I used that title because 1) this might have happened before RNA and certainly did before DNA and 2) can something be genetic if it is deterministic chemistry?
http://ift.tt/1LuVzFI
This paper is old news but I was reminded of the contents by page 136 of Nick Lane's "The Vital Question". He refers to the DNA chapter of "Life Ascending" where he discussed this work, but then only points out that it assumes a constant supply of precursor chemicals which an alkaline hydro-thermal vent system would have supplied.
The following may be the most succinct statement I've seen of why fourteen of the current amino acids likely made up the first code:
****
Mapping of the Modern Genetic Code onto a Simpler Doublet Code
The 20 amino acids encoded by the modern code do not saturate the bound for the number of amino acids that can be unambiguously encoded by three bases (64) because of the degeneracy of the code. The structurally and synthetically complex amino acids (Trp, Tyr, Phe, Lys, His, and Met) are distinguished from simpler amino acids only in the third position of their codons and are likely to have been added to the code relatively late (30). The 14 remaining amino acids can be specified by using a doublet code and, in fact, these amino acids nearly saturate the bound for the number of amino acids that can be unambiguously encoded by two bases (16). These properties are consistent with later addition of a third base to an established two-base association. They are also consistent with an emergence of the reading frame and tRNA concurrent with or later than the emergence of the triplet code. We do not seek to explain the full code, the mechanism for addition of the complex amino acids to the code, or the emergence of translation here, although the arguments we provide are a consistent foundation on which to pursue these questions later.
*****
This reminded me of papers on how ten of the amino acids are thermodynamically favored. Since I'm not willing to pay to get the full text I'm referring to the lpetrich post discussing them at SecularCafe:
http://ift.tt/1LuVzFK
One paper has the most abundant amino acids being:
Glycine, Alanine, Aspartate, Glutamate, Valine, Serine, Isoleucine, Leucine, Proline, Threonine
Which are all 7 metabolic steps (Isoleucine, Leucine) or less and none of them the ones requiring the third codon. Asparagine and Glutamine are Aspartate and Glutamate with a different side chain as the names suggest. That makes 12 of the 14. Arginine and Cysteine would complete the 14.
The oddity is that the 9 step Arginine is supposed to be an early AA while the 7 step Methionine is not. On the other hand, Copley etc paper posits Arginine replacing its simpler precursor Ornithine. Also, Methionine takes two steps more than Cysteine, the only other AA with sulfur.
Plenty of work needs to be done, but despite anything creationists might say it is not unreasonable for the genetic code to have arisen from plain chemistry.
http://ift.tt/1LuVzFI
This paper is old news but I was reminded of the contents by page 136 of Nick Lane's "The Vital Question". He refers to the DNA chapter of "Life Ascending" where he discussed this work, but then only points out that it assumes a constant supply of precursor chemicals which an alkaline hydro-thermal vent system would have supplied.
The following may be the most succinct statement I've seen of why fourteen of the current amino acids likely made up the first code:
****
Mapping of the Modern Genetic Code onto a Simpler Doublet Code
The 20 amino acids encoded by the modern code do not saturate the bound for the number of amino acids that can be unambiguously encoded by three bases (64) because of the degeneracy of the code. The structurally and synthetically complex amino acids (Trp, Tyr, Phe, Lys, His, and Met) are distinguished from simpler amino acids only in the third position of their codons and are likely to have been added to the code relatively late (30). The 14 remaining amino acids can be specified by using a doublet code and, in fact, these amino acids nearly saturate the bound for the number of amino acids that can be unambiguously encoded by two bases (16). These properties are consistent with later addition of a third base to an established two-base association. They are also consistent with an emergence of the reading frame and tRNA concurrent with or later than the emergence of the triplet code. We do not seek to explain the full code, the mechanism for addition of the complex amino acids to the code, or the emergence of translation here, although the arguments we provide are a consistent foundation on which to pursue these questions later.
*****
This reminded me of papers on how ten of the amino acids are thermodynamically favored. Since I'm not willing to pay to get the full text I'm referring to the lpetrich post discussing them at SecularCafe:
http://ift.tt/1LuVzFK
One paper has the most abundant amino acids being:
Glycine, Alanine, Aspartate, Glutamate, Valine, Serine, Isoleucine, Leucine, Proline, Threonine
Which are all 7 metabolic steps (Isoleucine, Leucine) or less and none of them the ones requiring the third codon. Asparagine and Glutamine are Aspartate and Glutamate with a different side chain as the names suggest. That makes 12 of the 14. Arginine and Cysteine would complete the 14.
The oddity is that the 9 step Arginine is supposed to be an early AA while the 7 step Methionine is not. On the other hand, Copley etc paper posits Arginine replacing its simpler precursor Ornithine. Also, Methionine takes two steps more than Cysteine, the only other AA with sulfur.
Plenty of work needs to be done, but despite anything creationists might say it is not unreasonable for the genetic code to have arisen from plain chemistry.
via International Skeptics Forum http://ift.tt/1GSCzh5
Aucun commentaire:
Enregistrer un commentaire