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Actually, the editing step, where spliceosomes separate mRNA exons from introns, wasn't apparent in the 1950's. But let's look at it for a moment. It's not yet entirely clear how the enzymes that do the editing do what they do, but it appears that the simple picture of cutting out the introns isn't quite adequate, either. It seems instead that these enzymes play a sophisticated role in constructing proteins out of assembled pieces, regardless of the order in which they appeared in the DNA sequence. With the help of these enzymes, one stretch of DNA can be responsible for hundreds of different proteins.
A recent study of chick development estimated that a single gene can make over 500 different proteins for "tuning" the hairs in the chick's cochlea.15 In a cochlea, part of the inner ear, each hair is sensitive to a different range of acoustic vibrations, and the sensitivity appears to be largely determined by the form of the protein produced by the chick's cSLO gene. For normal hearing, you want your cochlea to contain a collection of hairs sensitive to a wide variety of ranges, and that's apparently what normal development produces. However, there is no good explanation of why any particular cell chooses one cSLO variant over another. Perhaps it is just random; a chicken might be able to hear normally if all the cochlear hairs are tuned randomly, but we don't know that. If it's not random, what is the mechanism by which these hair cells organize themselves? No one knows. One possibility is that the cells actually respond to the sound waves that hit the cochlea, that the tuning of the ear is done after the ear is constructed. But this is just speculation.
A very similar mechanism works in our immune system, which needs variety to work. We need lots of different kinds of antibodies to throw at whatever antigens appear in our blood. Antibody variety is created by enzymes in your marrow who maniacally reorder the genes in a certain stretches of immune-system DNA to create an astronomically large number of possible antibodies.16 Lymphocytes armed with these new proteins go out into battle against these intruders. The successful ones--that find and engage the enemy--reproduce, which is how your immune responses become sensitized over time.
Even the "just-sitting-there" stage of DNA turns out to be more intricate than originally thought. DNA is inert, but it's not perfectly so, nor is it ultra-stable, like a crystal. It decays and falls apart from time to time, and copies are often imperfect. To fulfill its function as genetic material requires an assortment of repair genes to mend tears, excise ruined sections, edit poor copies and undo occasional mutations. Much of DNA's stability, then, is not static, but a kind of dynamic balance maintained by a crew of molecular repair technicians.
These repair technicians do more than just repair things. Sometimes, they deliberately screw them up. It appears, for example, that you can artifically breed bacteria to improve the fidelity of their DNA copies. Naturally occurring bacteria, the product of billions of years of evolution, haven't done so themselves, so the implication is that there may be some adaptive value in messiness. This may be simply because of an energy trade-off between fidelity and efficiency (that is, higher fidelity requires more energy, and if you've got repair technicians around, why bother?), or it may be that there is value in leaving the field open to occasional mutations. There is evidence that some bacteria produce messy versions of DNA polymerase when they are under environmental stress. These enzymes, dubbed "mutases," act like normal DNA polymerase, but the resulting copies have more mistakes than copies made with regular polymerase. That is, they seem to promote mutation among the bacterial offspring.
Promoting mutation may not, in fact, be a bad survival strategy: if you're having trouble, at least there's a chance that your daughters may have a better time of it. There even exist clues that the induced mutations may not be entirely random, but at this writing, what evidence there is remains inchoate.17
Human genomes contain codes for enzymes that resemble these mutases, though no one now knows what we do with them. It does appear that some lymphocytes, the agents of our immune system, use directed mutations, "somatic hypermutation," to fine-tune the immune response to antigens.18 The mutases may be part of that system, or they may be used in some other way entirely.
Barbara McClintock noted, back in the 1940's, that attentive analysis of classical (non-molecular) genetics results show that genes have some interesting ways of dealing with environmental stress. Among many other effects, she pointed out that chromosomes can combine and recombine, and that genetic elements are capable of simply moving from one chromosome to another, changing their effects with their position. The mechanisms for detecting stress, and the procedures by which the genome is rearranged, are not yet understood, but some of the effects are quite clear.19
The transcription step, making RNA from DNA, also has its intricacies. Regulatory sites have been known about since Jacob and Monod, but the picture has become successively more interesting. The history of what is known about a single gene is worth retelling. The lac gene, implicated in the processing of milk sugars by E. Coli bacteria, was discovered in 1947, and determined to be a single point on a chromosome. Fourteen years later, when Jacob and Monod published their discovery of gene expression controls, it was about the lac gene. With their new analytical tools, and an increased resolution view of the chromosome site, Jacob and Monod found that the gene corresponded to three different enzymes, and an operator site that controlled their expression. The operator that binds to the site only does so in the absence of lactose, and it acts to repress expression of the gene. Therefore, the gene products, used in digesting milk sugars, are only expressed in the presence of those sugars. It seemed a tidy picture. The whole complex was dubbed an "operon" to imply that it was a unit, albeit a complex one.
Thirty years later, increased scrutiny has filled out the picture. It's now known that the "operator site" found by Jacob and Monod is actually a composite of three repressor and three promoter sites that overlap each other (at least one of which overlaps the enzyme-encoding part of the gene) along with two sites activated by repressors from some entirely different gene group.20 The picture that emerges is one of a much more sophisticated mechanism than had been expected: not a simple switch, but a collection of interdependent switches, dependent on each other and on a host of different external stimuli.
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