447 Key terms autonomous elements for their movement because they do not encode the necessary proteins, including reverse transcriptase (for RNA elements) and transposase (for DNA elements). • How can humans survive given that up to 50 percent of the human genome is derived from transposable elements? Three major reasons. First, most of the transposable elements’ sequences are mutant and no longer capable of transposition. Second, the transposition of the few active elements in the genome is usually prevented by host regulatory mechanisms. Finally, the vast majority of transposable-element sequences in the human genome are in noncoding DNA including telomeres, centromeres, intergenic DNA, and introns. • How can the study of retrotransposons in yeast lead to improved procedures for human gene therapy? Yeast retrotransposons target their new insertions to so-called safe havens, regions of the genome with few genes. By understanding the underlying mechanisms, scientists may be able to devise new strategies to target genes for
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