"Junk" DNA

This week, instead of covering a genetic disorder or mutation, I decided to study "junk" DNA.  Referring to this sort of DNA junk DNA is actually a misnomer.  This DNA is a non-protein-coding variety of DNA that composes up to 95% of our DNA.  While most of the claims as to the function of this hazy topic are largely speculation, studies have been conducted in order to definitively discover the purpose of non-coding DNA.

The very existence of non-coding DNA is an interesting topic to speculate.  Studies in comparative genomics (the study of gene structure and function across several biological species) has revealed that several strands of non-coding DNA have been conserved in the genome for sometimes hundreds of millions of years.  The duration of these non-coding DNA sequences in biological species indicates that these strands of non-coding DNA must have experience significant evolutionary pressures as well as positive selection.  Take a comparative genomic study conducted between humans and mice for example.  The human species shared a common ancestor with mice until about seventy million years ago.  At this point the two species diverged.  Among the DNA that is shared between humans and mice, only about 20% of that DNA is actually coding DNA. () The rest is non-coding DNA that has been conserved in the human genome for millions of years. () Clearly these strands of non-coding DNA must possess a vital function that has allowed them to conquer evolutionary pressures for millions of years.

Recent studies in molecular biology have confirmed one extremely important function of non-coding DNA in biological systems.  Several sequences of non-coding DNA have been identified as cis-regulatory elements.  In other words, there are various strands of junk DNA that are responsible for regulating the expression of the protein-coding genes that surround them.  These cis-regulatory elements are almost always located on the same chromosome as the protein-coding sequences that they influence.  On a basic level, this regulatory elements have the ability to bind with transcription factors that influence gene expression.  These transcription factors are proteins that can either promote or repress the activity of RNA polymerase on specific genes.  (As I covered in my screencast on the central dogma of biology, RNA polymerase is responsible for the creation of mRNA, and subsequently protein creation.)  From there, things start to get a little complex.  There are several types of cis-

regulatory elements which include enhancers, silencers, and insulators.  Enhancers are responsible for positively regulation gene expression, while silencers perform the opposite.  Insulators are slightly more complicated in that they block the interaction between enhancers and promoters.  By doing so the insulators limit the region of genes that an enhancer can influence.  These are all vital functions that facilitate proper gene expression.  The activity of cis-regulatory elements is dictated by two types of inputs: transcription factors, and epigenetic modifications.  The output of these signals is the activity of the cis-regulatory module, which dictates to the transcriptional machinery how fast to manufacture and protein and whether to be turned on or off.

So, it has been confirmed that non-coding DNA actually possesses a very important, functional purpose in regulating gene expression.  But are there any other purposes hidden within the enormous amounts of non-coding DNA in the human genome?  Well, current research being conducted at UCSF is starting to reveal that non-coding DNA actually play a crucial role in brain development.  Dr. Alexander Ramos and his team have discovered a link between long noncoding RNA dubbed lncRNA and the development of pathways in the brain as well as the progressions of certain diseases.  While much of this work is unconfirmed, the research team at UCSF has located a region in the brain called the sub-ventricular zone.  This region of the brain is where neurons are degraded in Huntington's disease.  During the team's research they discovered approximately 2,000 lncRNA molecules located where chemical changes occur in chromosomes associated with this region of the brain.  It is therefore speculated that lncRNA may play a role in the chemical development of the brain by affecting gene expression in those chromosomal regions.

In conclusion, while not much information is currently understood pertaining to non-coding DNA, clearly the genetic element is a key player in biological systems.  As misconceptions continue to become debunked and new theories become developed, non-coding DNA is becoming an undeniably important role in living organisms.

Works Cited:

" Junk DNA - What is Junk DNA? ." THE MEDICAL NEWS | from News-Medical.Net - Latest Medical News and Research from Around the World . N.p., n.d. Web. 7 May 2013. <http://www.news-medical.net/health/Junk-DNA-What-is-Junk-DNA.aspx>.

"lncRNA Expression Analysis | NanoString Technologies." Digital Genomics for Pathway-based Translational Research and Health | NanoString® Technologies. N.p., n.d. Web. 27 May 2013. <http://www.nanostring.com/applications

Norris, Jeffrey. "Brain Development Is Guided by Junk DNA that Isn’t Really Junk." www.ucfs.com. N.p., 15 May 2013. Web. 15 May 2013. <www.ucsf.edu/news/2013/04/105126/brain-development-guided-junk-dna-isn’t-really-junk>.

"More junk DNA than we think, Ewan Birney :: DNA Learning Center." DNA Learning Center. N.p., n.d. Web. 7 May 2013. <http://www.dnalc.org/view/15294-More-junk-DNA-than-we-think-Ewan-Birney.html>.

For more information on non-coding DNA, watch this video: