tells you what species it comes from. The function of a gene is to produce a protein: nothing more, nothing less. The differences between species, or between individual organisms, lie only in the particular numbers and specifications and combinations of their genes.

Because the genetic code is universal, almost any cell in any organism can ''read" a gene and translate it into the relevant protein. Today, for example, the insulin used to treat thousands of people with diabetes is produced on an industrial scale in huge vats by bacteria that have been genetically engineered to carry the human insulin gene. This is the essence of biotechnology.

With an understanding of how cell mechanisms produce proteins, our journey which began with the discovery of cells themselves comes to rest. It took several generations of scientists to show us that:

• the properties of living things come from the properties of the proteins they contain
• the properties of proteins depend on the arrangement of amino acids making them up
• the arrangement of amino acids is determined by the sequence of nucleotides on a section of DNA—or in other words, a gene.

This has been a story about how we discovered some of nature's secrets and learned how organisms come to be the way they are. Biotechnology is using those secrets to alter organisms in very specific ways, and how it does that is the business of the next chapter.