The fate of DNA injected into mammalian oocytes and zygotes at different stages of the cell cycle.

The stability of exogenous DNA microinjected into the cytoplasm at different stages of the meiotic cycle and after pronuclear formation was examined in ungulate species. Metabolism of the injected 1201 base pair (bp) DNA was examined by Southern blotting. Similar levels of metabolism of the injected DNA were detected in pig, sheep and bovine oocytes before germinal vesicle breakdown, in which about 30-40% of detected DNA was ligated into higher-molecular-weight forms. Porcine metaphase oocytes and pronuclear zygotes showed a reduced ability to ligate the exogenous DNA. In contrast, sheep and bovine metaphase oocytes and zygotes showed increased levels of ligation and, at the pronuclear stage, generated significant amounts of extremely large (greater than 15 kbp) ligation products. These results are discussed in the context of maternal precursors and metabolic activities in the egg.