Episode 14: Junk DNA Luyện thi TOEIC

Episode 14: Junk DNA Luyện thi TOEIC Mobile Luyện thi TOEIC, B1, B2: 0974 459 158 - 0918 533 316,

Although the term “junk DNA” was already in use as early as the 1960s [10]–[12], the term's origin is usually attributed to Susumu Ohno [13]. As Ohno pointed out, gene duplication can alleviate the constraint imposed by natural selection on changes to important gene regions by allowing one copy to maintain the original function as the other undergoes mutation. Rarely, these mutations will turn out to be beneficial, and a new gene may arise (“neofunctionalization”) [14]. Most of the time, however, one copy sustains a mutation that eliminates its ability to encode a functional protein, turning it into a pseudogene. These sequences are what Ohno initially referred to as “junk” [13], although the term was quickly extended to include many types of noncoding DNA [15]. Today, “junk DNA” is often used in the broad sense of referring to any DNA sequence that does not play a functional role in development, physiology, or some other organism-level capacity. This broader sense of the term is at the centre of most current debate about the quantity—or even the existence—of “junk DNA” in the genomes of humans and other organisms.

It has now become something of a cliché to begin both media stories and journal articles with the simplistic claim that most or all noncoding DNA was “long dismissed as useless junk.” The implication, of course, is that current research is revealing function in much of the supposed junk that was unwisely ignored as biologically uninteresting by past investigators. Yet, it is simply not true that potential functions for noncoding DNA were ignored until recently. In fact, various early commenters considered the notion that large swaths of the genome were nonfunctional to be “repugnant” [10], [16], and possible functions were discussed each time a new type of nonprotein-coding sequence was identified (including pseudogenes, transposable elements, satellite DNA, and introns; for a compilation of relevant literature, see [17]).

Importantly, the concept of junk DNA was not based on ignorance about genomes. On the contrary, the term reflected known details about genome size variability, the mechanism of gene duplication and mutational degradation, and population genetics theory. Moreover, each of these observations and theoretical considerations remains valid. In this review, we examine several lines of evidence—both empirical and conceptual—that support the notion that a substantial percentage of the DNA in many eukaryotic genomes lacks an organism-level function and that the junk DNA concept remains viable post-ENCODE.

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