How does your DNA know where it is in your body and which body part to grow into?


How does your DNA know where it is in your body and which body part to grow into?

In: Biology

Some parts of your DNA are regulatory genes. They don’t build anything used for tissue or enzymes or anything like that. Instead, they build proteins that activate other parts of your DNA. So, say your DNA is trying to build an eyeball. It doesn’t immediately start making an eyeball. Instead, there’s a gene for “build an eyeball” and that gene turns on another gene that says, “build a retina” and *that* gene turns on *another* gene that says, “build a nerve cell” and so on.

Those regulatory genes are activated by any number of chemical signalers. Some of them turn themselves on and off. So for instance, to build fingers there is a signal that both turns on the “build a finger” gene *and* turns *off* the genes that build itself. So the more of this signal there is, the less of it will be made. So you end up with pockets of high levels of this signal, but they fall off really quickly. So your body builds a finger where those signals are strong, but not where none exist.

Similar signals pervade your body. Signals turn genes on or off which turn on or off other signals, which turn on or off the genes that code for the actual “stuff” that you’re made out of. Every type of cell tells every other cell what kind it is, and affects which parts of their DNA is activated, which feed back to that first cell and tells *it* what DNA to activate. Before your cells even start to differentiate, when you’re just an embryo barely starting to form, the cells still started with some signals that affect the growth of the cells from that point forward so they *do* begin to differentiate.

This is a fairly complex topic that involves two biological processes called cell differentiation and gene expression.

The first is the process by which embryonic and adult stem cells transform into whatever cell type or tissue is necessary and is done through a complex series of signalling molecules and proteins and biological pathways that affect the gene expression of that cell.

Gene expression is the means by which your cells turn on or off certain parts of your DNA. You have proteins and enzymes that will inhibit certain portions of your DNA and activate others. This then allows the cell to make only what is necessary for that cell type.

Early on, cells in an embryo “know” whether they are on the outside or inside, top or bottom. They then turn on certain genes and turn off others. This makes the cells increasingly specialized. The cells send signals to each other which become more and more complex, until you get certain tissues and organs in particular places and arms and feet where they are supposed to go.

The first part of your question: how does a cell (my edit, it’ll make things easier to explain) know where it is?

Cells use a lot of different ways to get an idea of where they are. It starts when an egg and a sperm meet and start growing into a baby.

At this stage, the baby is one big cell. It chooses a point on the cell as ‘up’. As far as I know, we don’t really understand how this happens in humans (at least we didn’t when I studied this). The cell organises a bunch of little cell machines around ‘up’, and they act as a kind of flag.

The cell then starts to split up into lots of other cells, keeping its little ‘up’ flag. Once there’s enough cells, they start talking to the cells next to them, to check in and see how they’re doing. Cells do this by making little machines that live on their surface. When two matching machines from two adjacent cells meet, they do a sort of hand shake. This changes the shape of the machine, and makes it swim back into the cell.

It will meet other machines whose job is to find swimming handshake machines, and take them to the DNA. (Or sometimes they’ll find the handshake machines, and then drop off a different machine at the DNA). The handshake machine then looks for a specific ‘sentence’ in the DNA, and then it latches on. While it’s latched on, it acts as a flag that says “hey read this gene”. This is how cells talk to their neighbours about where they are. This can happen quite fast – sometimes a signal is actually a question, and the gene that gets flagged is for another type of handshake machine. It’ll get made and stuck on the surface, which is how cells answer each other.

Cells can also do this kind of communication at a distance. There’s a kind of machine called a ‘morphogen’. A group of cells that know they’re in a specific, useful reference point – like the cells at ‘up’ in a tiny embryo – will make these morphogens and spit them out. Other cells have a set of handshake machines that are looking specifically for a morphogen. When a handshake machine gets bumped into by a morphogen it’s looking for, it’ll grab onto it, and pull it into the cell with it. This means that cells near ‘up’ will get lots of morphogens, and cells far away from ‘up’ will get very few. This is one way cells know how far they are from a specific point. They combine this with talking to each other, and checking that their neighbours also think they’re as far from ‘up’. All these signals latch on to the right places on DNA, and make more flags.

Now, as far as knowing what to turn into, there’s one more piece of the puzzle. We call these Transcription factors. These are little machines that act as master switches for a whole bunch of genes. Usually, these are genes that work together to make a specific type of cell or organ. So, for example, a heat sensing nerve needs DNA instructions on how to send signals like a nerve, how to build machines that sense heat, and how to stay alive and healthy in the body. It might use a handful of master control switches to turn on all those genes, and turn off all the irrelevant ones.

Those signals that I wrote about earlier? They’re all competing to turn on a particular Transcription Factor. The cell is constantly asking questions with its handshake machines, and checking in to see if its healthy. All these signals will fight to either block the transcription factor or flag it. When the flags win, and a transcription factor gene is turned on, the first thing it does is permanently turn off other transcription factors. Then it goes and turns on all the relevant genes.