5/26/2023 0 Comments Ifactor californiaThe frequency of transposition of I factors in inducer strains is low but is increased by several orders of magnitude in the female progeny of crosses between reactive females and inducer males ( 9). A few strains lack these active elements and are known as reactive strains ( 8). These include full length functional elements. The majority of strains, the so-called inducer strains, also contain 10–15 I factors in the euchromatic DNA of the chromosome arms. To transpose, the I factor is transcribed into a full length RNA that is reverse transcribed into a copy integrated at a new site in the genome ( 5– 7).Īll strains of Drosophila melanogaster studied so far contain 20–30 defective I factors per haploid genome located in pericentromeric DNA. Like other elements of this type, it has no terminal repeats but possesses a deoxyadenosine-rich sequence at the 3′ end of one strand and has two ORFs, the first encoding a nucleic acid binding protein and the second a putative reverse transcriptase ( 3, 4). The I factor is a non-long terminal repeat, or LINE-like, retrotransposon ( 3). The mechanisms that restrict transposition are understood poorly, although they include element encoded regulatory molecules ( 1) and interactions between elements and the products of host genes ( 2). There is probably strong selection for mechanisms that limit the frequency with which these elements transpose as transposition leads to an increase in the number of copies of an element and can cause insertional mutations and gross chromosome rearrangements. Transposable elements are a substantial component of eukaryotic genomes, often making up >15% of the total DNA.
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