Genetic engineering is the science of modification of genes in cells.
Genes are collections of DNA molecules that comprise the code that determines the function and reproductive pattern of cells. Together with hormones and other chemical messengers, developed and naturally recombined over millions of years, genes determine everything about an organism. Genetic engineers modify gene patterns in cells by implanting genes or DNA from one organism into another organism. For instance, Monsanto Corporation found a gene somewhere that made a plant resistant to a Monsanto weed-killer called Roundup. Monsanto then took that gene and implanted it into soybeans, instantly creating a new species of soybean that could be sprayed with Roundup weed killer without fear of killing the soybean plant.