Depending on whom you listen to, genetically modified crops are either ungodly Frankenfoods unfit for even a house pet or our only hope against famine in a post-climate-change world. Putting aside all the shouting, it’s interesting to examine how scientists modify plant genes. This is the story of how ordinary crops become transgenic crops.

The first step in the process is finding a useful gene. (In most cases, genetic modification involves adding genes to a plant rather than removing or disabling genes.) These discoveries can be serendipitous. Scientists around the globe are working furiously to map the species’s genomes. In some cases, they stumble across something useful, such as a gene that produces a fungus-fighting protein.

But multinational agricultural companies aren’t just hoping for some nerd in a university laboratory to get lucky. Instead, they’re spending huge sums hunting for genes that might fulfill pressing agricultural needs. This usually involves turning off or enhancing genes and observing the consequences. Does the change create pesticide tolerance? Does the fruit ripen more quickly? Those are indications that the gene could be promising.

Genes that enhance resistance to drought and heat are on the top of agribusiness wish lists, although researchers haven’t yet found anything particularly effective. That’s not surprising, considering that discovering useful genes is quite a bit harder than finding a needle in a haystack. Genetic code doesn’t have to come from another plant; it can come from just about anywhere: bacteria, fungi or even animals.

“Fundamentally, a strand of DNA is the same in animals and plants,” says Peter Pauls, a plant scientist at the University of Guelph in Ontario. “There are regulation systems around genes that make them work better in the organism they evolved in, but they code for the same protein in any organism.”

Corn stalks in test tubes. (Getty Images)

In other words, researchers are combing through the genome of the entire tree of life for a few promising base pairs. So looking for a drought-resistance gene is kind of like searching for a single needle that could be hidden in any haystack, anywhere on Earth.

When they find a candidate gene, researchers place it in a test tube with an enzyme that “amplifies” the sample, meaning they make millions of copies of it. Then comes the complicated part: inserting the gene into the target crop’s genome.

For the sake of illustration, let’s assume we’re working with corn. Companies focus almost exclusively on high-value crops such as corn, soybeans and wheat because this research is expensive. There’s not a lot of money in genetically modified fava beans.

There are a couple of ways to modify a corn genome. The slightly crude method is to place a bunch of the sample genes onto small particles of tungsten or gold, then shoot them into a corn cell. Suddenly flooded with genetic information, the plant cell can’t help but integrate some of it into its own genome.

More often, however, scientists use microbes called agrobacteria, which Pauls calls “natural genetic engineers” because of their behavior in the wild.

When an ordinary agrobacterium infects a corn cell, it commandeers the corn cell’s machinery by inserting a gene sequence that tricks the corn into making nutrients to support the spread of the agrobacterium.

Researchers have found that agrobacteria can be reprogrammed to insert just about any genes you want into a plant cell, instead of the ones that simply support the bacteria itself. They place their reworked agrobacteria in a nutrient bath with the corn cells and allow the bacteria to infect the corn, thereby adding the genes that might enhance drought tolerance or help fight fungi.

Scientists then face a problem: how to tell which corn cells have integrated the new genetic sequence. The process doesn’t always work, and you can’t recognize a genetically modified corn cell from its appearance.

To solve this problem, scientists attach the genes they’re testing to marker genes; these are genes whose only purpose is to mark the corn cells. The marker genes may, for example, confer antibiotic resistance on corn. Scientists then give the agrobacteria-corn mixture a solid dose of antibiotics, which would normally kill the corn. The samples that survive must have integrated the new genetic material — both the antibiotic resistance genes and those being tested — into their DNA.

At the end of all this, researchers hope to have a few cells of what is called transgenic corn. I say “hope,” because the success rate is rather low. Sometimes none of the cells take up the genes. In other cases, the genetic modification undermines the normal functioning of the corn.

Testing is basically Horticulture 101. The researchers grow several generations of corn plants from the genetically modified cell and test to see whether they have the desired new trait.

Companies have to do extensive safety testing before attempting to market the crops — for example, by feeding enormous amounts of the new product to mice to check for potential health hazards.

Jonathan Jones, a researcher at the U.K.’s Sainsbury Laboratory, wants you to remember one thing about genetically modified crops: “If you have a plant with 30,000 genes, and you add another, you have a plant that is 99.999 percent identical. It’s very unlikely that would make a difference for human health.”

That’s not likely to convince opponents, who worry not just about nutrition and toxicity but also about environmental issues. But when people start hollering about Frankenfoods, at least now you know how the monster was made.