Telomerase-independent mechanisms of telomere elongation

The ends of linear chromosomes must be elongated in a DNA-replication-independent fashion. For chromosome end elongation the majority of eukaryotes use a specialized reverse transcriptase, telomerase, which adds a short, tandemly repeated DNA sequence motif to chromosome ends. Chromosome elongation can also be achieved, however, by mechanisms other than telomerase. Such elongation events have been detected under conditions where telomerase has been inactivated experimentally and in the few organisms that naturally lack telomerase. We will summarize current knowledge on these telomerase-independent elongation mechanisms in yeast and mammalian cells and will discuss in more detail the telomere elongation mechanism by retrotransposons in Drosophila melanogaster.