Linear chromosomes carry on both ends repetitive DNA sequences called telomere. In the conventional model of semi-conservative DNA replication, the 3-end of a linear DNA strand cannot be fully replicated, resulting in a single-stranded 3 overhang at one end of the double-stranded DNA product. In this model, the length of the overhang is expected to be about the size of an RNA primer (about nine nucleotides for human cells). However, it has been found that the telomere overhangs in human cells can be as long as several hundred nucleotides. At present, the opinion regarding how such long overhangs are produced is controversial.