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I was doing biology today and we mentioned the fact that DNA is degenerate[1], but it struck me that there is no practical reason DNA couldn’t have more than four bases; in fact, it could pretty much come in any multiple of two. There’s no particular reason for it to be anything more than four on Earth though, as four bases provide all the redundancy you need in your code and any more would just be a waste of resources.

I however have my head in the sky and it struck me that one of the biggest things degenerate coding protects against is radiation exposure. So, say we had life developing near a very hot star; the cosmic rays would be significantly more powerful from our local sun and a lot more radiation would reach the surface. This means that mutation due to radiation is now a significant consideration and a species, not an individual, that can withstand constant irradiation without major gamete damage will be able to pass on their genes far more effectively.

In this situation greater redundancy would be a massive advantage so, and let me just whip out my calculator so we can see how much of an advantage:

On Earth we have four bases and three codons. That’s 43 = 64 combinations.

And we have 20 amino acids and 2 directional codons[2]. So 64/22 = 2.9 codes for each instruction.

Let’s round a little and say our safety factor is 3, so for every codon there are two different codes that will give the same result[3].

With a standard mutation changing one character that means there is a 2/9[4] (22%) chance that, once a mutation occurs, the new codon will code for the same amino acid.

Now, let’s try the same thing with six bases. 63 = 216 combinations.

The same number of objects too code for, just for arguments sake. So 216/22 = 9.8 unique codes.

Rounding again, that’s a safety factor of ten and so for each codon there are nine other codons which will give the same result.

Introduce a standard mutation and then there will be a 9/15[5] (60%) chance that the amino acid will stay the same, a marked improvement.

There is some hand waving here, as the codons are not going to be divided this simply, and substitution mutation is not the only form of mutation, or in fact always caused by radiation. But still this suggests that an alien evolving in a high radiation environment may well not have the same number of base pairs in their DNA.

Interestingly, this formula infers the more base pairs you have the more protected you are against random mutations, but you have to consider this may not be an advantage in evolutionary terms as eventually you would reach a point where no single mutation would have any effect and the species would effectively stagnate. Perhaps that is why we have four bases, so that we have a beneficial level of mutation, or perhaps the bases were established at a point where mutations were much less of an issue and they have persisted ever since.

Anyway, it’s something interesting to mull over at least.

Now I’ve just got to figure out how to get different base numbers into the books…

[1] That means multiple sequences code for the same thing for any of you non-biologists out there.

[2] Stop and Start.

[3] I know this isn’t quite right as some are coded by more than three and some less, but that’s not the point here.

[4] Each base could be changed into any other of the four, but only one base can be changed. So that’s a total of nine as we’re only varying one at a time and each has three possibilities.

[5] Each base could be changed into any other of the six. So that’s five and five and five. Fifteen.


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