So, first of all, Roundup is the name of an herbicide that is sold by Monsanto. I understand that everyone thinks Monsanto is the worst thing to ever happen to the planet, but lets put that aside for now and just focus on the science. Roundup is the trade name for a chemical called Glyphosate, it looks like this:
What this chemical does is it inhibits an enzyme (5-enolpyruvylshikimate-3-phosphate synthase, or EPSPS for short), which is involved in making the precursor to the biosynthetic pathways of the amino acids tyrosine, tryptophan, and phenylalanine. For reference, human beings do not have these pathways, we need to get our aromatic amino acids from our food. That means that if you were accidentally sprayed with Roundup, nothing would happen to you. This inhibition is competitive, which means that the glyphosate binds to the site in the enzyme where its substrate would go. Anyway, this basically inhibits plant growth because those amino acids are necessary for plants to grow. Idealy, the only plants that are able to grow in the presence of Glyphosate are those that have been genetically modified to be glyphosate resistant, or Roundup Ready.
Glyphosate-resistant plants have been genetically modified to over-express bacterial EPSPS. That means the plants express the bacterial form of the gene, which is resistant to the effects of the inhibitor, at amounts that are higher than normal gene expression for the plant. Scientists do this kind of over-expression all the time, including for something as simple as purifying proteins or for making vaccines. When we over-express a gene, we're basically asking the host organism to make as many copies of it as is possible. That's it, that's all there is to it. The bacterial gene isn't bad for humans or other animals, because a) we don't have the necessary biochemical pathway anyway, and b) like I said in my first post on GMOs, the food we eat every single day is covered in bacteria which means that at many, many points in your life, you've consumed bacteria that has EPSPS in its genome, and you've lived to tell the tale! Eating Roundup Ready plants is no worse than eating bacteria-covered plants, you're exposed to the same genes.
Ok, so most people argue that Roundup is a toxic chemical and they don't want to be eating crops that are completely doused in herbicides and pesticides. See, people think that because something kills a plant, it's automatically dangerous to people. But that's simply not true. Another commonly used herbicide is 2,4-D (aka 2,4-dichlorophenoxyacetic acid), which is a synthetic auxin. Auxin is a plant hormone that mediates plant growth and development. Sidebar: plants only have 5 types of hormones that basically do EVERYTHING! Plants are amazing!
Anyway, every time I debate with people about GMOs, the conversation is always derailed by the evil practices of Monsanto. And that's because as soon as you really understand the science behind GMOs, it's hard to argue against it. Monsanto is a company that wants to make money (GASP!), and so yes, it charges people to use its products. But that doesn't mean GMOs are bad or will kill you. Glyphosate and the products of its degradation have been shown to have low toxicity when ingested, inhaled, or applied to skin. It's not a toxin that kills everything it touches except for Frankenplants. It's a little bit less effective now because the improper and/or widespread use of glyphosate has led to the development of resistant weeds - this is the same problem we're starting to see with the widespread, improper use of antibiotics.