system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: From a single cell to a human - is every cell's behavior, task, etc all governed by DNA and more importantly, how does DNA interact with the cell to control it?
gpt
So the cool thing about biology is that we're all just "chemistry buckets". We're just a lot of very advanced chemistry, and chemistry dictates everything we are.

Proteins are a family of molecule that are actually very diverse. They can make structures, like cell walls and fingernails. But again -- just chemicals. They're not "smart". A protein has a job to do, and it does it. Keratin, which is one of the proteins that make up hair and nails, is shaped so that it lines itself up into structures. 

Other proteins carry "messages" from one place to another. Really that just means that some protein is sent into the blood, where it travels the whole body -- cause again, just a sack of chemistry -- and eventually it'll find a place that it fits. And when it does "fit" into something, that something may react, maybe by producing other chemicals.

Proteins also break down other materials (enzymes), can attack pathogens, and do lots more. 

DNA itself is the protein cookbook. It's also just a chemical; a huge one, but a chemical nonetheless. In labs, to inspect DNA, they actually grow more of it. You just take DNA, and throw it into a soup of other chemicals that replicate DNA (those same chemicals found in the body) and then you cook it, and boom, you have a lot more DNA.

Either way, there's another chemical that interacts with DNA. It latches onto a bit of DNA, takes its shape, falls away and then another chemical comes in and builds a protein from that shape. It's almost like taking a cast and then a mold of something you want to sculp. The DNA is huge and "codes" lots of different proteins -- all your proteins, in fact -- but the RNA only needs to interact with a section at a time. 

And remember still, nothing "smart" is happening here. It's just several chemical reactions, one after another. 

But because DNA is the template for every protein your body uses, it defines your whole body. There's one protein that causes the gold flecks around the ring of your iris (in your eye) -- some people have it, some people don't. But at no point do these biological tools make "decisions"; the DNA doesn't "control" a cell in the sense that it's directing traffic. It's just a very complex system of different chemicals interacting with each other and producing different effects.