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The Genetic Code

The genetic information in the mRNA is composed of an alternating sequence of the four bases adenine (A), guanine (G), cytosine (C) and uracil (U). This alternating sequence provides the unique code specifying each of the 20 amino acids naturally found in protein.
20 amino acids»


It was early realised that the only way codons for the 20 amino acids could be spelled out from the four available nucleotides was to use combinations of at least three bases, codons (43 = 64 combinations). In the beginning of the 1960s, the nature of the genetic code was experimentally tested using artificial mRNAs composed of only one base (homopolymers), or a composition of synthetic trinucleotides representing all 64 possible codon combinations. Using these techniques the genetic code was solved and the main contributors, Nirenberg and Khorana, were awarded the 1968 Nobel Prize in Physiology or Medicine.

 

 The Nobel Prize in Physiology or Medicine 1968
The genetic code»


In the genetic code each of the 20 amino acids is represented by at least one codon. Most of the amino acids are coded for by more than one codon. This is referred to as the degeneracy of the code.

The code has no comma (i.e. unlike written language there are no separators between words). After identification of the first codon, translation proceeds by taking the following nucleotides three at a time.

The genetic code is read by transfer RNAs (tRNA). Each tRNA has an anticodon that is complementary to the codon in the mRNA (see basepairing principles). However, due to the so-called wobble principle the cell manages with less tRNAs than would be expected from the number of anticodons required to match the codons in the code table. Each tRNA can also carry the amino acid specified by the codon (see amino acid activation).

Three of the codons do not code for any amino acid. These codons are instead interpreted as stop signals i.e. signals that define the end of the message (see termination). It is notable that there is no unique codon specifying the point were the message starts. Instead the codon coding for the amino acid methionine, AUG, has two functions: it can either specify incorporation of methionine into the growing protein chain (see elongation) or specify the starting point of the message (see initiation). The discrimination between the two functions of the AUG codon is coupled to the occurrence of two structurally different methionine-carrying tRNAs (Met-tRNAs), elongator Met-tRNAm and initiator Met-tRNAi.


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