Genetic modification, succinctly, is the introduction of genes from foreign organisms into the genome of one's organism of choice. This has been happening slowly and quietly for billions of years; when a virus infects a cell, it occasionally incorporates some of the cell's DNA into its own genome. The virus's descendents can then go on to infect and transform other cells, sometimes modifying their genomes. In the early 1980s, we learned to do this much faster and in a directed way. Agrobacterium, a genus of plant bacteria, naturally transfers DNA between it and its hosts, via small extra 'chromosome-like' piece of DNA called plasmids. So a scientist can choose a gene from one organism, synthesize a plasmid containing the gene, infect Agrobacterium with it, infect a plant with the Agrobacterium, and screen the cells or offspring of the plant for ones that have successfully incorporated the gene. You can now buy kits to do this; it's so easy I've done it myself (I genetically engineered a bacterium to overproduce a plant protein called azurin). If Agrobacterium won't infect the plant (it doesn't infect corn) we can use something called a gene-gun to fire bits of DNA through the plant cell wall, again screening for descendants that have incorporated the gene into their own genomes.
Aside from a commercially unsuccessful attempt to genetically engineer frost-resistant tomatoes, the earliest atttempts to genetically engineer crops involved trying to introduce herbicide resistance (remember that term). The most successful of these GM-crops have been 'Roundup-resistant'. These are based on a very simple but elegant strategy of introducing a gene, already native to Agrobacterium, for the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (or EPSP synthase). EPSP synthase is vital to all plants, because it's used in the pathway to make the essential aromatic amino acids phenylalanine, tyrosine, and tryptophan. Animals, however, don't need it, because we can't make aromatic amino acids, and have to get them in our diet. So if we can block, EPSP synthase, we can kill plants, but leave animals unharmed. And this is what Roundup, technically known as glyphosate and even more technically as N-(phosphonomethyl)glycine) does. It diffuses into the active site of the EPSP synthase enzyme, and sits there, tightly bound, preventing the enzyme from doing its job. Glyphosate is an extremely simple molecule, and is almost completely nontoxic to humans and other animals (naturally enough, since we don't have its target enzyme).
So now a farmer can plant his corn and soybeans on a field already cleared by glyphosate. He can even spray glyphosate on the field while his crops are growing, killing weeds but not his crops. The result is higher yield, and no need to use selective herbicides like atrazine, which are far more toxic to humans.
How could anyone oppose that? Well, as we'll see in the next post, Chuck Hassebrook did.