Imagine being the person who took the first gander ever at a genome, the instruction manual for life. Pretty amazing stuff. Now imagine ferreting out which bits do what and discovering that quite a lot of what you're looking at doesn't seem to do anything. It's just there, cluttering things up. In a word, it's junk. Quite a bit of the genomes of eukaryotic organisms is junk — non-coding DNA. Why is it there? No one really knows, but several explanations have been proposed, as the following sections explain.
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