Solving mysteries of DNA replication and frog cloning.

Compared to sperm nuclei, nuclei from adult somatic cells replicate inefficiently in frog egg extract. In this issue of Cell, Lemaitre et al. (2005) show that pre-exposure of erythrocyte nuclei to a mitotic extract removes this difference, reorganizes the chromatin into shorter loops, and allows replication at much shorter intervals along the DNA. Remarkably, these observations also explain an old mystery of why serial nuclear transplantation was so successful for cloning frogs.