In his book, Power, Sex, Suicide: Mitochondria and the Meaning of Life, Nick Lane takes a moment to clear up something about the relationship between DNA and cells: DNA doesn’t make cells. Only cells make cells.
When we talk about DNA being the “blueprint” for making an organism, a lot of people get the wrong idea. They think that the DNA can be plopped into a vat of chemicals, and as long as it’s the right kind of chemicals, it will contain all the information that it needs to create a living thing. Some people don’t think about it that graphically, of course, or in that much detail; however, they get the general impression that all of the information needed to create a cell “from scratch” (as it were) is right there, in the DNA. All that the DNA needs is the raw materials.
This is not the case. The information in the DNA can determine what proteins are made, and it can even determine where they are directed. But the process is more like fitting new puzzle pieces into an already-growing puzzle. The system is carefully tuned so that the new protein will interact with the existing cell wall, or cytoplasm, or whatever is already there in order to produce the next step in cell growth or cell division. A piece of DNA set in a pool of proteins will never create a cell, unless there is already a cell there. In an important philosophical sense, DNA by itself cannot make a cell: Only cells make cells.
I think this is an important philosophical point, and it goes beyond the “fun nerdy trivia” side. For one thing, Nick Lane points out that this has implications for Dawkins’ “Selfish Gene” concept. Although genes might be “the thing that gets passed on” from generation to generation, they only do it by bootstrapping themselves from the cell structure of one generation to the next. The DNA itself does not contain all of the information that is being passed on. If only cells can make cells, then does it really make sense to consider DNA the “basic unit” of reproduction and evolution? Or should it be the cell?
The fact that only cells make cells also has implications for discussions of “nature versus nurture.” For a lot of people, the idea of “nature” has become synonymous with “genetics” and DNA. They think that either there is a gene for a trait, or the trait is something that was learned or chosen deliberately. No other options. But in actuality “nature” is a much more complex thing. For a trait to be “in-born” it is constrained strongly by both the structure of the DNA of the organism and the structure of the cells that it comes from. This is true for every cell in the body, going right back to the sperm and the egg of the parents, and the cells that created them.
Finally, the fact that only cells make cells has strong implications for the way that we understand life itself, and our scientific models of life and evolutionary processes. In computer science today we have very oversimplified, even trivial, simulations of evolutionary processes. Most models assume that if you look at the “DNA information” (whatever information is being passed from one generation to the next) you know everything there is to know to create the next generation. This is very fundamentally true, for example, of genetic algorithms and genetic programming. When I did research at the Santa Fe Institute on using genetic algorithms to evolve cellular automata (CA’s), the representation of the “DNA” was a basic representation of the dynamic rules of the CA cells. I could look at the “DNA” code, and build a CA from scratch based only on that information.
But that’s not how real DNA works. DNA has to interact with a pre-existing cell in order to create a new one.
Does that complexity matter? Does it impact the way evolution progresses? Does it impact the definition of life? I don’t know. But, I don’t think we will ever know as long as we keep thinking of the relationship between DNA and cells in the wrong way.
So, it’s just something to keep in mind. The next time you talk about evolution as a metaphor, the next time you talk about “nature vs. nurture,” or the next time you talk about the Human Genome project, it might be fun to just keep this piece of trivia in the back of your head. See how it applies, and how it might change your thinking. DNA doesn’t make cells: only cells make cells.