A cloned Xenopus embryo overexpressed genes that were being actively transcribed in its parent cell at the time the nucleus was transferred, according to a study published in this week's PNAS. This suggests that the embryo "remembers" what type of cell its nucleus came from, according to study co-author John Gurdon. He and Ray K. Ng of the University of Cambridge report that genes specific to the cell type of a transferred nucleus are turned on in the wrong tissues of some cloned embryos at early stages of development.

"The idea that active gene transcription can be stable through pretty dramatic cell changes is not new," Paul Wade of the National Institute of Environmental Health Sciences told The Scientist. "What's new is that it survives nuclear transfer."

When a somatic nucleus is transferred to an oocyte, factors in the cytoplasm induce an erasure of the differentiated cell program...

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