If I understand correctly, DNA determines which proteins are replicated within a cell, and therefore determine it’s function. So, it’s not as though DNA “knows” every cell in your body and builds out from a blueprint, rather your cells use DNA as instructions for how to respond to chemical stimuli.
This makes sense when talking on a small scale, like something in the environment triggering production of a specific enzyme. But what determines the creation and shape of complex organs or structures like eyes or fingers? What do those “instructions” look like? (“Keep building finger cells until blood circulation is low, then we have to start building fingernails”)?
In: Biology
The nucleotides in DNA thymine, cytosine, guanine and adenine are grouped into batches of three which act like a byte in a computer program for assembling a protein, each of these codons represents a start, an amino acid or a stop instruction resulting in the correct sequence of amino acids being assembled to complete the protein. https://youtu.be/DfaPwWCvN5s
The keyword you’re looking for is HOX genes (sometimes called homeobox genes). These are, broadly, a set of instructions that tell stem cells how to differentiate into different cell types, based on the cells surrounding them and the chemical signals they’re receiving. Mutations in HOX genes lead to deformities, including milder ones like extra fingers and toes, but also appendages growing where they absolutely should not be, or the absence of some structures entirely. For example, they can tweak certain genes in fruit flies to cause them to grow extra legs where their antennae would normally be.
It’s a good question. The general term for all the stuff that tells cells what to do beyond the DNA is “epigenetics.” It’s super complex and there’s definitely a lot of stuff that’s still not fully understood.
However, the rough idea of how it happens is that every cell has all the different sets of possible instructions for being a heart cell, a bone cell, etc. Each cells “decides” what to turn into based on signals from the other cells around it: hormones, physical contact, etc. Almost like how chat gpt finishes sentences.
Sometimes tumors have hair and teeth and stuff when these signals get messed up.
DNA, alone, does not have this information. Let me show you, a bit simplified, the components you need for development.
So the first thing is that DNA has two layers of information. The first layer as you mention the protein coding. The second one is as you mention as well, the reactivity it comes in a form of “switches” or regulator elements that can decide whether a protein is needed or not.
Now the thing is that this regulatory information does not only apply to environmental responses that you called “small scale” in your question. In fact every gene is always regulated. The regulating signal can come from inside the body (like, a hormone) or from the outside. The bodily development is regulated by signals in the body, but the origin of those signals are the mother.
You see there’s another layer of information each cell has. The origin of these information goes back to the egg. The mother sends signal molecules into the egg, but these signals come from one point and does not have time to dissolve. So basically you have a concentration gradient of signals, within the egg cell that creates a difference from top to bottom. The first few steps in your development is basically building walls within the egg, and each room created by these walls are new cells. And each cell has its own concentration of signal molecules.
Now, you basically need two axes in a living being, top-bottom and front-back, and the cross section of these two tell each cell what they need to develop. For a fine resolution each cell can communicate with its neighbors. These concentration imbalances (we call it “gradients”) and the fine tuning regulate the genes similarly to what you call “small scale”, which creates a program to develop each organ at their spot.
One important thing is that DNA is not a blueprint in a way you imagine a blueprint of a house. It is more like a set of instructions for blind workers that are the cells. Imagine you could theoretically build a house with preprogrammed instructions like “on day 5 bring a bucket of mortar 5 steps north and leave it there”. Nobody needs to see the whole as long as everyone is doing their jobs. This is how we develop.
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