Isn't that kind of like saying that the usefulness of words and letters is entirely dependent on the words and letters being formed into coherent thoughts and put on paper?
No. Words are not material things. They do not exist in the physical world. Words are dependent on our minds subjectively perceiving them. The base pairs that comprise a DNA helix are materially important - like an organ in the body. Without DNA the cell will eventually die. DNA moleculues are to the nucleus of a cell what the brain matter is to the skull for the entire body.
No. The usefulness of words and letters is in constructing sentances that are arranged according in a syntax whereby ideas can be communicated. If I want a glass of water, I can communicate that in one of a myriad ways (including binary), but the idea remains the same. Its not like there's an idea being conveyed in DNA. And besides, if you change the letters, words and syntax of DNA you get a different message entirely or just pure gibberish. Totally unlike any language known to man.