Mammalian telomeres and telomerase.

New features of mammalian telomeres and telomerase have been identified. Telomeres form t-loops, which engage the 3' single-stranded DNA overhang in an interaction with double-stranded telomeric repeats. Mammalian telomerases contain an RNA H/ACA motif and associated protein(s) shared with H/ACA family of small nucleolar ribonucleoproteins. Essential roles for telomerase in the sustained viability of cultured tumor cells and in the normal proliferative capacity of human somatic cells have been demonstrated.