Often in nature things are not what they seem. A rock on the seafloor may be a poisonous fish; a beautiful flower in a garden may be a carnivorous insect lying in wait for prey. This misleading appearance extends to certain components of cells, including chromosomes—the strings of linear DNA that contain the genes. At one time, the DNA at the ends of chromosomes seemed to be static. Yet in most organisms that have been studied, the tips, called telomeres, are actually ever changing; they shorten and lengthen repeatedly.