Chromosomes Ancestral grass 70 million years Maize Sorghum Rice Barley ~ Figure 13-25 Transposable elements in grasses are responsible for genome size differences. The grasses, including barley, rice, sorghum, and maize, arose from a common ancestor about 70 million years ago. Since that time, the transposable elements have accumulated to different levels in each species. Chromosomes are larger in maize and barley, whose genomes contain large amounts of LTR retrotransposons. Green in the partial genome at the bottom represents a cluster of transposons, whereas orange represents genes. The ability of some transposons to insert preferen-tially into certain sequences or genomic regions is called targeting. A remarkable example of targeting is illustrated by the R1 and R2 elements of arthropods, including Drosophila. R1 and R2 are LINEs (see Figure 13.23) that insert only into the genes that produce ribosomal RNA. In arthropods, several hundred rRNA genes are organized in tandem arrays (Figure 13-26b). With so many genes encoding the same product, the
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.