Universal genetic code

THE GENETIC CODE IS DEGENERATE, UNAMBIGUOUS, NONOVERLAPPING, WITHOUT PUNCTUATION, UNIVERSAL... 

One important question is that of the order in which the basic mechanisms of evolution processes, leading eventually to the emergence of life, occurred. As far as the development of the genetic code is concerned, it is not clear whether the code evolved prior to the aminoacylation process, i.e., whether aminoacyl-tRNA synthetases evolved before or after the code. A tRNA species which is aminoacy-lated by two different synthetases was studied if this tRNA had important identity elements such as the discriminator base and the three anticodon bases for the two synthetases, this would be evidence that the aminoacyl-tRNA synthetases had developed after the genetic code. Dieter Soil s group, which is experienced in working with this family of enzymes, came to the conclusion that the universal genetic code must have developed before the evolution of the aminoacylation system (Hohn et al, 2006). 

A further (mathematical) model for the evolution of the genetic code was devised by Carl Woese and co-workers. This dynamic theory provides information on the evolvability and universality of the genetic code. One conceptual difficulty was due to the fact that it had been overlooked that the genetic code was highly communal... 

RNA messenger and transcription according to the universal genetic code. 

One of the groups of theories about the origin of the genetic code states that the code has to be the way it is, and is therefore universal, for stereochemical" reasons. In other words, phenylalanine, f. ex. must be represented by the triplets UUU and UUC because phenylalanine is somehow stereochemically related to these two codons 52,53,56,57) This seems likely, since steric fit is an essential property of the processes of replication, transcription and translation. That doesn t mean that one has conclusive evidence for such a statement. It only means that the theoreticians are groping in such a direction. 

With few exceptions, the genetic code is universal. Codons in different organisms have identical meanings, specifying the insertion of one of the canonical 20 amino acids or translational termination. Although amino acids different from the canonical 20 have been discovered in some organisms, only two amino acids,... 

Life is strikingly unified at the molecular level one genetic code, limited universal sets of key molecules, common metabolic pathways. 

Table 12.1. The genetic code presented in this table is very nearly universal. There are isolated exceptions in the genome of mitochondria, which is described later in this chapter. Beyond that, the genetic code has been expanded to include codons for two unusual amino acids that occur in a modest number of proteins. These amino acids are selenomethionine, in which an atom of selenium replaces the sulfur atom of methionine, and pyrrolysine, a cychzed form of lysine. For details, see A. Ambrogelly, S. Palioura, and D. Soil, Nat Chem Biol 3 29-35 (2007). 

As the genetic code provides 4 = 64 codons for the 20 amino acids, there are several synonymous codons for most amino acids— the code is degenerate. Three triplets do not code for amino acids, but instead signal the end of translation (stop codons). Another special codon, the start codon, marks the start of translation. The code shown here is almost universally applicable only the mitochondria (see p. 210) and a few microorganisms deviate from it slightly. 

Clearly this means a complete rejection of the fundamental Darwinian principle of common descent. Also, he rejects mutation and natural selection as the mechanisms that produced species. Is this view also contrary to the universality of biochemistry, and in particular the monophyletic origin of life, to which most biochemists today would subscribe Probably yes but of course if one assumes an absolute determinism, then the laws of chemistry and physics would produce the same products at each different start. This goes against the notion of frozen accident and the unique origin of the genetic code. So, there was never a time on Earth with only one kind of species, and the development of species was parallel rather than sequential. Of course all these ideas are substantiated by arguments and data - for these, the reader should refer to the original sources. 

C, G or T. This is not sufficient to encode the 20 possible amino acids. In triplets of 3 positions, there are 64 possible combinations. Hence, the system uses triplets, called codons. The code for each protein starts with an ATG (start codon) and ends with a TAA, TAG or a TGA (stop codons). The code is almost universal only mitochondria and ciliated protozoa have a different genetic code. 

These variations tell us that the code is not quite as universal as once believed, but that its flexibility is severely constrained. The variations are obviously derivatives of the normal code, and no example of a completely different code has been found. The limited scope of code variants strengthens the principle that all life on this planet evolved on the basis of a single (slightly flexible) genetic code. 

The standard genetic code words are universal in all species, with some minor deviations in mitochondria and a few single-celled organisms. 

Universality The genetic code is virtually universal, that is, the specificity of the genetic code has been conserved from very early stages of evolution, with only slight differences in the manner in which the code is translated. [Note An exception occurs in mitochondria, in which a few codons have different meanings than those shown in Figure 31.2.]... 

Characteristics of the genetic code include specificity, universality, and redundancy, and it is nonoverlapping and commaless. 

A number of studies suggested that the genetic code as worked out for E. coli might be universal. 

As often happens, a well-established conclusion may have to be modified. There are exceptions to the universal genetic code in mitochondrial DNA and in some protozoa (Chapter 5).45... 

In what ways is the genetic code not quite "universal " What is meant by "editing" of mRNA ... 

Parker, J., Errors and alternatives in reading the universal genetic code. Microbiol. Rev. 53 273, 1989. A summary of the current knowledge of alternative mechanisms of translation. 

Explain this statement The universal genetic code is not quite universal. ... 

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