The structure at the end of a eukaryotic chromosome formed by the DNA strand of the chromosome being extended by the enzyme telomerase, which is present in germ-line and cancer cells, and which adds a species-specific G-rich sequence. The DNA ends in a 3’′ overhang; this strand turns back onto itself, forming a telomere- (t-)loop and displaces its own up-stream sequence , forming a displacement- (D-)loop, and hybridises with the complementary strand. The t- and D-loops are stabilised by telomere-binding proteins. The telomere, although not replicated by DNA polymerase, provides an anchorage for the RNA primer during replication. Without this extension, replication would shorten the chromosome each time that the cell divides. In fact this is exactly what happens in most cells, where telomerase is absent, the step-wise shortening of the chromosome creating a biological clock which counts down to eventual cell death.