Remodeling replication

Cloned nuclei are plucked from a comfortable, differentiated cell and thrust into an egg primed for embryonic development. For a frog nucleus, part of that switch is a conversion into embryonic DNA replication mode, say Jean-Marc Lemaitre, Marcel Méchali, and colleagues (CNRS, Montpellier, France). The absence of this switch may partially explain the woeful, 2–3% success rate of nuclear transfer cloning.Most researchers trying to improve cloning efficiency have concentrated on epigenetics—how to change modified chromatin proteins from a differentiated to an embryonic activity pattern.

But a frog's embryonic divisions differ from mature divisions in another sense: they take only 30 minutes each, in part because they use many more replication origins. In the new experiments, cloned nuclei replicated slowly and with fewer replication origins. Putting the nuclei first into a frog mitotic extract, however, boosted the number of origins and the speed and efficiency of replication in the subsequent cell cycle.

The switch was associated with closer spacing between origins, a randomization of attachment sites to the nuclear matrix, and shorter loops of DNA between nuclear matrix attachment sites. In each cycle the attachment sites were converted to a more dispersed, mature form during DNA replication, but were rerandomized during mitosis.

Many mammals lack rapid early cell divisions but, says Méchali, “origins may be fixed in different places in different cell types.” For efficient cloning, “it may be important to fix them into a pattern appropriate for embryogenesis.”

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