I have always been somewhat confused by the manner in which the word “junk” has been attached to the repeating sequences of DNA in our genes which – as far as was known – had no function, and also for high risk securities which offer high returns.
There is a new paper in PLOS Genetics called The Case for Junk DNA (which is a little beyond me) but there is also an eminently readable commentary by Carl Zimmer. My take-aways from Zimmer’s piece are:
- Genomes are the pattern for life.
- Genomes contain genes.
- Genes are made up of DNA.
- Our DNA is a string of units called bases.
- Our cells read the bases in a stretch of DNA–a gene–and build a molecule called RNA with a corresponding sequence.
- The cells then use the RNA as a guide to build a protein.
- Our bodies contain many different proteins, which give them structure and carry out jobs like digesting food.
- The human genome contains about 20,000 protein-coding genes.
- Protein-coding genes only make up about 2 percent of the human genome.
- In the 1950’s the non-coding 98% began being called “junk” genes.
- Functions performed by some of these “junk” gene are constantly being found. The ENCODE project has assigned some bio-chemical function to about 80% of the genome.
- Having large amounts of truly “junk” DNA is a protection against mutation (by making most mutations of the junk portion of no consequence). Evolution requires “junk”. A junk-free genome would be too vulnerable to mutations to survive (mutational meltdown). This suggests that humans need about 90% junk DNA to avoid mutational meltdown.
- Junk portions are also important for evolution since protein-coding genes can evolve from these non-coding regions.
- Much of our genome is made up of viruses, and every now and then, evolution has used those viral genes.
From all of this I come to the layman’s understanding that about 2% of our genome is made up of about 20,000 active protein-coding genes, another 10 – 30% has some active bio-chemical function (such as switching genes on of off), some unknown portion is passive material which could feasibly be activiated and the remainder is the buffer material which both provides protection from rampant mutation and provides a pool resource for future evolution.
Junk bonds are risky investments, but have speculative appeal because they offer much higher yields than safer bonds. Companies that issue junk bonds typically have less-than-stellar credit ratings, and investors demand these higher yields as compensation for the risk of investing in them. A junk bond issued from a company that manages to turn its performance around for the better and has its credit rating upgraded will generally have a substantial price appreciation.
Now as it becomes clear that not all sections of non-protein-coding DNA are entirely useless, I begin to see an analogy between “junk DNA” and “junk bonds”. A high – but manageable – risk but giving high yield on the one hand and a high – but manageable – genetic redundancy giving high evolutionary appreciation on the other.
Junk DNA is the genome’s hedging instrument.
Vertical species evolution (rather than horizontal evolution for mere survival) is then probably dependent upon achieving some optimum balance between genome size, coding DNA and junk DNA.
This is my attempt to apply a similar description to junk DNA,
Junk DNA are the genome’s hedge instruments and have evolutionary appeal because they offer a much wider range of evolutionary possibilities. Species that build up massive genomes with very high levels of junk DNA typically lie lower on the evolutionary hierarchy and evolve horizontally rather than vertically. When junk DNA in a species high up on the vertical scale (mammals) achieves a balance with the coding genes and the size of the genome, the species will have its rating upgraded and will generally have a substantial evolutionary appreciation.
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