Figure 1.

Schematic representation of telomere structure in A) Organisms with telomerase. The strand with the RNA template sequence, AATCCC, is shown. A single repetitive unit of the mammalian telomere sequence is shown in italics and underlined. B) Drosophila. The coding strand (equivalent to template strand) is shown. Centromere lies to the right in both cases.

Thin arrows above retrotransposon diagrams indicate 5′? 3″ direction of each element. Black arrow underneath the HeT-A element indicates that its transcription unit includes sequence from two adjacent elements (see discussion of HeT-A’s unusual promoter),. Striped short lines underneath HeT-A elements indicate sequences in the transcription unit that are duplicated because they derive from the 3′ end of two elements. 5′, and 3′ indicate untranslated regions, AAA, indicates oligo (A) adenine characteristic of non-LTR retrotransposons, gag, gag gene, pol, pol gene.