Telomere-to-centromere ratio of bovine clones, embryos, gametes, fetal cells, and adult cells.

In 1997, Dolly, the first animal cloned from an adult cell, was born. It was announced in 1999 that Dolly might be aging faster than normal because her telomeres were shorter than age-matched control sheep. Telomeres, a repeated DNA sequence located at the ends of linear chromosomes, allow for base pair loss during DNA replication. Telomere shortening acts as a "mitotic clock," leading to replicative senescence. By using whole cell lysate and slot-blot analysis, we determined the telomere-to-centromere ratio (T/C) for bovine gametes, embryos, fetal tissues (brain, heart, lung, kidney, uterus, ovary, and skin), adult donor cells, and cloned embryos. Our data indicates a consistency in T/C among the various fetal tissues. The T/C of sperm is significantly lower than in oocytes. The T/C decreases from the oocyte to the 2-8-cell stage embryo, increases dramatically at the morula stage, and decreases at the blastocyst stage. Our data shows no significant difference in T/C between cloned embryos and in vitro fertilized (IVF) embryos, but there is a significant difference between cloned embryos and adult donor cells. In conclusion, the enucleated bovine oocyte has the ability to reestablish the telomere length of adult somatic cell donor nuclei.