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Non-coding DNA can be helpful in generating useful mutations that can go on to become new features/functionality of an organism. Non-coding DNA also indirectly reduces chances of mutation of useful/functional genes.

So if an organism has a larger proportion of non coding DNA it'll be safer from mutation. Humans have 80%-90% non-coding DNA. But this number is far lesser in many other species especially plants. Some plants have as few as only 3% non-coding DNA.

My question is, do these species with less non-coding DNA undergo evolution faster? Because their functional genes are more susceptible to mutations (which would be disadvantageous on average I guess). Would it be fair to say that evolution tries to keep expanding this proportion of buffer DNA? Lastly, is my question related to C-value paradox?


My assumption that non-coding DNA indirectly offers adaptive advantage (evolutionarily speaking) seems to be questioned. Let's consider the following case:

Let A and B be two genomes of equal size. Let A have 80 % non-coding DNA and 20% coding DNA. And let B have the opposite i.e., 20% non-coding DNA and 80% coding DNA. Let both A and B be exposed to the same amount of UV radiation for the same amount of time. All conditions being the same, let's assume that this causes the same amount of mutation in both A and B, say 10%. Now what is the probability that this mutation happened in non-coding DNA (which means it doesn't harm the organism) in each of A and B? Evidently, the probability is higher in A meaning it's safer from mutation caused due to UV radiation. So A has an advantage of surviving over B.

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  • $\begingroup$ Humans have 80%-90% non-coding DNA Wasn't it 98% as per as my limited knowledge is concerned? $\endgroup$ – Failed Scientist Aug 28 '18 at 15:52
  • $\begingroup$ @FailedScientist 98% was the popular understanding which is now changing due to broadening of the definition of what is 'functional' part of DNA sequence. $\endgroup$ – yathish Aug 28 '18 at 16:03
  • $\begingroup$ “Non-coding DNA also indirectly reduces chances of mutation of useful/functional genes” — It does?! How? The rate of mutation doesn’t change because of the size of the genome. $\endgroup$ – Konrad Rudolph Aug 29 '18 at 10:02
  • $\begingroup$ @yathish “functional” ≠ “coding”. Regardless of the definition of “functional”, the percentage of coding (= protein-coding) DNA has barely changed. $\endgroup$ – Konrad Rudolph Aug 29 '18 at 10:03
  • $\begingroup$ @KonradRudolph Let's assume mutation is bad and it can happen anywhere on the genome with equal probability. If non-coding DNA is 95% of the genome then the probability of mutation happening over coding DNA is only 5%. I understand functional!=coding.. it's a broader definition. $\endgroup$ – yathish Aug 29 '18 at 10:07
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Whether or not species with less non-coding DNA evolve faster, the assumption that this could be explained by their genomes being more susceptible to mutation would seem to me to be based on a false premise.

Let us examine the logic of the assumption by making an analogy to drawing of tickets from an urn in a lottery. If you buy one ticket (coding region), and only one ticket is drawn (there is a single mutation) from the urn (the whole genome), the probability of winning (your gene suffering a mutation) decreases as the number of tickets (the size of the genome) increases.

So the question is “does mutation follow this model?”, i.e. are the number of mutations in a genome independent of its size? To answer this, let us look at two examples of known mechanism of mutation.

  • Misreading during DNA replication: Here (and in other types of misreading) the error frequency is, quite logically, related to the number of bases replicated i.e. it has a specific (if very low) value of n per 1,000,000 basesnot n per genome.
  • UV radiation: Here the source of the radiation is external and its flux will be proportional to the area it strikes — hence the larger the target area the more interactions with thymine causing dimerization. Although our linear-centric view of DNA tends to ignore its area, the larger the genome, the larger the target. The only way I can see the non-coding DNA buffering the coding DNA from radiation is if it somehow formed a protective shell around the latter. I know of no evidence that this is so.
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  • $\begingroup$ It seems my question is unclear. May I request you to read a portion I added in the question as an edit? Assuming mutation is harmful on an average, my argument is that having more non-coding DNA in a way shields the coding DNA from mutation. There doesn't need to be physical protection - I'm only talking in terms of probabilities (averages). Note that I've also assumed the size of genomes under comparison to be same. $\endgroup$ – yathish Aug 29 '18 at 10:19
  • $\begingroup$ @yathish — Again, I question your assumptions. I do not think it valid to compare two genomes of equal size, or if you do you are asking a different question. You clearly talk about increasing the amount of non-coding DNA, not the coding DNA. For this you have to compare two sets of coding genes of equal size, but associated with different amount of non-coding DNA — something like Drosophila, with a genome with a similar number of genes (ca. 18,000) but more non-coding DNA. And for that my argument holds. $\endgroup$ – David Aug 29 '18 at 10:32
  • $\begingroup$ I was definitely assuming the size of genome to be immaterial. I wanted to highlight the PROPORTION of non-functional DNA and not the amount of it. Would my argument hold for species of comparable genome sizes but with widely varying proportions of coding to non-coding DNA? $\endgroup$ – yathish Aug 29 '18 at 10:43

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