In genetics, the term junk DNA refers to regions of DNA that are non-coding. Some of this noncoding DNA is used to produce noncoding RNA components such as transfer RNA, regulatory RNA and ribosomal RNA.
However, other regions are not transcribed into proteins, nor are they used to produce RNA molecules and their function is unknown. The importance of these non-coding regions has been a key focus in genetic research for many years, particularly in terms of evolution.
Comparative genomics studies have shown that some of this junk DNA is highly conserved, sometimes over hundreds of millions of years, which suggests that these noncoding regions have undergone positive selection throughout evolution and confer an advantage in terms of preserving or minimizing certain biological traits.
For example, the human genome and the mice genome diverged from a common ancestor around 65 to 75 million years ago. However, the protein-coding DNA only makes up about 20% of the conserved DNA in these genomes, with the remainder of the conserved DNA found in noncoding regions.
Researchers have also found that some sequences in the junk DNA act as genetic “switches,” which determine where and when genes get expressed.
One comparative study that looked at more than 300 prokaryotic and 30 eukaryotic genomes estimated the minimum amount of noncoding DNA that a eukaryotic organism requires. In humans, the estimated minimum amount was around 5% of the whole genome.
Some regions of the noncoding DNA may also be essential for chromosome structure, the function of centromeres and play a role in cell division (meiosis). Some noncoding DNA sequences also determine the location where transcription factors can attach and control transcription of the genetic code from DNA to mRNA.