Telomere length shortens upon culture of fibroblast cells in vitro. But what happens to chromosomal ends and telomerase activity when nuclei from these aged cells are used to clone animals by somatic nuclear transfer? In the January 30 Proceedings of the National Academy of Science Betts et al. report on telomeres and telomerase activity in early and late-passage donor fibroblasts and recipient cloned bovine embryos (Proc Natl Acad Sci USA 2001, 98:1077-1082). Despite the reduction in telomere length in older bovine fibroblasts and ES-like cells, telomerase is reprogrammed in the early cloned blastocysts and telomere length is restored whatever the nuclei source. These results support earlier studies showing that bovine cloning rejuvenates old cells, in contrast to the telomere shortening that has been observed in cloned sheep.

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