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The genetic code was once believed to be universal: [20] a codon would code for the same amino acid regardless of the organism or source. However, it is now agreed that the genetic code evolves, [21] resulting in discrepancies in how a codon is translated depending on the genetic source.
Four novel alternative genetic codes were discovered in bacterial genomes by Shulgina and Eddy using their codon assignment software Codetta, and validated by analysis of tRNA anticodons and identity elements; [3] these codes are not currently adopted at NCBI, but are numbered here 34-37, and specified in the table below. The standard code
In particular, the genetic code clusters certain amino acid assignments. Amino acids that share the same biosynthetic pathway tend to have the same first base in their codons. This could be an evolutionary relic of an early, simpler genetic code with fewer amino acids that later evolved to code a larger set of amino acids. [84]
The 20 amino acids that are encoded directly by the codons of the universal genetic code are called standard or canonical amino acids. A modified form of methionine ( N -formylmethionine ) is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria and plastids (including chloroplasts).
This is the standard genetic code (NCBI table 1), in amino acid→codon form. By default it is the DNA code; for the RNA code (using Uracil rather than Thymine), add template parameter "T=U". Also listed are the compressed codon forme, using IUPAC nucleic acid notation. It's referenced in a couple of places, so have a single master copy.
This is the standard or universal genetic code. This table is found in both DNA Codon Table and Genetic Code (And probably a few other places), so I'm pulling it out so it can be common. By default it's the DNA code (using the letter T for Thymine ); use template parameter "T=U" to make it the RNA code (using U for Uracil ).
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In eukaryotes, there are only 21 proteinogenic amino acids, the 20 of the standard genetic code, plus selenocysteine. Humans can synthesize 12 of these from each other or from other molecules of intermediary metabolism. The other nine must be consumed (usually as their protein derivatives), and so they are called essential amino acids.