Friday, March 13, 2009
Original Triplet
On page 82 it says that "bases may be changing but their translated meaning is not." So because the triplet pairs can be mutated but still code for the same protein then are the triplets that we know and study today different then those that existed a million or more years ago? For example, on page 82 it says that TTA codes for leucine but also TTG, CTA, CTT, CTC, and CTG code for leucine, so my question is did the code for leucine used to be something very different than the TTA code that we recognize today? If so, why has this dramatic change happened, in the future will TTA no longer code for leucine but a whole new and different triplet base code for leucine?
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In Carroll’s discussion of immortal genes, he focuses on the redundancy of the genetic code, which enables natural selection to “purge the variant from a population” (83). In other words, the genetic code of a population does not acquire injurious mutations because certain mutations act as neutral mutations- they cause no change in the genetic code. One reason that there are 6 different codes for the leucine amino acid is because, as Carroll states, there are far more synonymous mutations than there are nonsynonymous mutations (a consequence of purging): “there is a 1:3 favor of synonymous changes” (80). Purifying selection is what maintains the constant of leucine.
ReplyDeleteBecause Carroll states that purifying selection holds the greatest impact for immortal genes, those that have been maintained over all domains of life, then it can be deduced that the sequence of TTA, and its counterparts, is an immortal gene. Leucine is one of the 14 amino acids that has been “effectively immortal” (82) and has been maintained through evolution. In general, immortal genes have been maintained across all domains of life because they most likely code for the machinery involved in decoding mRNA, a process that all organisms take part in.
Leucine, although not involved with mRNA decoding, still holds the amount of clout of that type machinery, warranting it status as immortal. It is an essential amino acid, C4H9CH(NH2)COOH, obtained by the hydrolysis of protein by pancreatic enzymes during digestion and necessary for optimal growth in infants and children and for the maintenance of nitrogen balance in adults.
Most likely the code TTA did not always code for purely the amino acid Leucine. This is because the initial genetic code was most likely comprised of “doublets” instead of triplets. According to the biosynthetic theory, the evolution of the modern genetic code began with a rudimentary code that coded for fewer amino acids. Thus, there is a separation between old and new amino acids, which would explain how TTA evolved to code for leucine. To continue, according to the different redundancies in the code presented on page 82, the one thing all the 6 different version of the triplets have in common is that the second, or middle letter, is always a T. This “second row” redundancy holds true for most triplet sequences. Thus, there may have been a time in the evolution of the genetic code in which the third section was not needed- a doublet preceded a triplet. If this holds true, than the rudimentary code for the “TTA” triplet, and its counterparts, is a TT or _T triplet. This doublet most likely did not code for a purely leucine amino acid, because the 16 possible amino acids that could be created by this doublet mechanism had to some way incorporate all 20 amino acids (or in some way give rise to them). In addition, it is generally agreed that Trp, Gln, Asn, and Tyr are the “newer amino acids.” Therefore, the primordial leucine is an older amino acid whose doublet had to code for leucine and something else in order to give rise to the modern genetic code (for example, leucine could be connected to the nucleotides C and U). This would make sense because uracil is the biosynthetic precursor of cytosine and thymine so the third spot, occupied by the place holder U, could be replaced by either T or C, which does account for the redundancy observed in the code.
Yet another way to explain the evolution of the TTA code is that the ancient genetic code was comprised of triplets, but the ancient encoding machinery only had the capability to decode in doublets. However, this means that our current reading frame is very inefficient: already a quaternary doublet code can encode 16 amino acids (or 15 plus a termination codon). For just four further amino acids a third letter is necessary.
The next step for genetic evolution, following the scheme already observed, would be that the reading frame evolved from a triplet reading frame to a quadruplet reading frame. This type of evolution would require that we needed another set of amino acids to carry out our metabolic functions. The change from two to three, most likely occurred as a result of energy demands. Because humans are kind of in this lull, where our energy demands are being met and are risk of predation is not as stark, I don’t see a need to evolve more amino acids than we have now. Over an extended period of time, if the phenomenon of global warming completely misbalances our current atmospheric composition, then maybe mankind must adapt its cellular respiration processes to harness carbon dioxide as a driving force to synthesize ATP. Again, the way TTA would be affected is that the product leucine would be bound to a place holder nucleotide, like U in the previous example; this would make it easy for the leucine to morph into a new amino acid to meet new energy needs. The key thing to remember is that that basic TTA will most likely code for a leucine counterpart because of the unifying redundancy of the second row.
Source: http://www.imb-jena.de/~sweta/genetic_code2/evolution.html