Key Concepts of Section 8.3

• Transposable DNA elements are moderately repeated sequences interspersed at multiple sites throughout the genomes of higher eukaryotes. They are present less frequently in prokaryotic genomes.

• DNA transposons move to new sites directly as DNA; retrotransposons are first transcribed into an RNA copy of the element, which is then reverse-transcribed into DNA (see Figure 8-8).

• A common feature of all mobile elements is the presence of short direct repeats flanking the element, generated as the result of staggered cuts in the target-site DNA that are filled in by a DNA polymerase during transposition (see Figure 8-10).

• Enzymes encoded by transposons themselves catalyze insertion of these sequences at new sites in genomic DNA.

• Although DNA transposons, similar in structure to bacterial IS elements, occur in eukaryotes (e.g., the Drosophila P element), retrotransposons are generally much more abundant, especially in vertebrates.

• LTR retrotransposons are flanked by long terminal repeats (LTRs) similar to those in retroviral DNA; like retroviruses, they encode reverse transcriptase and integrase. They move in the genome by being transcribed into RNA, which then undergoes reverse transcription in the cytosol, nuclear import of the resulting DNA with LTRs, and integration into a host-cell chromosome (see Figure 8-14).

• Non-LTR retrotransposons, including long interspersed elements (LINEs) and short interspersed elements (SINEs), lack LTRs and have an AT-rich stretch at one end. They are thought to move by a nonviral retrotransposition mechanism mediated by LINE-encoded proteins involving priming of reverse transcription by chromosomal DNA (see Figure 8-17).

• SINE sequences exhibit extensive homology with small cellular RNAs and are transcribed by the same RNA polymerase. Alu elements, the most common SINEs in humans, are sequences of about 300 bp found scattered throughout the human genome at about 1.6 million sites (see Figure 8-4a).

• Some interspersed repeats are derived from cellular RNAs that were reverse-transcribed and inserted into genomic DNA at some time in evolutionary history. Processed pseudogenes derived from mRNAs lack introns, a feature that distinguishes them from pseudogenes, which arose by sequence drift of duplicated genes.

• Mobile DNA elements most likely influenced evolution significantly by serving as sites for homologous recombination during unequal crossing over, leading to gene and exon duplication (see Figure 8-2) and exon shuffling (see Figure 8-18), and by mobilizing adjacent DNA sequences (see Figure 8-19).