First let me try to state the argument as clearly and charitably as I can. DNA is a design simply because it is a code for us! That is, because every single cell in my body has a complete genome, and because that complete genome carries all the necessary information to build my body, DNA must be a design. Prima facie, it meets all the criteria of information: it is medium independent (I can change the DNA molecule into a string of As, Ts, Gs, and Cs and it will still retain its information-carrying capacity), and it stands for something more than itself (i.e. my body). DNA, put simply, is a language--a code for me--and that means that it is a design. DNA has semantic content in the same way that the English language does: all those chemicals have a meaning, and that meaning is me. Any design implies a designer, and thus DNA had a designer. Humans didn't design themselves, so something else must have done the job. God is the most likely culprit.
I trust a design theorist would find this formulation acceptable--now I'm going to tell you why it's not. A common extension of this metaphor is to refer to DNA as a "blueprint for you;" I think this metaphor is pedagogically useful, so let's adopt it for the purposes of this discussion. A blueprint, to be sure, represents a building, but before we're going to decide if my DNA represents me in the same way, we're going to have to be clear about what precisely we mean by 'represent' here; it seems that there are at least two primary senses in which we might be using the term, so let's consider DNA in light of each of them.
First, by 'blueprint B represents building X' we might mean something like what we mean when we say that a map represents a nation: that is, that the blueprint corresponds to a building in that each line can be matched with a wall, door, or similar structure in reality. But wait, this does not seem entirely adequate: to adapt Hilary Putnam's famous example, we can imagine an ant crawling in the sand which, by pure chance, traces with its tracks a perfect duplicate of the original blueprint for the Eiffel Tower. It does not seem right here to assert that the ant has created a blueprint for the Eiffel Tower (for a more detailed argument for this, see Putnam's Reason, Truth, and History, pages 1-29). Representation in this sense--in the sense of a map of New York or a painting of Winston Churchill--requires more than mere correspondence: it has to come about in the right kind of way. How exactly to define "the right kind of way" is a deep question, and it is not one that I intend to pursue here. Suffice it to say that the right kind of way involves agentive production by beings with minds at least something like ours (minds that are themselves capable of semantic representation); other methods might produce things that look very much like representation, but this resemblance is not sufficient.
Here, then, is the problem for the the intelligent design advocate attempting to endorse the first horn of this dilemma: using it to demonstrate God's existence is straightforwardly question-begging, as we saw above. Arguing that DNA represents in the same sense that a map represents terrain or a portrait represents a person requires the assumption that DNA was produced agentively by a being with minds like ours; this assumption is precisely what the design-theorist wants to prove, making this line of argumentation invalid. As I said, though, there is a second horn of the dilemma that the design-theorist might instead endorse--let us return now to our blueprint metaphor and see if DNA fares any better here.
The second way we might intend to use 'blueprint B represents building X' is what we might call "the instructive sense." This is the case if building X has not yet been constructed: blueprint B represents not because it corresponds to anything in reality, but because it contains instructions for how one should proceed when constructing building X. What does it mean, though, for one thing to contain instructions for the creation of another? Consider computer programming: when I type something like the following into a compiler:
cout << "Hello World!"; return 0;
am I writing instructions for the creation of something? Prima facie, this looks just like the blueprint case, but there's an important (and relevant) difference here: in typing the code into a compiler, I'm not making instructions for the program's creation, but rather creating the program itself. That is, the "code" for the program just is the program looked at in a certain way (e.g. through a decompiler); to watch someone write a computer program and then say "Well yes, I've seen you write the instructions for the program, but when are you going to make the program itself?" would make you guilty of a category mistake, it seems--again, writing the program just is writing the code. Program and code are identical. This case, I think, is instructive. We'll see how in just a moment.
Let's set aside the philosophical struggle with this second horn for a moment and remind ourselves what DNA actually is and how it works. DNA consists of two long polymers composed of phosphate groups and sugar groups conjoined by organic esters. Attached to this scaffolding, four "bases"--adenosine, ctyosine, thymine, and guanine--do the real "work" of DNA: adenosine always attaches to thymine, and cytosine to guanine, meaning that the entire sequence of both sides can be deduced from just one given half. DNA's primary function in the body is to regulate the creation of proteins, which in turn regulate virtually all of the body's functions. DNA does this by creating strands of RNA through the process alluded to above; units of three base pairs at a time on the relevant portions of the RNA (there are huge parts of our DNA that seem to play no active role in anything) then interact with cellular objects called ribosomes, which produce corresponding proteins. This is obviously a very basic account of how the protein creation process happens, but it should suffice for our purposes here.
The most salient part of the above, it seems, is the emphasis on causation. The entire process of protein synthesis can be done without involving an agent at all. In this way, DNA stands in sharp contrast to the blueprint from our earlier discussion--the sense in which we're using 'instruction' when we're discussing blueprints (at least in the second horn of the dilemma) necessarily includes a concept of conscious builders; to put it more generally, instructions must be instructions for someone to follow. DNA, then, is somewhat more like a computer program than it is like a blueprint in the second sense: rather than being instructions for something's creation, it _just is_ that something viewed from a lower level. Still, though, there is an important element of disanalogy here--to assert that DNA is just like a computer program would be to assert that it represents in the first sense we discussed. This assumption, we saw, leads to a fallaciously circular line of reasoning, and thus is unacceptable. As with a blueprint, if we make the comparison between a computer program and DNA, we must be careful to remember that it is just a metaphor. This, I think, is the central point that I am making: while the blueprint metaphor is apt in many ways, we must take care to remember when we use it that it is just a metaphor--while DNA and blueprints share things in common, there are important difference that prevent the two from being completely equated, no matter which sense of 'represent' we're using.
We've said a great deal here about why thinking of DNA as representing organism is incorrect, so let's take a moment to sketch a positive argument and suggest what the correct way to think about DNA might be. Let's begin by remembering that DNA causes protein synthesis, which causes other necessary organic functions. If we keep this observation squarely in mind, DNA's metaphysics aren't all that difficult to articulate: DNA, mediated by other chemicals and environmental considerations, regulates the causal chain that leads to the occurrence of all the various functions we mean by the cluster-concept 'life.' These include, but are not limited to, metabolism, reproduction, cell growth, cell regeneration, gas exchange, and many, many others. DNA is just one link--albeit a very important link--in the naturalistic chain that both causes and is constitutive of life.
As I said, I'm aware that there are a great many holes here that need to be plugged before this theory is really solid, but I think the rough outlines are clear. Thoughts?