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 97% of the human genome has been designated as junk, including most sequences within introns and most intergenic DNA. While much of this sequence is probably an evolutionary artifact that serves no present-day purpose, some of it may function in ways that are not currently understood. Recent studies have, in fact, suggested functions for certain portions of what has been called junk DNA. The "junk" label is therefore recognized as something of a misnomer, and many would prefer the more neutral term "noncoding DNA".

Related Topics:
Molecular biology - DNA - Chromosome - Genome - Human genome - Introns - Misnomer

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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 of the remainder, if any, remains under investigation. Most of it can be identified as repetitive elements that have no known biological function (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".

Related Topics:
Functional genomics - Genes - RNA genes - Regulatory regions - Plant - Animal - Repetitive elements - Lineage - Phylogeny

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It is notable that 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 Fugu rubripes pufferfish genome is only about one tenth the size of the human, yet seems to have a comparable number of genes. Most of the variance appears to lie in what is now known only as junk DNA. This puzzle is known as the "C-value enigma" (7).

Related Topics:
Genome size - Amoeba - Pufferfish - C-value enigma

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