Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Mobile Genetic Elements
Mobile Genetic Elements in Eukaryotes

Eukaryotic MGEs make up about 10–30% of The Genome. They are dispersed throughout the genome, though they can occasionally concentrate in specific chromosomal regions. MGEs can move both within a single chromosome and between different Chromosomes. However, MGE Transpositions occur very rarely, with a single transposition often spanning many thousands of individuals. MGEs are responsible for A number of genetic phenomena, which will be discussed in greater detail later. In eukaryotes, several classes of MGEs are distinguished: Transposons, retroposons, and Retrotransposons.

Transposons

Eukaryotic transposons are structurally similar to prokaryotic MGEs. Their flanks are bounded by inverted repeats that are essential for transposition. The best-studied eukaryotic transposons are the P element in Drosophila (Fig. 10.6) and the Ac element in maize (Fig. 10.7). Present in 30–50 copies within genomes, they contain the transposase Gene. This gene has an interrupted Structure consisting of exons and introns. The RNA transcribed from it undergoes splicing. The spliced mRNA serves as a template for the synthesis of transposase, the protein responsible for mobilizing the elements from one genomic site to another. When transposons integrate into a new DNA site, target site duplication occurs (Figs. 10.6, 7). This is why these MGEs are flanked by short direct repeats. It is worth noting that during transposition, the P element typically inserts into a specific site with the canonical sequence: GGCCAGAS.

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Fig. 10.6. Drosophila P element. IR – inverted repeats, 31 bp in size; SDR – short direct repeats generated by target site duplication during transposon insertion, 8 bp in size.

Fig. 10.7. Maize Ac element. IR – inverted repeats, 11 bp in size; SDR – short direct repeats generated by target site duplication during transposon insertion, 8 bp in size.

The movement of a transposon can result in its integration directly into a gene. Naturally, this leads to gene inactivation. Conversely, the excision of a transposon from a gene restores its original structure and reactivates it. In some cases, transposon excision may be imprecise, leading to alterations in the Introduction/19.html">Primary Structure of the gene that can affect its function.