'Junk' DNA distinguishes humans from other species

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In a paper published in Genome Research on November 4, 2008, scientists at the Genome Institute of Singapore (GIS) discovered that what was previously believed to be 'junk' DNA is in actual fact one of the important ingredients that distinguishes humans from other species. More than half of the DNA in humans used to be referred to as 'junk' because it consists of copies of nearly identical sequences called repeats. A major source of these repeats is from internal viruses that have inserted themselves throughout the genome at various times during mammalian evolution.

 

Using the latest sequencing technologies, however, the GIS researchers were able to demonstrate that many transcription factors (master proteins that control the expression of other genes) bind specific repeat elements. The researchers show that from 18 to 33% of the binding sites of five key transcription factors with important roles in cancer and stem cell biology are embedded in distinctive repeat families. Over evolutionary time, these repeats were dispersed within genomes of different species, creating new regulatory sites throughout the genome. This means that the set of genes controlled by these transcription factors is likely to be significantly different from species to species and may be a major driver for evolution. This also proves that these repeats are anything but 'junk DNA', as they provide a great source of evolutionary variability and they might hold the key to some of the important physical differences that distinguish humans from all other species.

 

Another important consideration arising from this research is the functional importance of portions of the genome that are rich in repetitive sequences, something which many previous studies had ignored.

 

"Because a lot of the biomedical research use model organisms such as mice and primates, it is important to have a detailed understanding of the differences between these model organisms and humans in order to explain our findings. Our research findings imply that these surveys must also include repeats, as they are likely to be the source of important differences between model organisms and humans," said Dr Guillaume Bourque, Senior Group Leader and lead author of the research paper. "The better our understanding of the particularities of the human genome, the better our understanding will be of diseases and their treatments."

 

Prof Raymond White, Rudi Schmid Distinguished Professor at the Department of Neurology, University of California, and Chair of the GIS Scientific Advisory Board said, "The findings by Dr Bourque and his colleagues at the GIS are very exciting and represent what may be one of the major discoveries in the biology of evolution and gene regulation of the decade. We have suspected for some time that one of the major ways species differ from one another - for instance, why rats differ from monkeys - is in the regulation of the expression of their genes: where are the genes expressed in the body, when during development, and how much do they respond to environmental stimuli? What the researchers have demonstrated is that DNA segments carrying binding sites for regulatory proteins can, at times, be explosively distributed to new sites around the genome, possibly altering the activities of genes near where they locate. The means of distribution seem to be a class of genetic components called 'transposable elements' that are able to jump from one site to another at certain times in the history of the organism. The families of these transposable elements vary from species to species, as do the distributed DNA segments which bind the regulatory proteins.

 

"This hypothesis for formation of new species through episodic distributions of families of gene regulatory DNA sequences is a powerful one that will now guide a wealth of experiments to determine the functional relationships of these regulatory DNA sequences to the genes that are near their landing sites," added Prof White. "I anticipate that as our knowledge of these events grows, we will begin to understand much more how and why the rat differs so dramatically from the monkey, even though they share essentially the same complement of genes and proteins."

 

Source: A*STAR

[freeztar]

Hold up...I thought biological evolution was dead? :confused: