Human beings are amazing machines. Very complex, very self-sufficient. Truly in many ways “the paragon of animals!” as Shakespeare once put it.
And consider the human mind. The most complex thing in the known universe, at least according to some commentators.
Yet all of the aforementioned is encoded microscopically on DNA, in a space so small, you could copy it on a head of a pin several thousand times.
How could something so complex be written so apparently small? And as I understand it, most of DNA isn’t even used anymore. It is a throwback to a time when we had traits we simply don’t use anymore.
Also, I just have to ask. Does this mean that we will have computers some day, with the computational capacity of a human being, that can also fit several times on the head of a pin?
The so-called “junk” that forms the great majority of the genome derives from selfish genetic elements - bits of DNA that once proliferated across the genome simply because they were good at making extra copies of themselves in other places in the genome. They never originally encoded any beneficial trait for the organism. Virtually none of this material still has this original capacity to proliferate across the genome - in that sense it is just inert material that gets copied down the generations along with everything else, mutating over time.
It’s a huge open question how much of this “junk” now does anything for the organism.
All of this mass of DNA, originally useless from the organism’s perspective, has been mutating for millions of years, providing a vast substrate for evolution to occur. So it could potentially now have evolved many useful functions for the organism. We could easily recognize if there were a new protein, so it’s not anything as obvious as that. But it may be that it performs more subtle regulatory functions - how much of the existing proteins are produced, and when and where.
The DNA doesn’t contain a blueprint for the human brain. DNA isn’t a blueprint, it’s a recipe. The recipe contains instructions to build certain proteins, and there are various and extremely complicated mechanisms for turning that production on and off.
So there’s no instruction to build a neuron here, wire together with 50 other neurons in such and such a way, and on to the next neuron. As you imagine, that specification for every neuron in the human brain would be gigantic.
But the information isn’t stored that way. Instead there’s a method for taking a cell and duplicating it, the standard mitosis that drives cell replication. And then there’s a method for turning a generic stem cell into a neural stem cell. And then there’s a method for turning a neural stem cell into the various types of neural cells. And then there’s a method for cranking that neuron production for various times. Run it for a short time and you get a small brain. Run it longer and the brain is bigger. Then the neurons somehow know how to connect to each other in ways that produce the various reflexes and whatever that the brain produces. Start simple and you’ve got the brain of a fish. Add more complex behaviors and you’ve got a mammal brain. Elaborate certain parts of the mammal brain and you’ve got a human brain. Of course we really don’t know very well how this all works at the cellular level.
But at no point was there a map or model of the human brain contained in the DNA.
As for your broader question, it’s a very good one. Clearly, the DNA does not directly encode the states of a hundred billion neurons and produce a fully-formed brain. And how just how the information in the DNA becomes the information in a mental trait is largely unknown.
But you may be underestimating the amount of information that can be stored in DNA. If there are millions of functional regulatory elements in the DNA, the combinatoric effect of varying each of them independently, each in multiple states, equates to a vast amount of information.
The important question isn’t about the physical size of the DNA, it’s about how much information it encodes. The human genome contains about 3 billion base pairs. Each base pair represents two bits of information (since there are four nucleotides). So human DNA contains about 6 billion bits, or 750 megabytes. That is indeed a remarkably small amount of information to encode a human being, but the number is somewhat misleading because every three base pairs (6 bits) encodes one entire amino acid. Another clue to the fact that the sheer size of the DNA isn’t as important as one might think is the fact that many “lower” organisms have DNA much larger than human DNA. For example, wheat DNA is about 20 times the size of human DNA, and the DNA of the shrub Paris japonica is about 50 times the size of human DNA.
The end result (i.e. a living, thinking human being) may seem complex but it’s actually just trillions of iterations of the same thing, with only a few hundred variations. Look up fractals to understand this concept better
Changes from one variation to another can be triggered by environmental factors (e.g. chemical surroundings, types of neighboring cells, etc.)
DNA is supported in it’s functions by other cell structures. These structures are capable of performing their function without input from the DNA, so DNA only stores info about making those structures, and how to pass directions to those structures (just like a department manager doesn’t need to know how to perform every function in their department, only how to hire people and give directions (and what directions to give) to the people that do know how to do various functions)
The human genome contains about 725 megabytes worth of information.
There are many other factors, but I’ll let other dopers address those.
