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The Genetic Code: the Clues that Point to Teleology

September 19, 2011

The Genetic Code: the Clues that Point to Teleology

Recently, I have been writing on the subject of irreducible complexity, so now I will…well…take a break from that topic, and instead I will focus my attention on the genetic code. What is the genetic code?

It’s not standard practice to use Wikipedia to define stuff, but I will do precisely that. Wikipedia describes the “genetic code” in this manner:

“The genetic code is the set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins (amino acid sequences) by living cells. The code defines how sequences of three nucleotides, called codons, specify which amino acid will be added next during protein synthesis.”

That’s a pretty neat trick that life uses to make proteins. A molecule of deoxyribonucleic acid (DNA) is transcribed into messenger RNA (mRNA), and mRNA is then translated by machinery of the cell into the final protein product composed of a sequence of amino acids. The fact that life uses coded information to construct protein molecules is certainly suggestive that there was teleology behind the origin of the genetic code. I mean, c’mon, outside of human engineering and biology, nothing else is encoded by information. The formation of ice crystals follow exact chemical and physical laws. The formation of mountains is not encoded. It occurs through physical, geological processes. Now, just because life happens to be built around coded information isn’t proof that the genetic code arose through a teleological mechanism. It is, however, a hint – a suspicious clue – that maybe, just maybe, the genetic code is the product of teleology. If the mere existence of a biological code hints that it may have been designed, perhaps we can look beyond this and look at the properties of the genetic code and see if that strengthens our suspicions even further?

Below is a table (taken from here) that shows what RNA codons code for what amino acids.

 [Note: a glitch in the system prevents the image from appearing in this article]

Note that there are 64 codons and 20 amino acids. The first feature of the genetic code to take note of is that it is redundant. In general, more than one codon codes for an amino acid. In fact, usually about three different codons code for one amino acid. This redundancy helps protect against the effects of a mutation that changes a base pair: if a base pair is changed, often the resulting codon will be a codon that codes for the same ole’ amino acid.

        Something that is particularly intriguing about the genetic code is that the mapping of codons à amino acids does not seem to be arbitrary. Consider the amino acid glycine (Gly), for example. It is coded by four codons: GGU, GGC, GGA, and GGG. Interestingly, the only difference between those codons is the third position. The first two positions are composed of guanine (G) bases. What if the four codons that coded for glycine was arbitrary, such that UUU, GGG, AAU, and CGA coded for glycine? In such a scenario, it is extremely likely that any point mutation to one of the four codons will result in a completely different amino acid. For example, if UUU was mutated to UUG, this would not code for glycine in our hypothetical example. Or if UUU was mutated to UGU, this still would not code for glycine. In fact, it would probably take two simultaneous point mutations in order for there to be a synonymous mutation. To change UUU to a codon that would still code for glycine, we would have to change the UUU to AAU, for example, and that change would require two base pair changes. The pattern displayed by the actual mapping of codons to amino acids deepens our suspicions that the genetic code was designed. Can chance account for this optimal mapping of the genetic code? Not really. While there are numerous hypotheses out there about how the RNA molecule might have arisen through non-teleological processes, none of these hypotheses address the problem of how the genetic code became optimal, and how it acquired such an optimal pattern of mapping of codons to amino acids.

  In fact, a study was conducted on the optimality of the genetic code (Freeland et al., 2000) that concluded:

            “If our definition of biosynthetic restrictions are a good approximation of the possible variation from which the canonical code emerged, then it appears at or very close to a global optimum for error minimization: the best of all possible codes.”

  The mere existence of a code within life provides a hint that that code may have been designed; the fact that the code is extremely optimal strengthens this suspicion. Are there other clues that further point in the direction of teleology? Perhaps, and I will discuss this question in a later article. For the moment, however, I will offer one prediction that, if confirmed, would be yet another clue in favor of a teleological origin of the genetic code.

Consider that some amino acids are encoded by only two codons. On the basis of the hypothesis that life was designed by a rational mind, I predict that the amino acids that are encoded by only two codons are very chemically similar to the amino acids that use similar codons. For example, histidine is encoded by the codons CAU and CAC, while glutamine is encoded by the codons CAA and CAG. Thus, I would predict that these two amino acids are very chemically similar, so that if CAU happened to mutate to CAA, a chemically similar amino acid would replace histidine, and so there would be unlikely that that mutation would be deleterious.

          Do note that this is not a robust prediction: this prediction is made on the lack of my knowledge of whether or not the above prediction is correct. The prediction above may already be a known fact, but I am not aware of that. I am making this prediction mostly just to test my suspicions – to play around with the clues and see what we get. We will see how my “prediction” plays out in a future article.

References:

Freeland, et al. Early Fixation of an Optimal Genetic Code. Molecular Biology and Evolution, 2000, 17:511-518.

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