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 98.5% 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 may function in ways that are not currently understood. In fact, recent studies have suggested functions for certain portions of what has been called junk DNA. Moreover, the conservation of some "junk" DNA over millions of years of evolution may imply an essential function. The "junk" label is therefore recognized as something of a misnomer, and many would prefer the more neutral term "noncoding DNA".

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".

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" or, more conventionally, the "C-value paradox" (7).

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Hypotheses of origin and function

There are many hypotheses, none conclusively established, for how junk DNA arose and why it persists in the genome:

  • These chromosomal regions could be the remains of ancient pseudogenes, which have been cast aside and fragmented during evolution. A related hypothesis suggests that the junk represents the accumulated DNA of retroviruses.
  • Junk DNA may act as a protective buffer against genetic damage and harmful mutations. For example, a high proportion of nonfunctional sequence makes it unlikely that a functional element will be destroyed in a chromosomal crossover event, possibly making a species more tolerant to this important mechanism of genetic recombination.
  • Junk DNA might provide a reservoir of sequences from which potentially advantageous new genes can emerge. In this way, it may be an important genetic basis for evolution.
  • Some junk DNA could simply be spacer material that allows enzyme complexes to form around functional elements more easily. In this way, the junk DNA could serve an important function even though the actual sequence information it contains is irrelevant.
  • Some portions of junk DNA could serve presently unknown regulatory functions, controlling the expression of certain genes and/or the development of an organism from embryo to adult.
  • Junk DNA may serve other, unknown purposes. For example, some non-coding RNAs have been discovered in what had been considered junk.
  • Junk DNA may have no function. For example, recent experiments removed 1% of the mouse genome and were unable to detect any effect on the phenotype³. This result suggests that the DNA is, in fact, non-functional. However, it remains a possibility that there is some function that the experiments performed on the mice was merely insufficient to detect.
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Evolutionary conservation of "junk" DNA

Comparative genomics is a promising direction in studying the function of junk DNA. Biologically functional sequences, as the theory goes, tend to undergo mutation at a slower rate than nonfunctional sequence, since mutations in these sequences are likely to be selected against. For example, the coding sequence of a human protein-coding gene is typically about 80% identical to its mouse ortholog, while their genomes as a whole are much more widely diverged. Analyzing the patterns of conservation between the genomes of different species can suggest which sequences are functional, or at least which functional sequences are shared by those species. Functional elements stand out in such analyses as having diverged less than the surrounding sequence.

Comparative studies of several mammalian genomes suggest that approximately 5% of the human genome has evolved under purifying selection5 since the divergence of the mammals. Since known functional sequence comprises less than 2% of the human genome, it appears that there may be more functional "junk" DNA in the human genome than there is known functional sequence.

A surprising recent finding was the discovery of nearly 500 ultraconserved elements², which are shared at extraordinarily high fidelity among the available vertebrate genomes, in what had previously been designated as junk DNA. The function of these sequences is currently under intense scrutiny, and there are preliminary indications2,6,9 that some may play a regulatory role in vertebrate development from embryo to adult.

It must be noted that all present results concerning evolutionarily conserved human "junk" DNA are expressed in highly preliminary, probabilistic terms, since only a handful of related genomes are available. As more vertebrate, and especially mammalian, genomes are sequenced, scientists will develop a clearer picture of this important class of sequence. However, it is always possible, though highly unlikely, that there are significant quantities of functional human DNA that are not shared among these species, and which would thus not be revealed by these studies.

On a theoretical note, it is often observed that the presence of high proportions of truly nonfunctional "junk" DNA would seem to defy evolutionary logic. Replication of such a large amount of useless information each time a cell divides would waste energy. Organisms with less nonfunctional DNA would thus enjoy a selective advantage, and over an evolutionary time scale, nonfunctional DNA would tend to be eliminated. If one assumes that most junk DNA is indeed nonfunctional, then there are several hypotheses for why it has not been eliminated by evolution: (1) the energy required to replicate even large amounts of nonfunctional DNA is in fact relatively insignificant on the cellular or organismal scale, so no selective pressure results; (2) the aforementioned possible advantage of having extra DNA as a reservoir of potentially useful sequences; and (3) retroviral or transposon insertions of nonfunctional sequence occurring faster than evolution can eliminate it. These are all hypotheses for which the time scales involved in evolution may make it difficult for humans to rigorously investigate.

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Creation-evolution controversy

The patterns of mutation and rearrangement in nonfunctional sequence analyzed in comparative genomics studies provide strong scientific evidence for common descent, since these patterns tend to reflect the phylogenetic tree. Hence, the question of whether junk DNA is really junk has played a minor role in the creation-evolution controversy. Some advocates of creationism and intelligent design (except for proponents of Theistic evolution) contend that no DNA is junk, or that such junk DNA demonstrates only deterioration rather than macroevolution [1]. Another claim made by creationists is that the theory of evolution caused scientists to assume most DNA was functionless, stifling research into the functions of junk DNA [2].

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References

  1. Gibbs W.W. (2003) "The unseen genome: gems among the junk", Scientific American, 289(5): 46-53. (A review, written for non-specialists, of recent discoveries of function within junk DNA.)
  2. G. Bejerano et al. "Ultraconserved Elements in the Human Genome". Science 304:1321-1325, May 2004. Discussed in "'Junk' DNA reveals vital role", Nature (2004).
  3. M.A. Nobrega, Y. Zhu, I. Plajzer-Frick, V. Afzal and E.M. Rubin (2004) "Megabase deletions of gene deserts result in viable mice", Nature, 431: 988-993.
  4. Mattick, John S. (2004) "The Hidden Layer of Noncoding RNA: a Digital Control System Underpinning Mammalian Development and Diversity", HGM Symposium 2004 Session 4/16.
  5. Mouse Genome Sequencing Consortium. "Initial sequencing and comparative analysis of the mouse genome". Nature 420:520-562, December 2002.
  6. Woolfe, A., et. al. "Highly conserved non-coding sequences are associated with vertebrate development". PLoS Biology 3:e7, January 2005.
  7. Wahls, W.P., et. al. "Hypervariable minisatellite DNA is a hotspot for homologous recombination in human cells" Cell 60(1):95-103 (1990).
  8. Gregory, T.R. (Ed.) (2005) The Evolution of the Genome, Elsevier, San Diego.
  9. Sandelin, A., et. al. "Arrays of ultraconserved elements span the loci of key development genes in vertebrate genomes". BMC Genomics 5(1):99, December 2004.


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See also

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External links