DNA is the storage vehicle for genetic information, but it doesn't directly do the work of building proteins itself. DNA is the boss. The workhorse is a similar nucleic acid, RNA (ribonucleic acid), which carries out DNA's instructions. Essentially, RNA assembles proteins, one amino acid at a time, using the sequence of nucleotides along a strand of DNA (that is, a gene) as its guide.

A protein molecule is made by a gene in two stages. First, an RNA copy of the gene is made. Transcribed from a template of DNA, the RNA copy has a nucleotide sequence complementing that of the gene. Then the RNA moves to another part of the cell, where its nucleotide sequence is translated into a sequence of amino acids to build a protein.

The cell's tiny protein-assembly plant works in much the same way in all organisms. That is why genetic engineers can take genetic instructions from one organism and add them to another, or even write their own new instructions.

Outside of its cell, there is no distinction between a human gene, a cat gene, a wheat gene, or a bacterial gene. There is nothing intrinsic to a gene, in other words, that