junk Dna
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Although very catchy, the term "junk DNA" repelled mainstream researchers from studying noncoding genetic material for many years. After all, who would like to dig through genomic garbage? Thankfully, though, there are some clochards who, at the risk of being ridiculed, explore unpopular territories. And it is because of them that in the early 1990s, the view of junk DNA, especially repetitive elements, began to change. In fact, more and more biologists now regard repetitive elements as genomic treasures. It appears that these transposable elements are not useless DNA. Instead, they interact with the surrounding genomic environment and increase the ability of the organism to evolve by serving as hot spots for genetic recombination and by providing new and important signals for regulating gene expression.
Genomes are dynamic entities: new functional elements appear and old ones become extinct. And so, junk DNA can evolve into functional DNA. The late evolutionary biologist Stephen Jay Gould and paleontologist Elisabeth Vrba, now at Yale University, employed the term "exaptation" to explain how different genomic entities may take on new roles regardless of their original function—even if they originally served no purpose at all. With the wealth of genomic sequence information at our disposal, we are slowly uncovering the importance of non-protein-coding DNA.
In fact, new genomic elements are being discovered even in the human genome, five years after the deciphering of the full sequence. Last summer developmental biologist Gill Bejerano, then a postdoctoral fellow at the University of California, Santa Cruz, and now a professor at Stanford University, and his colleagues discovered that during vertebrate evolution, a novel retroposon—a DNA fragment, reverse-transcribed from RNA, that can insert itself anywhere in the genome—was exapted as an enhancer, a signal that increases a gene's transcription. On the other hand, anonymous sequences that are nonfunctional in one species may, in another organism, become an exon—a section of DNA that is eventually transcribed to messenger RNA. Izabela Makalowska of Pennsylvania State University recently showed that this mechanism quite often leads to another interesting feature in the vertebrate genomes, namely overlapping genes—that is, genes that share some of their nucleotides.
These and countless other examples demonstrate that repetitive elements are hardly "junk" but rather are important, integral components of eukaryotic genomes.Risking the personification of biological processes, we can say that evolution is too wise to waste this valuable information.
https://www.scientificamerican.com/article/what-is-junk-dna-and-what/