Putting recombinant DNA into bacteria is also, in effect, a simple method of cloning genes. One or two bits of recombinant DNA aren't much use if your aim is to turn out large amounts of the gene product. You need identical copies of the gene in millions of cells. The easiest way to achieve this is to engineer the gene into a few cells and let them multiply, duplicating the new gene along with their own DNA each time they divide in two.

I've talked about genes until now as if the mere presence of a given gene in a cell is enough to make the cell carry out that gene's instructions. If you think about it, that obviously can't be true. Every cell in your body has the same genes, but the cells aren't all alike. Genetic potential isn't the same as genetic fate. Every cell has far more genes than it uses, and only a proportion of the genes are actually "turned on," or expressed, at any one time, making one cell a heart cell and another a brain cell. Understanding the mechanism of gene expression is critical to controlling the outcome of genetic engineering.

In bacteria, certain genes are turned on or off according to the conditions in which the microorganisms are growing. For example, the bacteria Escherichia coli can use either of two sugars, lactose or glucose, for energy. They need enzymes to release energy from these sugars, and their enzyme production is encoded in their genes. If the bacteria are grown in an environment with both sugars, they prefer glucose, and express the genes for the enzymes that let them use that sugar. Digestion of lactose requires one extra enzyme, and only when the glucose