Feb. 12, 2005 While the vast majority of the world's genetic researchers focus on the five to ten percent of the human genome that is actually a blueprint for a useful molecule, Wojciech Makalowski is studying the "junk."
In particular, Makalowski, associate professor of biology and head of the Computational Evolutionary Genomics Lab at Penn State, studies DNA sequences called transposons, which make up more than a third of all the "non-coding" DNA in every cell of our bodies. Transposons, as Makalowski describes them, are like "little autonomous entities that live in the genome."
With their special DNA codes, they can "make offspring and move around," that is, they can induce the cell to make a copy of their sequence and then reinsert that copy into another part of the genome. If the new copy happens to land in the middle of a coding sequence, however, it can cause a fatal mutation. For this reason, some scientists have considered transposons genetic parasites that breed at the expense of the genome they live in.
But Makalowski is sticking up for the little guys, arguing that they are more helpful than harmful — a kind of genetic symbiont.
What good comes from transposons? Basically, the genetic machinery that gives transposons the ability to copy and paste themselves throughout the genome can be useful if it lands in the right place, Makalowski explains. For example, one day, more than ten million years ago, a new copy of Alu, our most common transposon, jumped in front of a dormant gene called theta-globin, a cousin of the famous hemoglobin that carries oxygen in our blood. Since Alu carries a bit of code that is necessary for gene expression, and theta-globin needed such a signal to be transcribed, Alu wound up turning on the gene that had been unused for millions of years. Today, we can find traces of theta globin in the blood of both humans and great apes, where it has little function, but the potential to evolve into a valuable gene. Its presence in our bodies is a piece of living proof that the action of transposons can be constructive.
Perhaps a more important function of transposons comes from their ability to change the way DNA code is edited before the information is used to build a useful protein. DNA in the cell’s nucleus is first copied and then exported to the protein manufacturing centers. However, this rough draft of the genetic plan is full of junk code that special molecules — like little editors — have to trim out, leaving only the sections of code that are actually used for building the final molecule. These little editors can bind to the special bit of code between the useful and junk DNA, called "splicing sites," and cut the strand. When certain transposons jump into the mix, however, they can introduce new cutting sites and interfere with old ones. By this means, new pieces of DNA get into the final draft of the code, and new variations of proteins are manufactured. Even a tiny change can have a huge impact (for good or ill) on a protein’s properties.
The more often these inserts happen, the more likely a beneficial change will occur. The more beneficial changes in a creature's DNA, the more likely that creature will be able to adapt to a changing environment. Thus, Makalowski argues that transposons are worth the risks. Over the long term, he suggests, they may have helped us become the people we are today — and they'll certainly help us again in the future.
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