[Why does the DNA code contain 4 letters?]

The answer to this question is not yet known. There are two ways to express information, i.e., to reduce the number of letters in alphabet (n), which simplifies the decoding machine, but leads to longer informational sequences, or to increase n, which shortens sequences, but complicates the informational machine. The compromise between these two possibilities would be to obtain the minimum of one of summary informational component's parameters. The summary component is the sum of corresponding decoding machine's and the program's parameters. In this work it was demonstrated that DNA four-letter code is optimal, for it allows the minimal volume of summary cell informational contents. But it is so only for the most simple DNA. Our calculations may indirectly show that such DNA (and not more complicated) was the object of "projecting" at one of the biological evolution's early stages.