This question has been in my mind for some time …

Scientists determined that in frogs, if one transplanted the nucleus of a cell from a later developmental stage into an enucleated oocyte, the embryo is not likely to survive. Therefore it was suggested that as the frog embryo gets older the chromosomes become more restricted. However, for cloning Dolly the nucleus of a somatic cell from an adult sheep was used. Although only one oocyte survived out of the 434 oocytes, how was it possible that this nucleus reverted back to an embryonic stage?