Nowe informacje na temat przydatności transpozonów i „śmieciowego DNA”.
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Scientists have called it „junk DNA.” They have long been perplexed by these
extensive strands of genetic material that dominate the genome but seem to
lack specific functions. Why would nature force the genome to carry so much
Now researchers from Princeton University and Indiana University who have been
studying the genome of a pond organism have found that junk DNA may not be so
junky after all. They have discovered that DNA sequences from regions of what
had been viewed as the „dispensable genome” are actually performing functions
that are central for the organism. They have concluded that the genes spur an
almost acrobatic rearrangement of the entire genome that is necessary for the
organism to grow.
It all happens very quickly. Genes called transposons in the single-celled
pond-dwelling organism Oxytricha produce cell proteins known as transposases.
During development, the transposons appear to first influence hundreds of
thousands of DNA pieces to regroup. Then, when no longer needed, the organism
cleverly erases the transposases from its genetic material, paring its genome
to a slim 5 percent of its original load.
Laura Landweber (Photo: Denise Applewhite)
„The transposons actually perform a central role for the cell,” said Laura
Landweber, a professor of ecology and evolutionary biology at Princeton and an
author of the study. „They stitch together the genes in working form.” The
work appeared in the May 15 edition of Science.
In order to prove that the transposons have this reassembly function, the
scientists disabled several thousand of these genes in some Oxytricha. The
organisms with the altered DNA, they found, failed to develop properly.
Other authors from Princeton’s Department of Ecology and Evolutionary Biology
include: postdoctoral fellows Mariusz Nowacki and Brian Higgins; 2006 alumna
Genevieve Maquilan; and graduate student Estienne Swart. Former Princeton
postdoctoral fellow Thomas Doak, now of Indiana University, also contributed
to the study.
Landweber and other members of her team are researching the origin and
evolution of genes and genome rearrangement, with particular focus on
Oxytricha because it undergoes massive genome reorganization during development.
In her lab, Landweber studies the evolutionary origin of novel genetic systems
such as Oxytricha’s. By combining molecular, evolutionary, theoretical and
synthetic biology, Landweber and colleagues last year discovered an RNA
(ribonucleic acid)-guided mechanism underlying its complex genome rearrangements.
„Last year, we found the instruction book for how to put this genome back
together again — the instruction set comes in the form of RNA that is passed
briefly from parent to offspring and these maternal RNAs provide templates for
the rearrangement process,” Landweber said. „Now we’ve been studying the
actual machinery involved in the process of cutting and splicing tremendous
amounts of DNA. Transposons are very good at that.”
The term „junk DNA” was originally coined to refer to a region of DNA that
contained no genetic information. Scientists are beginning to find, however,
that much of this so-called junk plays important roles in the regulation of
gene activity. No one yet knows how extensive that role may be.
Instead, scientists sometimes refer to these regions as „selfish DNA” if they
make no specific contribution to the reproductive success of the host
organism. Like a computer virus that copies itself ad nauseum, selfish DNA
replicates and passes from parent to offspring for the sole benefit of the DNA
itself. The present study suggests that some selfish DNA transposons can
instead confer an important role to their hosts, thereby establishing
themselves as long-term residents of the genome.