Telomeres Do D-Loop–T-Loop

Rarely it seems does a new result emerge, loop around, and bite you from behind. But a collaboration between Jack Griffith’s and Titia de Lange’s laboratories to look at the binding of telomere proteins to telomeric DNA has done just that. Griffith and de Lange show, in this issue of Cell, that rather than ending as linear DNA molecules as was previously thought, telomeres in mammalian cells end as large terminal loops (Griffith et al. 1999). Following the nomenclature of three-stranded DNA displacement loops, or D loops, and RNA displacement loops, or R loops, described by electron microscopy, they call these new structures “t loops” for telomere loops. The long stretches of the double-stranded telomere DNA are looped around, and the single-stranded terminus is tucked back inside the double-stranded DNA molecule—thus protecting the chromosome terminus (Figure 1). These results force us to rethink the classical view of telomere structure and function developed over the past 20 years and provide a context for a new synthesis.