A snapshot of the human genome. (a) This pie chart shows the proportions of various types of sequences in the human genome. The classes of transposons that represent nearly half of the total genomic DNA are indicated in shades of gray. LTR retrotransposons are retrotransposons with long terminal repeats. Long interspersed nuclear elements (LINEs) and short interspersed nuclear elements (SINEs) are special classes of particularly common DNA transposons (see Chapter 14). (b) The approximately 20,000 protein-