Last century, the world was upended when science breached the inner sanctum of atoms and created the A-bomb. Today we are hacking into the inner sanctum of living cells and the consequences, for better and worse, spell the end of life as we know it.

Today’s drama began several years ago, when genetic engineers from Harvard, MIT, and U.C. Berkeley co-developed powerful gene-editing tools that now make it possible for us to directly monkey with DNA at will. The tools go by the name CRISPR, which stands for – are you ready? – Clustered Regularly Interspaced Short Palindromic Repeats.

In plain English, CRISPR is making possible scenarios once considered science fiction. They are unprecedented developments ranging from utopian to dystopian.

First, the good: using CRISPR and other tools to treat genetic diseases, of which there are thousands – everything from cystic fibrosis, hemophilia, and breast cancer to diabetes, autism, and even obesity.

Patients with Duchenne Muscular Dystrophy (DMD), for instance, have a defective gene that fails to produce enough dystrophin, a key muscle protein. Recently, doctors at Nationwide Children’s Hospital in Columbus, Ohio, injected healthy genes into four DMD boys and obtained very encouraging results .

Second, the sketchy: using gene-editing tools to mix and match disparate species. Please note: this is not your grandfather’s selective-breeding practice.

As I write this, renegade genetic engineers in California and Minnesota are creating pig-human and sheep-human chimeras whose vital organs are human. They are doing it against the wishes of and without any funding from the National Institutes of Health (NIH). But they’re reportedly motivated by a noble cause: to create a limitless source of body parts for patients in need of them.

But what if the human cells in these illicit chimeras proliferate beyond their vital organs – to their brains, for instance? The resulting creatures, possibly capable of human thought, could no longer be considered just pigs.

“We are not near the island of Dr. Moreau, but science moves fast,” warns David Resnik , an ethicist at the NIH. “The specter of an intelligent mouse stuck in a laboratory somewhere screaming ‘I want to get out’ would be very troubling to people.”

Hiromitsu Nakauchi – a Stanford University biologist who is trying to make human-sheep chimeras – concedes such a thing could happen. “If the extent of human cells is 0.5 percent, it’s very unlikely to get thinking pigs or standing [two-legged] sheep,” he says. “But if it’s large, like 40 percent, then we’d have to do something about that.”

Finally, the unnerving: using CRISPR to create novel organisms from scratch. “We are going from reading our genetic code to the ability to write it,” declares renowned geneticist Craig Venter . “That gives us the hypothetical ability to do things never contemplated before.”

To that end, geneticists recently launched Genome Project-Write (GP-Write), a global initiative that intends to redesign existing genomes and synthesize living organisms sprung entirely from the human imagination.

I love science and trust in its motivating desire to understand the universe and improve the human condition.

But if we’ve learned anything from our hacking the atomic nucleus, it’s this: the sketchy and unnerving consequences – such as nuclear power plants and H-bombs – severely overshadow the good ones, such as nuclear medical devices that treat cancers and routinely save lives. Witness our current nail-biting efforts to keep nukes out of the hands of North Korea and Iran.