The complex truth about “unwanted DNA”


Imagine the man a genome like a string stretching the length of a football field, with all the genes that encode proteins grouped at the end near your feet. Take two big steps forward; all the information about proteins is already behind you.

The human genome has three billion base pairs in its DNA, but only about 2% of them encode proteins. The rest seems like pointless swelling, an abundance of duplication of sequences, and genomic dead ends, often called “unwanted DNA.” This stunningly economical distribution of genetic material is not limited to humans: Even many bacteria seem to secrete 20% of their genome into a non-coding filler.

Many mysteries still surround the question of what non-coding DNA is and whether it is really useless garbage or something more. The smallest parts of it turned out to be biologically vital. But even beyond the question of its functionality (or lack thereof), researchers are beginning to assess how noncoding DNA can be a genetic resource for cells and a breeding ground where new genes can develop.

“Slow, slow, slow” junk DNA terminology [has] he started dying, “said Christina Sisu, a geneticist at Brunel University in London.

Scientists carelessly mentioned “unwanted DNA” as early as the 1960s, but they adopted the term more formally in 1972, when geneticist and evolutionary biologist Susumu Ono used it to claim that large genomes would inevitably carry sequences. , passively accumulated over many millennia that do not encode any proteins. Soon after, researchers gained solid evidence of how much of this garbage is in the genomes, how diverse its origins are, and how much of it is transcribed into RNA, even though protein drawings are missing.

Technological advances in sequencing, especially over the past two decades, have done much to change the way scientists think about noncoding DNA and RNA, Sisu said. Although these non-coding sequences do not carry protein information, they are sometimes formed by evolution to different targets. As a result, the functions of the different classes of “junk” – insofar as they have functions – become clearer.

Cells use part of their non-coding DNA to create a diverse menagerie of RNA molecules that regulate or support the production of proteins in various ways. The catalog of these molecules continues to expand with small nuclear RNA, microRNA, small interfering RNA and many more. Some are short segments, usually less than two dozen base pairs, while others are an order of magnitude longer. Some exist as double strands or fold on themselves into hairpins. However, all of them can selectively bind to a target, such as a messenger RNA transcript, to stimulate or inhibit its translation into a protein.

These RNAs can have a significant impact on the well-being of the body. Experimental exclusions of certain miRNAs in mice, for example, have caused disorders ranging from tremor to liver dysfunction.

The largest category of non-coding DNA in the genomes of humans and many other organisms consists of transposons, segments of DNA that can change their location in the genome. These “jumping genes” tend to make many copies of themselves – sometimes hundreds of thousands – throughout the genome, says Seth Cheatham, a geneticist at the University of Queensland in Australia. The most prolific are the retrotransposons, which propagate efficiently by making their own RNA copies, which are converted back into DNA elsewhere in the genome. About half of the human genome consists of transposons; in some maize plants, this figure rises to about 90 percent.

Non-coding DNA also appears in the genes of humans and other eukaryotes (complex cell organisms) in intron sequences that interrupt protein-coding exon sequences. When genes are transcribed, exon RNA binds together into mRNA, while much of the intron RNA is discarded. But some of the intron RNA can turn into small RNAs that are involved in protein production. Why eukaryotes have introns is an open question, but researchers suspect that introns help accelerate the evolution of genes by making it easier to redirect exons to new combinations.



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