In genetics, the term “junk DNA” refers to regions of DNA that are non-coding. DNA contains instructions (coding) that are used to create proteins in the cell. However, the amount of DNA contained inside each cell is vast and not all of the genetic sequences present within a DNA molecule actually code for a protein. In fact, over 98% of the human genome is non-coding DNA.
More recently, biologists have produced a stunning inventory of previously hidden switches, signals and sign posts embedded throughout the entire length of human DNA which are helping them better understand human inheritance and disease. While our protein-coding genes change very slowly over time, the non-coding elements of our DNA turn over much faster. In fact, though we share many of our protein-coding genes with other species (e.g., mice), the regulatory part of our genome is much more complex.
The Encyclopedia of DNA Elements (ENCODE) - a consortium to build a
comprehensive parts list of functional elements in the human genome.
It appears that non-coding DNA can be broken down into 3 components:
- Regulatory regions for genes
- Areas that code for RNA molecules that are used by our cells for functions other than coding for proteins
- Non-functional DNA
Our genes operate in a complex regulatory network. A significant amount of our non-coding DNA has do with gene regulation–that is, controlling the expression or activation of genes. They act like switches, turning genes on or off; allowing them to code for proteins or deactivating them altogether.
Over the history of our species, retroviruses have managed to set up housekeeping in our cells and propagate when they infect a human egg. Over time, these retroviruses get inactivated by mutations and are relegated to a non-functional status. This deactivated viral DNA can later become the spawning ground for new coding genes.
Non-functional DNA also serves as a buffer against harmful mutations. Studies have shown that we are generally born with anywhere from 50-175 mutations in our DNA. The fact that so much of our DNA is non-coding means that these mutations likely will occur in an area where it does no harm.
As scientists have accumulated large databases of human genomes and studied the variations in them, they have discovered that the areas of the genome linked to many diseases occur in these non-coding regions. A team of researchers recently linked variation in a non-coding region of DNA to breast cancer, prostate cancer and brain tumors. For example, they found a single DNA letter change that seems to have great impact on the development of breast cancer. This single letter change occurs in an ultra-sensitive region that is central to a network of many related genes. Such discoveries illustrate that figuring out what is going on with our “junk DNA” may lead to a much better understanding of disease processes and their genetic underpinnings.