Junk DNA goodbye: a review article (Introduction)

by David Turell , Thursday, September 02, 2021, 01:59 (968 days ago) @ David Turell

DNA seems to have function in most places:

https://www.quantamagazine.org/the-complex-truth-about-junk-dna-20210901/

"Many mysteries still surround the issue of what noncoding DNA is, and whether it really is worthless junk or something more. Portions of it, at least, have turned out to be vitally important biologically. But even beyond the question of its functionality (or lack of it), researchers are beginning to appreciate how noncoding DNA can be a genetic resource for cells and a nursery where new genes can evolve.

"Abstractions navigates promising ideas in science and mathematics. Journey with us and join the conversation. “Slowly, slowly, slowly, the terminology of ‘junk DNA’ [has] started to die,” said Cristina Sisu, a geneticist at Brunel University London.

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"Cells use some of their noncoding DNA to create a diverse menagerie of RNA molecules that regulate or assist with protein production in various ways. The catalog of these molecules keeps expanding, with small nuclear RNAs, microRNAs, small interfering RNAs and many more. Some are short segments, typically less than two dozen base pairs long, while others are an order of magnitude longer. Some exist as double strands or fold back on themselves in hairpin loops. But all of them can bind selectively to a target, such as a messenger RNA transcript, to either promote or inhibit its translation into protein.

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"By far the biggest category of noncoding DNA in the genomes of humans and many other organisms consists of transposons, segments of DNA that can change their location within a genome. These “jumping genes” have a propensity to make many copies of themselves — sometimes hundreds of thousands — throughout the genome, says Seth Cheetham, a geneticist at the University of Queensland in Australia. Most prolific are the retrotransposons, which spread efficiently by making RNA copies of themselves that convert back into DNA at another place in the genome. About half of the human genome is made up of transposons; in some maize plants, that figure climbs to about 90%.

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"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. But Cheetham warns that some pseudogenes may not be “pseudo” at all. Many of them, he says, were presumed to be defective copies of recognized genes and labeled as pseudogenes without experimental evidence that they weren’t functional.

"Pseudogenes can also evolve new functions. “Sometimes they can actually control the activity of the gene from which they were copied,” Cheetham said, if their RNA is similar enough to that of the working gene to interact with it. Sisu notes that the discovery in 2010 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.

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"But how much of this DNA therefore qualifies as true “junk” in the sense that it serves no useful purpose for a cell? This is hotly debated. In 2012, the Encyclopedia of DNA Elements (Encode) research project announced its findings that about 80% of the human genome seemed to be transcribed or otherwise biochemically active and might therefore be functional. 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.

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"In the future, researchers may be less and less inclined to describe any of the noncoding sequences as junk because there are so many other more precise ways of labeling them now. For Sisu, the field’s best way forward is to keep an open mind when assessing the eccentricities of noncoding DNA and RNA and their biological importance. People should “take a step back and realize that one person’s trash is another person’s treasure,” she said."

Comment: This is a carefully drawn article, but as I have presented here most non-coding DNA has some alternative function so the 80% estimate of function from ENCODE is reasonable. The argument about the importance pf 'junk' is that chance mutations in evolution should produce lots of junk. That is not true from this evidence, so that suggests DNA may be designed as I believe.