In the murky darkness, blue and green blobs are dancing. Sometimes they keep decorous distances from each other, but other times they go cheek to cheek — and when that happens, other colors flare.

The video, reported last year, is fuzzy and a few seconds long, but it wowed the scientists who saw it. For the first time, they were witnessing details of an early step — long unseen, just cleverly inferred — in a central event in biology: the act of turning on a gene. Those blue and green