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| Modern Genetic Analysis Chapter 13. Transposable Genetic Elements Transposable Elements in Drosophila Figure 13-16. Summary of the structures of three classes of Drosophila transposable elements. The copia-like elements carry long direct terminal repeats. Each repeat makes up about 5 percent of the length of the element. These repeats are shown on an expanded scale below the element to illustrate the presence of short, imperfect inverted repeats () at the ends of each long direct repeat and the presence of a few base pairs of duplicate target sequence () flanking the element after insertion. Such duplication of the target sequence is a virtually universal feature of trans-posable element insertion. The different genomic copies of the family elements are very similar in structure to one another. The FB elements make up a family of heterogeneous but cross-homologous sequences, ranging in size from a few hundred base pairs to several kilobases. Each FB element carries long terminal inverted repeats. In some cases, the entire element consists of these inverted repeats. In other cases, a central sequence is located between the inverted repeats. The inverted repeat sequences themselves are internally repetitious, having a substructure made up primarily of 31-bp tandem repeats. The number of these 31-bp tandem repeats can differ not only between FB elements, but also between the termini of a single FB element. The P elements have a very different structure from that of the copia-like and FB elements. P elements carry perfect terminal inverted repeats of 31 bp. A fraction of the P elements (about one-third in the one strain examined) are very similar in sequence to one another and are 2.9 kb in length. The remainder of the P elements are more heterogeneous, but all appear to have structures that are consistent with their having been derived from the 2.9-kb element by one or more internal deletions. DNA sequence analysis of the 2.9-kb element reveals a gene coding for transposase, composed of four exons and three introns. (From G. Robin, in J. A. Shapiro, ed., Mobile Genetic Elements, pp. 329361. Copyright © 1983 by Academic Press.)

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