Less than five percent of our genome is responsible for synthesizing proteins. This five percent has long been attributed with determining our physical appearance while the rest was thought to be useless. In 1972, Susumu Ohno coined the term ‘junk DNA’ to categorize this majority, but this title is quickly losing its applicability. Junk DNA no longer seems to be junk.
Before researchers began uncovering the secrets of junk DNA, one of the most popular hypotheses likened its persistence in our genome to the process of natural selection. Some genes won the evolutionary race while others died out. These dead genes constitute junk DNA. They’re essentially a vestigial structure.
Wisdom teeth are an example of vestigial structures to which most can relate. This extra set of molars developed in early humans with the purpose of allowing them to chew rougher foods. With the invention of cutlery, humans gained the ability to soften food before ingesting it; thus, wisdom teeth lack the same necessity they once had. Our DNA retains the sequence to form wisdom teeth regardless of their uselessness, making them vestigial or lingering; we don’t use the wisdom teeth, yet they are there. This wouldn’t constitute junk DNA because of the fact that protein synthesis still takes place, but junk DNA is classified similarly.
Rather than the structure produced by the DNA being vestigial, the DNA was thought to be vestigial in and of itself. Just like wisdom teeth, the sequencing lost its function. Perhaps it had some use for a distant ancestor, but holds no bearing in the current organism. Therefore, it’s essentially junk: taking up space while providing no benefit. They are dead genes.
Though, simply occupying space wasn’t enough to force creatures to get rid of the junk DNA. It instead remained inside the genome and compounded over time. Our bodies just skip over it when making proteins, so erasing it would take more evolutionary effort than it’s worth.
This was largely the thought process behind the term ‘junk DNA’ being coined. Admittedly, there may be some DNA in our genome that fits this description, but scientists are beginning to doubt its existence. Not knowing the functionality of a gene doesn’t prove it lacks one. This junk DNA exhibits strangely repetitive qualities. If it was truly junk, we likely wouldn’t see such a level of organization.
Additionally, researchers have established that junk DNA evolves significantly faster than sequences that synthesize proteins. This recently discovered behavior doesn’t make sense under the pretext that junk DNA is an evolutionary remnant. Since it changes even faster than encoding DNA, it lacks the stagnation necessary to be called vestigial.
Scientists believe that junk DNA might instead have some bearing in speciation. Bananas and humans share around 50 percent of their DNA, but are wildly different organisms. Chimpanzees are even more closely related, sharing over 98 percent of their genome with humans. Still, they are very distinct creatures. Could two percent, or even 50 percent in the case of bananas, cause such a variance in phenotype?
In reality, these percentages ignore all junk DNA within the organisms. Humans, chimpanzees, and bananas have vastly different sequences of junk DNA. Due to this, researchers are exploring the possibility that said junk DNA is responsible for such large divergences.
The term ‘junk DNA’ has also been inappropriately adopted to encompass all forms of noncoding DNA: DNA that doesn’t produce proteins. Some types of noncoding DNA already have proven purposes such as making ribonucleic acid or RNA.
Due to its misleading connotation and the general disapproval towards it in the scientific community, the term ‘junk DNA’ is ready to be thrown away.
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