Gene expression

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 runs out do the bacteria switch on their genes for producing this additional enzyme. By tying gene expression to environmental cues, the bacteria don't waste energy and materials making products they don't need.

The control of gene expression in multicelled organisms is much more complex and not yet fully understood. The cluster of undifferentiated cells that make up an embryo soon after fertilization must quickly begin expressing different genes to produce different body tissues and organs. The cells in a particular tissue or organ may also switch genes on or off at different times during growth. For example, the cells in testicles or ovaries don't switch on the genes that result in sex hormone production until the organism reaches puberty.

The switch mechanisms that regulate gene expression include groups of genes called regulatory genes. Unlike the genes discussed so far — which we must now call structural genes — regulatory genes do not code for enzymes or other proteins. Their function is to either promote or inhibit the sequence of events by which a structural gene is translated into a product. Promoter regions are located adjacent to structural genes on a strip of DNA. When genetic engineers transplant genes for making products, they must include the switches that control gene expression as well as the genes themselves.

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