The question is, how do four different nucleotides translate into 20 amino acids and thousands of different proteins? It's not a big problem, really, just a matter of coding. Think of the dots and dashes of the Morse Code giving alphabetic instructions for writing out King Lear and you'll have an idea of how it can be done.

Like the dot-dot-dot and dash-dash-dash signifying "S" and "O" in Morse code, the genetic code is organized in groups of three. That is, a sequence of three adjacent nucleotides is a code for each amino acid. For example, the amino acid glycine is coded by the sequence GGA. Each triplet of nucleotides is called a codon. Since four nucleotides can be put together in groups of three in 64 different ways (4 x 4 x 4 = 64), there are more than enough codons to encode all 20 amino acids. (In fact, most amino acids are encoded by more than one codon.)

The complete genetic code linking each of the 64 codons to an amino acid was finally cracked by the research of Har Gobind Khorana and Marshall Nirenberg in 1967. As a result of their work, they were able to draw up a universal decoder chart showing the correlations between codons and amino acids—a correlation identical in virtually all organisms.

I said before that genes are instructions for making proteins. I can now refine that definition and say that a gene is a segment of DNA with a unique sequence of nucleotides, encoding information for assembling particular amino acids into a particular protein.