Taming the genome's 'jumping' sequences

The human genome is fascinating. Once predicted to contain about a hundred thousand protein-coding genes, it now seems that the number is closer to twenty thousand, and maybe less. And although our genome is made up of about ...

The evolution of bird-of-paradise sex chromosomes revealed

Birds-of-paradise are a group of songbird species, and are known for their magnificent male plumage and bewildering sexual display. Now, an international collaborative study involving the University of Vienna, Zhejiang University ...

Study of Arctic fishes reveals the birth of a gene—from 'junk'

Though separated by a world of ocean, and unrelated to each other, two fish groups—one in the Arctic, the other in the Antarctic—share a surprising survival strategy: They both have evolved the ability to produce the ...

Why wasn't the human genome shredded long ago?

In the January 7th edition of Communications Biology, researchers at InsideOutBio argue that an unusual form of DNA with a reverse twist may have helped thwart the invasion of the human genome by junk DNA. This mechanism ...

Platforms for investigating lncRNA functions

To aid in the discovery and understanding of lncRNA biology, newly published work from Richard and Eichhorn in SLAS Technology features the technological platforms and methodology presently used to identify the roles of lncRNA ...

Scientists discover a role for 'junk' DNA

Researchers at the University of Michigan Life Sciences Institute and the Howard Hughes Medical Institute have determined how satellite DNA, considered to be "junk DNA," plays a crucial role in holding the genome together.

Plant genes may lack off switch, but have volume control

Scientists at the University of California, Davis have discovered that DNA sequences thought to be essential for gene activity can be expendable. Sequences once called junk sometimes call the shots instead.

Rare evolutionary event detected in the lab

It took nearly a half trillion tries before researchers at The University of Texas at Austin witnessed a rare event and perhaps solved an evolutionary puzzle about how introns, non-coding sequences of DNA located within genes, ...

DNA dumpster diving

Since the 1960s, it's largely been assumed that most of the DNA in the human genome was junk. It didn't encode proteins—the main activity of our genes— so it was assumed to serve no purpose. But Assistant Professor of ...

Sex, genes, the Y chromosome and the future of men

The Y chromosome, that little chain of genes that determines the sex of humans, is not as tough as you might think. In fact, if we look at the Y chromosome over the course of our evolution we've seen it shrink at an alarming ...

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Junk DNA

In evolutionary biology and molecular biology, junk DNA is a provisional label for the portions of the DNA sequence of a chromosome or a genome for which no function has been identified. The term was introduced in 1972 by Susumu Ohno, but is as of 2008 somewhat outdated, being used mainly in popular science and in a colloquial way in scientific publications. For some sequences once classified as junk DNA, functions have been found, and others are subject to ongoing research. About 95% of the human genome has once 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 junk DNA may 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 appropriate 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). About 80% of the bases in the human genome may be transcribed, but transcription does not necessarily imply function.

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". For example, recent experiments removed 1% of the mouse genome and were unable to detect any effect on the phenotype. This result suggests that the DNA is nonfunctional. However, it remains a possibility that there is some function that the experiments performed on the mice were merely insufficient to detect. This can also be evidence for reconstructing ancestral lineages.

While overall genome size, and by extension the amount of junk DNA, are correlated to organism complexity, there are many exceptions. For example, 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.

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