The Use of Non-Coding Dna in the Human Genome

THE USE OF NON- CODING DNA IN THE HUMAN GENOME

The regions of the human genome that do not have any apparent protein coding function are known as non- coding DNA. Initially, these non- coding parts of the genome were dismissed by the scientific fraternity as useless and thus termed as ‘junk DNA’. However, this view has been changed largely over the years. Perhaps the most important reason for this change is the sheer volume of non- coding DNA that is present in our cells. After the completion of the human genome project it was made clear that about 98% of the DNA in a human cell has no apparent protein coding function. The importance of the non- coding DNA in our cells can be assessed by the fact that the only genomic features that increased in number as animals became more complicated were the regions of non- coding or junk DNA. This observation points out that there is a chance that the key to evolutionary complexity lies with junk DNA.

It is becoming evident that non- coding DNA has many different functions. A portion of the non- coding DNA is responsible for forming specific structures in the chromosomes and thereby preventing our DNA from untying and getting damaged. Two of such important structures formed by the non- coding DNA are the centromeres and the telomeres. Other non- coding regions function as insulations which help in restricting gene expression to specific regions of chromosomes. Telomeres are specific structures formed by a large chunk of non- coding DNA that is present at the end of each of our chromosomes. The length of the telomere gets shortened with each round of replication. In absence of telomeres, the ends of the chromosomes will not be protected and they will become susceptible to exonuclease action or will get attached to free and sticky ends of other chromosomes. Another specialized structure, the centromeres is also extremely important for proper cell division. Centromeres are specific structures that are formed within the chromosomes. These are the structures where the spindle fibers bind and pull individual chromosomes to the two opposite poles of the cell during cell division. Centromeres are formed of non- coding DNA that are tightly packed and bound to a specific protein called CENP-A.

However, a huge amount of the non- coding DNA present in humans is not simply structural. Though these regions do not code for any particular protein, they code for RNAs that are essential for gene regulation and other important cellular functions. In addition to the non- coding DNA which is present in the centre and the terminals of the chromosomes, each functional, protein coding gene also has chunks of non- coding DNA inside them. These regions which do not ultimately appear in the protein coding mRNA are known as introns. These regions are critical for regulating how the messenger RNAs are used by the cells, and create yet another level of control, thereby regulating the amount of protein that is finally produced by the gene. Another possible role for the non- coding regions of the genome can be explained by the insulation theory. The protein coding regions of the genome are extremely important. Any mutations or replication errors in these regions can have deleterious effects as it may lead to production of a faulty protein which can ultimately result in cell death. Having large chunks of non- coding DNA might help in absorbing the mutations that are caused by evolutionary pressure.