Repetitive regions at the very ends of chromosomes are called telomeres, and they're found in a wide range of eukaryotic species, from human beings to unicellular protists. Telomeres act as caps that protect the internal regions of the chromosomes, and they're worn down a small amount in each round of DNA replication. Unlike bacterial chromosomes, the chromosomes of eukaryotes are linear (rod-shaped), meaning that they have ends. These ends pose a problem for DNA replication. The DNA at the very end of the chromosome cannot be fully copied in each round of replication, resulting in a slow, gradual shortening of the chromosome. In most cases, the primers of the Okazaki fragments can be easily replaced with DNA and the fragments connected to form an unbroken strand. When the replication fork reaches the end of the chromosome, however, there is (in many species, including humans) a short stretch of DNA that does not get covered by an Okazaki fragment—essentially, there's no way to get the fragment started because the primer would fall beyond the chromosome end
