| | | What A Difference Context Makes |
First, to clear up a persistent myth, there is nothing "auto-catalytic" or "self-reproducing" about DNA. DNA alone in a test tube stays alone. It doesn't make copies of itself, and it doesn't make anything else either. It just sits there. Actually, DNA is about the most inert biochemical substance around, which is why people can find useful scrapings of it from ancient mummies, old bones, or crime scenes. It doesn't degrade easily. In order to get the DNA to do anything, like reproduce itself, you need to add some enzymes that do the work, and some more nucleotide bases for them to work with. You need helicase to unwind the double helix, an assortment of binding proteins to keep the strands from winding back up together, DNA polymerase to make the copy on one side, and DNA primase, DNA ligase, and DNA polymerase to make the copy on the other side. Each of these enzymes requires a handful of helper enzymes and coenzymes to do its work.
In other words, to get our DNA to replicate, we've had to add information to the test tube, in the form of specific quantities of enzymes, coenzymes, nucleic acid bases, and sugars: a recipe not contained in the DNA itself. This information is present in the nucleus of a cell, but in an implicit form, and therefore not easily recognizable as information.
What else can we make with our DNA strand? Well, perhaps it codes for a protein. We could try to make that protein. First, of course, we have to transcribe the DNA onto a messenger-RNA (mRNA) template, which would then be used to make the protein. This process is roughly comparable to replicating the DNA. You need to make sure there are enough bases, and you need an enzyme called RNA polymerase, along with an assortment of eight or ten different "general transcription factors," molecules whose jobs vary from marking the start point of the transcription to providing the energy the polymerase needs to do its job. A new recipe; more information
Now we have an RNA template we can use to make a protein. But before we can make anything useful out of it, we have to edit it. DNA consists of protein-encoding parts called "exons" interspersed with parts whose function is still mysterious: "introns." There are enzymes whose job is to remove the introns, and splice the exons together. These are the "spliceosomes." Now the mRNA we've edited must be applied to some ribosomes, supplied with amino acids, dozens more enzymes, and a supply of transfer RNA (tRNA) to translate the base code of the mRNA into the protein's peptide chain, a linked train of amino acids.
At each step of the way, the recipe for the new protein becomes a little more intricate and a little more subtle. Subtle enough that as of this writing, the last couple of steps in this chain are not so readily reduced to information. Scientists can make short protein-like chains, linking together short chains of amino acids, but have difficulty reliably making many real proteins, which often contain hundreds of amino acids, and may contain multiple chains. To turn DNA into protein, biologists typically enlist the aid of some living organism: a native speaker. To make a protein, they splice the DNA into the genes of some bacteria, and let it do the dirty work. We know more or less how it all works, but somehow the detail--the translation into a language we understand--remains elusive. Even the steps we can master in vitro are generally much more easily done with the help of engineered bacteria.
Much talk about genetic information seems to depend on the image of the cell as little more than a growing medium for "information." But cells contain a sophisticated mixture of molecular components necessary to express DNA--as well as the mechanisms for creating and maintaining those components--and characterizing them as neutral sites, where DNA expression "happens" hardly seems right.
The mixture is actually even more sophisticated than this outline implies. At this point, we've created a protein from a strand of DNA. We've recreated what Francis Crick dubbed the "central dogma of biology": DNA makes RNA makes proteins. But he formulated this dogma in the 1950's. What have we learned since about how these processes really work inside growing cells? What other details are there to cloud the crystalline information of the DNA strand?
| | | What A Difference Context Makes |