MUSEUMS have always been a branch of show business, and seldom has the connection been so clear as in the National Museum of American History's new exhibit on molecular biology.

"The Search for Life," produced and directed by New York theater people, is a show that closed before it opened and then was reborn after sneak previews and a major overhaul. If it turns out to be a hit, it will be going on the road.

Molecular biology? Yes. This is a show whose playbill includes a glossary and a bibliography. It's about natural and unnatural selection, jumping genes, recombinant deoxyribonucleic acid, the alpha helix, mechanisms of biological inheritance and some of the environmental, medical, legal, social, ethical, economic and global implications of genetic technology.

It is, in short, about the "bioburst," the revolution in the life sciences that promises and threatens to give mankind absolute dominion over every living thing on the planet, including ourselves.

We have wrested from nature the role of creator, altering genetic material and transplanting it between species. From designing "better" plants and animals the next step will be developing entirely new species. We are now repairing defective human genes, and may one day be fashioning new people, or whatever they might call themselves; under present American law such creatures might well be patentable property.

It makes for pretty heavy theater, but museum director Roger Kennedy makes no apology for the lack of fluff and oversimplification.

"The subject is as important and profound as any in human history," he said. "You can't do it in baby talk."

But you can do it in style, of which this Peter Wexler production has plenty. The setting is a framework of aluminum girders that suggests a vast spiraling DNA model. The visitor wends past displays arranged like subgroups of the genetic code, accompanied (and subtly urged along) by a computer-controlled progression of lighting and narration. It's a "work in progress," designed to accommodate the new discoveries that seem to come along weekly.

If you follow the stage cues, the performance lasts 15 minutes, but you can drop off the tour at any point if you want to reflect on such things as the hardships of women in science. Here is the triumph of Nobel laureate Barbara McClintock, who lived to see the vindication of her stubborn and indefatigable work on "jumping genes," which had been ignored or scorned for half a century by many of her male colleagues. There is the melancholy story of Rosalind Franklin (1920-58), whose X-ray diffusion research led James Watson and Francis Crick to their Nobel for puzzling out the structure of DNA. Franklin died knowing she would be denied her share of the credit for the key discovery that made manipulation of the essence of life conceivable and therefore possible.

A little farther along, in the midst of this celebration of science, comes a section on the American eugenics movement, an eerie reminder of the perversions that the increase and diffusion of knowledge make possible. This effort to put a scientific sheen on racism led to immigration quotas and "clean gene" contests in the U.S.; in Germany it became the intellectual justification for the Holocaust.

The centerpiece of the exhibit is the "Cell Theatre," meant to give the feeling of being inside a cell while genetic replication is taking place, as well as a sense of the scale of the organization of matter from the submolecular to the universal.

But the meatiest and most stimulating part of the exhibit comes afterward, in the exit passageway lined with television monitors. Each endlessly plays back taped segments of newsfilm, agribusiness promos and talking-head exposition on the many promises and problems of genetic technology. Edited with consummate skill, the segments teach and tease, and the half-hour potpourri includes much thoughtful commentary by the scientists in whose hands our future may lie. It is somehow reassuring to know that they're scared too. THE SEARCH FOR LIFE --

Through March at the National Museum of American History.