What Is Junk DNA?
Junk DNA is a somewhat outdated name for DNA that is not a gene or associated with one.
When it was coined in 1972 by Susumu Ohno, junk DNA referred to the DNA that scientists did not understand. It was believed that at least some of this junk DNA was useful, since it makes up 98 percent of human DNA.
Since then we have learned that junk DNA is not junk because some of it is needed to survive. Junk DNA is now more commonly called noncoding DNA, which are the components of an organism's DNA that do not encode protein sequences.
Junk DNA in Humans
Your DNA has the instructions for making and running you coded in genes. Genes are the two percent of DNA that is not junk. So, what is the other 98 percent of junk DNA?
One way to think about this is to imagine your DNA is a cookbook with the recipes for making you, in the form of genes. This massive cookbook only has two pages of complete recipes -- your genes -- out of each one hundred pages. The other 98 pages are junk DNA that help your body 'read' the recipes at the right time, in the right place, and in the right amounts. They ensure your body doesn't make mistakes.
Some of these 98 pages are old recipes that didn't quite work out, or are old recipes that got copied and turned into new recipes. Some old recipes faded over time and became illegible. There are even some pages with recipes that have viruses inserted in them, and recipes that have inserted themselves into multiple places in the cookbook!
Most of the known functions of junk DNA appear to involve gene regulation. Noncoding DNA sequences can turn a gene on or off, turn it up or down, or even influence which version of a gene is used.
The Function of Junk DNA in Humans
Junk DNA in humans is actually very useful. Some researchers believe 80 percent of junk DNA does something, somewhere in your body. Other researchers suggested that the functional proportion of junk DNA in humans is under 25 percent. While the exact proportion is an active area of research, at least some junk DNA is not junk.
The following are a few examples of different types of DNA, with various functions, that have been lumped into the category of junk DNA.
- Introns: An intron is a non-coding DNA sequence located inside a gene. In DNA, an intron is cut out before the instructions in the gene are carried out (the RNA is translated into a protein). Introns may help control how often a gene is read by turning genes on and off.
- Telomeres: Telomeres are like aglets, those plastic tips on shoelaces. They protect the ends of chromosomes from degradation during the copying of genetic material like aglets keep your shoelaces from fraying. Telomeres may also hold some clues to how humans age and develop cancer.
- Satellite DNA: These are highly repetitive and short sequences of DNA that make up important structural components of chromosomes.
- Non-coding RNA genes: These are sequences of DNA that code for RNA molecules that don't become proteins. Instead, they can become other molecules such as transfer RNAs (tRNAs), which carry amino acids during translation.
- Gene regulatory sequences: These sequences of DNA control the genes by turning them on and off, up or down. Unlike introns, they're not located within genes.
- Pseudogenes: A pseudogene looks like a normal gene, but it's not functional and can't code for a protein. Pseudogenes may regulate the activity of normal genes.
Can You Activate Junk DNA?
Much of non-coding DNA is transcribed into RNA or binds proteins, and whether it is active is dependent on what cells are doing. One area that researchers are studying is how to use junk DNA to treat cancer. For example, researchers are activating transposable elements (TEs), which are DNA sequences that can change positions to create a new mutation or reverse a previous one. These noncoding DNA sequences can also affect human resistance to anti-cancer drugs, so activating them may help treatment by reducing resistance.