4–14 Mobile pieces of DNA—transposable elements— that insert themselves into chromosomes and accumulate during evolution make up more than 40% of the human genome. Transposable elements of four types—long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), long terminal repeat (LTR) retrotransposons, and DNA transposons—are inserted more-or-less randomly throughout the human genome. These elements are conspicuously rare at the four homeobox gene clusters, HoxA, HoxB, HoxC, and HoxD, as illustrated for HoxD in Figure Q4–4, along with an equivalent region of chromosome 22, which lacks a Hox cluster. Each Hox cluster is about 100 kb in length and contains 9 to 11 genes, whose differential expression along the anteroposterior axis of the developing embryo establishes the basic body plan for humans (and for other animals). Why do you suppose that transposable elements are so rare in the Hox clusters?
Figure Q4–4 Transposable elements and genes in 1-mb regions of chromosomes 2 and 22 (Problem 4–14). Blue lines that project upward indicate exons of known genes. Red lines that project downward indicate transposable elements; they are so numerous (constituting more than 40% of the human genome) that they merge into nearly a solid block outside the Hox clusters. (Adapted from E. lander et al., Nature 409:860–921, 2001. with permission from macmillan Publishers ltd.)
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