“Junk DNA” is a term historically attributed to non-protein coding DNA, which many experts previously assumed to be genetically unimportant. An ongoing research initiative has suggested this is not the case, sparking calls for the term “junk DNA” to be retired. We take a look at the facts.
What is junk DNA?
The human genome consists of 3.42 billion nucleotides neatly packaged in 23 pairs of chromosomes. The human genome is comparable in size with that of most mammals, with cows carrying 3.65 billion nucleotides and mice 3.45 billion.
Some creatures, however, have been found to carry significantly more nucleotides, with the genome of the red vizcacha rat featuring 8.21 billion and the marbled lungfish carrying a staggering 130 billion.
DNA is crucial to protein synthesis, but not all DNA codes for proteins. In fact, scientists have known for some time of the existence of “selfish DNA,” apparently existing for itself without contributing to the fitness of the organism it forms part of. In fact, about 98% of human DNA is noncoding in nature.
The geneticist Susumu Ohno coined the phrase “junk DNA.”
He first used the phrase in 1972 to describe all DNA that does not code for proteins. Ohno noted that these noncoding genes appeared in repeating segments that occurred randomly throughout the genome. He and many leading scientists of the time deemed them to be of little scientific significance.
Mainstream scientific researchers showed little interest in junk DNA over the years that followed. Indeed, geneticists deemed noncoding DNA to be of low scientific value, up until the 1990s. It is only relatively recently that biologists have begun to understand the potential significance of non-coding DNA and its interactions with the surrounding genomic environment. Scientists now believe these genetic sequences are intrinsic to human evolution, increasing the ability of the organism to genetically adapt and providing important signals to regulate gene expression.
The human genome is incredibly dynamic, evolving new functional elements while allowing others to die out. Scientists now believe that “junk” DNA is capable of evolving into protein-coding DNA. The late biologist Stephen Jay Gould collaborated with paleontologist Elisabeth Vrba, coining the phrase “exaptation” to describe how different genomic elements may assume new roles. Scientists are still unravelling the riddle of noncoding DNA.
A major breakthrough dispelled the theory that noncoding DNA has no value.
The Encode Project, an international collaboration between some of the world’s leading geneticists, discovered that within our junk DNA are more than 10,000 genes that actually code for certain elements that control our protein-coding DNA. The project found that about 18% of our DNA regulates that 2% of DNA that codes for proteins. This represents the most significant shift in scientific understanding of the function of DNA since the human genome was first sequenced at the dawn of the 21st century.
The results of the Encode Project have had a huge impact on the world of genomics, changing scientific understanding of how genes operate. The findings are of particular interest to those researching treatments for genetic diseases like Crohn’s disease, diabetes, and heart disease, which are partially caused by genetic anomalies.
Dr. Ewan Birney, associate director of the European Bioinformatics Institute, explained in a 2012 interview with Britain’s Guardian newspaper that since the Human Genome Project was completed in 2003, scientific interest in junk DNA has increased, with many suspecting that non-protein coding DNA plays a bigger role than first assumed.
While virtually all of the cells within the human body contain a copy of our genome, the unique genetic blueprint for making a person, not all protein-coding DNA is utilized in every cell at any given time. Protein-coding is site-specific. For example, hair cells utilize the genes that make keratin, while bone-marrow cells use those that code for hemoglobin. The bone marrow cell knows it needs to generate red blood cells rather than hair proteins, and the hair cell knows it needs to produce keratin rather than hemoglobin.
Scientists now suspect that this control or regulatory function lies within the part of our DNA previously written off as mere “junk.”
The Encode Project changed scientific attitudes about noncoding DNA.
A series of papers published in Nature revealed the findings of the 400-strong team of leading scientists at the conclusion of the Encode Project. Based on these papers, researchers believe around 20% of our genes perform regulatory functions, though a few suspect that figure may be as high as 50%.
Many leading scientists are now calling for the retirement of the term “junk DNA” as a throwaway phrase that effectively diverted attention from an extraordinarily valuable aspect of modern genomics. Whereas geneticists previously placed sole value on the 2% of genes that could directly generate proteins, today scientists are coming to understand the value of regulatory genes in evolutionary terms.
The human genome is incredibly complex. The Encode Project has been hailed as a major advancement in terms of promoting understanding of the role of non-coding genes, dispelling assumptions and providing vital insight into this long-overlooked area of the genome.