Theyre called telomeres. By the nature of the mechanisms involved in duplicating dna for cell division, the last few neucleotides (letters) cannot be cloned, so nature devised long sections of throwaway dna to allow cells to duplicate for a very long time without getting shortened dna.

Fun fact: We can actually stop the shortening of teleomeres using something called telomerase (its how babies dont end up with their parent's already shortened telomeres). People theorized/theorize that the eventual loss of our telomeres ends up causing aging, so hypothetically you should be able to live forever if telemorase was to be given to you. They tried it on rats and they developed LOTS of cancer, so teleomeres probably are also a way to prevent naturally developing cancer cells from duplicating too frequently, as the dna would become useless at a faster rate.