How much are we, we? How much of our DNA actually consists of ‘human DNA’ that encodes proteins? What is our excess baggage?
If we would lay our DNA from one cell end by end it would be about 180 cm (6 ft). About 1.5% of all that DNA is coding for functional proteins. The rest of this 98.5% of this DNA is partly for gene regulations and remnants of ages of evolution. Because these pieces of “junk-DNA” are evolutionary remnants they are also found in other organisms. On this basis we could also compare how closely related we are to other organisms.
So what is DNA exactly composed of? We know our DNA has genes that are encoding for protein but these are not one hundred percent efficient. The genes we have encode for introns and exons. The introns are eventually spliced out and are thrown away, thus they do not code for anything. These introns are about 24% of our DNA.
The human chromosomes also has transposable elements (TE) which consists about 45% of our genome. These are pieces that can ‘move about’ in our genome and often originated from viruses.
One example of these TE’s are long interspersed elements (LINEs) and consists of 20% of the genome. Ancient and successful, this element is about six thousand nucleotides long and encodes proteins for transposition. Inside these LINEs there are ALU (Arthrobacter luteus) sequences. This ALU sequence is located within these LINEs and is about 10% of our genome.
Another form of transposable elements are transposons. Transposons are DNA sequences that are present in very large numbers in the chromosomes of nuclear genomes. Not only are they present a lot in genomes but some are able to replicate themselves independent of the rest of the genome and insert themselves on other locations. In plants this is often visible in the colour of flowers or fruits. When, for example, a transposon has shifted from place and nested itself in a gene that was in use. If for example the transposon would nest itself into a gene encoding colour, it would change the appearance of the flower.
About 20% of our genomes is structural materials, mostly as a grappling point to pull the DNA apart during mitosis and meiosis after replication.
And last but not least, repeated sequences. Simple sequence repeats (SSRs) are about 3% of our genome. These SSR’s are simple repeats of -CA- or -TAA- thousands of times. Imagine it like a broken record. There are also duplicated sequences ,like SSR’s but with larger pieces of DNA, which takes about 7% of the total genome. So in total repeated sequences about 10% of the total genome.
So the next time you feel alive and like a good human being, just remember how much of your DNA makes you, you. Afterall, we share about 50% of our DNA with bananas.
Transposon line W138 The Petunia Lab at Radboud University, Nijmegen –
txt.writer.com – Dr. George Johnson on “Very little of the human genome is devoted to being human”.
Barbara McClintock and the Discovery of Jumping Genes (Transposons)