DNA-strand breaks in chromosomes of early mouse embryos as detected by in situ nick translation and gap filling.

The nick translation and gap filling procedures, without external addition of nicking enzymes, were performed in situ on fixed chromosomes of mouse preimplantation and postimplantation embryos and of bone marrow in order to detect possible DNA single-strand breaks (nicks and (or) gaps). All chromosome preparations were made using the same technique. Nick translation of chromosomal DNA with DNA polymerase I (Pol I) or gap filling with the Klenow fragment of Pol I in the presence of biotinylated-dUTP, demonstrated a regular absence of label on chromosomes of postimplantation embryos and bone marrow. No difference in sensitivity was found between the holoenzyme and the Klenow fragment. In preimplantation embryos, the chromosome reactivity in nick translation was highest at the blastocyst stage and varied according to cleavage divisions of the zygote.