He considered the frequency of deleterious mutations — harmful changes or breaks to the double helix — our genome acquires over time, along with fertility rates. Because those mutations can be lethal, Graur estimates in a paper in Genome Biology and Evolution that no more than a quarter of our genetic code can be functional — any more and we would accumulate deadly mutations at an unsustainable rate.
Questions and much debate remain around junk DNA. If Graur is right, a vast portion of it might just be scratch pages that protect the useful stuff from mutations. Register or Log In. The Magazine Shop. Login Register Stay Curious Subscribe. Much of our genome has no apparent purpose. Newsletter Sign up for our email newsletter for the latest science news. Sign Up. Already a subscriber? Want more? More From Discover. Recommendations From Our Store. Stay Curious.
View our privacy policy. Why eukaryotes have introns is an open question, but researchers suspect that introns help accelerate gene evolution by making it easier for exons to be reshuffled into new combinations. A large and variable portion of the noncoding DNA in genomes consists of highly repeated sequences of assorted lengths. The telomeres capping the ends of chromosomes, for example, consist largely of these. It seems likely that the repeats help to maintain the integrity of chromosomes the shortening of telomeres through the loss of repeats is linked to aging.
But many of the repeats in cells serve no known purpose, and they can be gained and lost during evolution , seemingly without ill effects. One category of noncoding DNA that intrigues many scientists these days is the pseudogenes , which are usually viewed as the remnants of working genes that were accidentally duplicated and then degraded through mutation. As long as one copy of the original gene works, natural selection may exert little pressure to keep the redundant copy intact.
Akin to broken genes, pseudogenes might seem like quintessential genomic junk. Pseudogenes can also evolve new functions. Sisu notes that the discovery in that the PTENP1 pseudogene had found a second life as an RNA regulating tumor growth convinced many researchers to look more closely at pseudogene junk.
Because dynamic noncoding sequences can produce so many genomic changes, the sequences can be both the engine for the evolution of new genes and the raw material for it. Researchers have found an example of this in the ERVW-1 gene, which encodes a protein essential to the development of the placenta in Old World monkeys, apes and humans. This is hotly debated. However, this conclusion was widely disputed by scientists who pointed out that DNA can be transcribed for many reasons that have nothing to do with biological utility.
Primarily it is the code that guides the binding of the DNA-packaging proteins known as histones. Yes, and in the long stretches of non-coding DNA we see information in excess of mere repeats, tandem repeats and remnants of ancient retroviruses: there is a type of code at the level of preference for the GC pair of chemical DNA bases compared with AT. This reading machine carries a folding machine with it that places a kind of peg at each D, kinking the message by degrees in a plane.
In eukaryotic genomes, the GC sequence bias proposed to be responsible for structural condensation extends into non-coding sequences, some of which have identified activities, though less constrained in sequence than protein-coding DNA.
There it directs their condensation via histone-containing nucleosomes to form chromatin. Analogy between condensation of a word-based message and condensation of genomic DNA in the cell nucleus. Panel A: Information within information, a sequence of words with a variable fourth space which, when filled with particular letters, generates a further message.
One message is read by a three-letter reading machine; the other by a reading machine that can interpret information extending to the 4 th — variable — position of the sequence. This is an analogy for the principle of genomic 3D compression via chromatin, as depicted in panel B: a fluorescence image via Fluorescence In-Situ Hybridization — FISH of the cell nucleus.
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