The complicated truth about “junk DNA”

Imagine human The genome is a string as long as a football field, and all the genes encoding proteins are gathered near the end of the foot. Take two big steps forward; all protein information is now behind you.

There are 3 billion base pairs in the DNA of the human genome, but only about 2% of them encode proteins. The rest seems to be meaningless expansion, a large number of sequence repetitions and dead ends of the genome are usually marked as “junk DNA.” This surprisingly frugal distribution of genetic material is not limited to humans: even many bacteria seem to use 20% of their genome for non-coding fillers.

Many mysteries still revolve around the question of what non-coding DNA is and whether it is really worthless garbage or other things. At least, part of it has extremely important biological significance. But even in addition to the problem of its function (or lack of function), researchers have begun to realize how non-coding DNA becomes a genetic resource for cells and a nursery where new genes can evolve.

“Slowly, slowly, slowly, the term’junk DNA’ [has] Began to die,” said Christina Sisu, A geneticist at Brunel University in London.

As early as the 1960s, scientists casually mentioned “junk DNA”, but they used the term more formally in 1972, when geneticist and evolutionary biologist Oh Ye Jin used it to argue that large genomes inevitably contain The sequence, passively accumulated for many thousands of years, did not encode any protein. Soon thereafter, researchers obtained conclusive evidence of how abundant this junk is in the genome and how diverse its sources are. Despite the lack of protein blueprints, how much is still transcribed into RNA.

Sisu said that advances in sequencing technology, especially in the past two decades, have greatly changed the way scientists think about non-coding DNA and RNA. Although these non-coding sequences do not carry protein information, they are sometimes formed by evolving toward different purposes. Therefore, the functions of various types of “garbage”—as long as they have functions—become more and more clear.

Cells use some of their non-coding DNA to create various RNA molecules that regulate or assist protein production in various ways. The catalog of these molecules keeps expanding, Small nuclear RNA, MicroRNA, Small interfering RNA There are a lot more. Some are short fragments, usually less than two dozen base pairs long, while others are an order of magnitude longer. Some exist in double strands or fold back in the form of hairpin loops. But they can all selectively bind to targets, such as messenger RNA transcripts, to promote or inhibit their translation into proteins.

These RNAs can have a major impact on the health of organisms.For example, the experimental shutdown of certain microRNAs in mice can induce a variety of diseases, including Trembling arrive Liver dysfunction.

To date, the largest non-coding DNA categories in the genomes of humans and many other organisms include Transposon, DNA fragments that can change their position in the genome.These “jumping genes” tend to make many copies of themselves in the entire genome-sometimes hundreds of thousands of copies, say Seth Chisholm, A geneticist at the University of Queensland, Australia.The most prolific is Retrotransposon, It effectively spreads by making its own copy of RNA and converting it back to DNA at another location in the genome.about Half of the human genome is made up of transposons; In some corn plants, this figure climbs to around 90%.

Non-coding DNA also appears in the genes of humans and other eukaryotes (organisms with complex cells), and its intron sequence interrupts the protein-coding exon sequence. When the gene is transcribed, the exon RNA is spliced ​​into mRNA, and most of the intronic RNA is discarded. But some intronic RNA can become small RNA Involved in Protein productionWhy eukaryotes have introns is an open question, but researchers suspect that introns help accelerate genetic evolution by making it easier for exons to recombine into new combinations.

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