons; also known as Miniature Inverted-repeats Transposable Elements or MITEs.
- Retrotransposons(Class I) that
- first transcribe the DNA into RNA and then
- use reverse transcriptase to make a DNA copy of the RNA to insert in a new location.
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.
In the process, they may
- cause mutations
- increase (or decrease) the amount of DNA in the genome.
These mobile segments of DNA are sometimes called "jumping genes".
There are three distinct types:
- Class II Transposons consisting only of DNA that moves directly from place to place.
- Class III Transposons; also known as Miniature Inverted-repeats Transposable Elements or MITEs.
- Retrotransposons(Class I) that
- first transcribe the DNA into RNA and then
- use reverse transcriptase to make a DNA copy of the RNA to insert in a new location.
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