Transposable Elements and Plant Molecular Genetics

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The Beginnings of Plant Molecular Genetics

Plant molecular genetics evolved from classical genetics (such as the work of Gregor Mendel). Classical genetics has formed the fundemental genetic principles on which molecualar genetics is based. These are:

  • Detailed molecular analysis
  • Applications in biotechnology

    • Transposable Elements (transposons)

    A transposon is a "jumping gene1". Transposons were fist identified by Barbar McClintock in the late 1930's and early 1940's when she worked on unstable mutants of maize. The transposons were causing the mutant instability, as they 'jump' into another gene, generally causing a loss of function2. They then may jump back out of the gene, and a restoration of function may occur. Transposons occur naturally in many plant species. For example, transposons are responsible for changes in flower colour in snapdragons.

    Insertion

    When a transposon enters a new 'host' gene it causes a duplication of a small part of the host DNA3 sequence. The size of this duplication is a characteristic feature of each type of transposon.

    Excision

    When a transposon jumps out of a host gene it ofte leaves behind the duplicated DNA sequence. If the excision process is accurate, the duplicated DNA left behind is a record of the visit of the transposon. This is known as a footprint.

    Stable Mutants

    Altough less is known about stable mutants than unstable mutants, they are probably more common. There are two main types of transposon caused stable mutants;

  • Gene is permanantly mutated by a footprint. For examply, the tranposon inserts into the DNA sequence encoding new amino acides of an enzyme. Excisision leaves behind a footprint. The footprint produces a frameshift mutation changing all of the amino acids downstream of this site, causing a total loss of function.
  • Internal deletion of transposons. For example, the transposon insertion process results in a loss of some of the transponon's own DNA. This deletion leads to the loss of transposon function, as it can no longer make the 'transtosase' enzyme that it needs to be able to jump. Excision can therefore not occur and the host gene is permanantly mutated.

    • Transposon Tagging

    1. This is a method of isolating mutant genes which have not previously been analysed at the DNA sequence level.
    2. New genes 'cloned' in this way can be manipulated to produce improved plants.
    3. The method relies on their insertion of transposons into a gene of interest causing a mutant phenotype
    4. The transposon DNA sequence is known and its presence inside a gene effectively 'labels' or 'tags' the gene of interest.
    5. DNA 'probes' specific to the transposon can be used to isolate DNA 'flanking' the transposon. This flanking DNA should be part of the gene of interest.
    6. DNA 'probes' based on the flanking DNA should allow the isolation and sequencing of the previously unknown gene of interest.
    7. Transposons have been artificialy transferred to new host plant species, so that they can be used for transposon tagging of important genes.
    1A gene being a unit of heredity composed of DNA2When a gene losses its function, it can no longer code for the specific protein that it would normally produce3deoxyribonucleic acid

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