Credit: Illustration by Chris Buzelli

By the late 1960s, molecular biologists had erected an overarching explanation of how genes work--their substance, their structure, their replication, their expression, their regulation or control. Or at least they had done so in outline, for prokaryotes, the simplest single-celled organisms (which include bacteria), and for the viruses, called bacteriophages, that prey upon them. The leaders of the field were now looking to a far more difficult problem: doing it all over again for higher organisms.

Select from the choices above to read the entire article.