JunkDNA

From Activating Evolution

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[edit] Junk DNA

In molecular biology, "junk" DNA is a collective label for the portions of the DNA sequence of a chromosome or a genome for which no function has yet been identified. About 98% of the human genome has been designated as "junk", including most sequences within introns and most intergenic DNA. While much of this sequence may be an evolutionary artifact that serves no present-day purpose, some is believed to function in ways that are not currently understood. Moreover, the conservation of some junk DNA over many millions of years of evolution may imply an essential function. Some consider the "junk" label as something of a misnomer, but others consider it apposite as junk is stored away for possible new uses rather than thrown out; others prefer the term "noncoding DNA" (although junk DNA often includes transposons that encode proteins with no clear value to their host genome).

Broadly, the science of functional genomics has developed widely accepted techniques to characterize protein-coding |genes, RNA genes, and regulatory regions. In the genomes of most plants and animals, however, these together constitute only a small percentage of genomic DNA (less than 2% in the case of humans). The function, if any, of the remainder remains under investigation. Most of it can be identified as repetitive elements that have no known biological function for their host (although they are useful to geneticists for analyzing lineage and phylogeny). Still, a large amount of sequence in these genomes falls under no existing classification other than "junk".

Overall genome size, and by extension the amount of junk DNA, appears to have little relationship to organism complexity: the genome of the unicellular Amoeba dubia has been reported to contain more than 200 times the amount of DNA in humans".

The pufferfish Takifugu rubripes genome is only about one tenth the size of the human genome, yet seems to have a comparable number of |genes. Most of the difference appears to lie in what is now known only as junk DNA. This puzzle is known as the C-value enigma or, more conventionally, the C-value paradox.