Biology / Computing / Genetics / Science

Writing in DNA

“Scientists say all our genetic information is coded on DNA. But DNA is an acid! How can you write information in acid? You can write information on a hard drive, but not a liquid!”

I was “dared” to try to answer “just one” of a list of stupid questions meant by fundamentalist Christians to stump people with a basic knowledge of science, like teachers at a fairly low grade level. (Without using the phrases “that’s a false analogy”, “that’s not how it works”, or “you just don’t understand science”.) This one shouldn’t test me too much, but isn’t as easy a target as “if humans evolved from chimpanzees, why are chimpanzees still around?”

DNA is indeed an acid: deoxyribose nucleic acid, to be precise. Information isn’t written on a single component of DNA, though. A single guanine on its own means nothing: it’s only when DNA is strung together into the double helix structure familiar from a thousand images that it becomes information. Just as you can’t reproduce a song by singing a single note that isn’t even written on a stave, you can’t make a protein — the basic building block of a living organism — out of a single nucleic acid. Instead, information about the protein comes out of the sequence of the bases, one after the other, just as the melody of a song read from a manuscript comes out of singing (or playing) the notes in order, knowing which note is which by where it’s positioned on a stave.

In this analogy, the stave for DNA is the codon: a group of three bases. And the value of the note for DNA is the order of the three bases within that codon, to continue the analogy. Just as you know by looking at the music — even though it’s written, and not aloud — that you need to sing middle C for four beats, you know from reading a sequence of DNA that goes thymine-cytosine-adenine (TCA) that you’ll get a piece of RNA with the bases adenine-guanine-uracil (AGU), and that means that the growing protein adds an amino acid called serine.

Even information in a hard drive isn’t literally written like words on a page or a screen, so it’s really just trying to make you think there’s something silly about the idea of a series of different chemicals being translated (and that’s literally the word, like translating words from one language to another) to produce a message. When you write information to your hard drive, you’re really writing “off-on-off-on-off-off-on-on” and so on in order to specify the data. Little sectors of your hard drive are either magnetised or not, so when your hard drive needs to be read, the reader records “off” as “0” and “on” as “1”. So you get 01010011. That way, you can build up sentences (and much more complex information).

So that’s how you can “write” information on an acid, or a hard drive — or at least, that’s one way.

(Sorry, folks, I know; if you’re reading this, you’re probably already in the choir and I’m preaching to you. A dare’s a dare, though.)

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