By changing an organism's genes, scientists can give the organism and its descendants different traits. For thousands of years, breeders of plants and animals have used breeding methods to produce favorable combinations of genes. Beginning in the 1970's, scientists developed ways to reintroduce individual genes into cells or into plants, animals, or other organisms. Such techniques alter the heredity of the cells or organisms. How genes are reintroduced into cells.
Genes lie within cells on tiny, threadlike structures called chromosomes.
A molecule of DNA may contain thousands of genes. DNA stores within its chemical structure the information that determines an organism's hereditary properties. The physical structure of DNA is much the same in all organisms. The DNA molecule is shaped like a twisted rope ladder, called a double helix. A pair of bases forms each rung.