Codons
<p>The genetic code (which includes the codon) serves as a basis for establishing how genes encoded in DNA are decoded into proteins. A critical interaction in protein synthesis is the interaction between the codon in messenger RNA (mRNA) and the anticodon in an aminoacyl-transfer RNA (aminoacyl-tRNA). </p>
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<b>A codon is a triplet </b>of adjacent nucleotides in mRNA that specifies an amino acid to be incorporated in a protein. Because the codon can be made from three of the four possible ribonucleotides, there are 4 to the power 3 or 64 combinations, leading to 64 different codons. The first letter of the codon is at the 5′-end, while the last letter is at the 3′-end. For example, 5′-AUG-3′. Because we have 64 combinations we have code redundancy and can use some as stop codes or use 2 codons only or use some extra ones (reason??) up to 6. </p>
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The amino acid sequence of a protein will be specified by the sequence of contiguous codons in the mRNA template. The initial codon in the mRNA establishes the reading frame and defines the protein's initial amino acid.</p>
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There are three types of codons. There is an initiation codon, AUG, which signifies the initial amino acid (and also codes for methionine residues in internal positions) in the protein. There are 61 codons, including AUG, that designate individual amino acids. The remaining three codons (UAA, UAG, and UGA) are termination codons (also called stop codons or nonsense codons), which do not code for amino acids, but signal the end of the mRNA message and provide the "stop" signal for protein synthesis. 61 of the 64 code for proteins. 3 are stop codons and AUG does start and Methionine.</p>
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Two amino acid residues, tryptophan and methionine, have unique codons—UGG and AUG, respectively. All other amino acids may be coded for by more than one codon, such that the code is said to be degenerate. This degeneracy is not uniform, but varies according to the particular amino acids. For example, three amino acids (arginine, leucine, and serine) have six codons, five amino acids have four, isoleucine has three, and nine amino acids have two. The first two letters of each codon provide the primary determinant in the specificity. For example, the codons for amino acid valine are GUU, GUC, GUA, and GUG. The open reading frame of the mRNA, which extends from the AUG codon to the termination codon, establishes the protein that is to be synthesized.</p>
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<img src="/3308e9eb-23fc-5079-8057-218815500a42.png"/> <img src="/b343008b-e2ac-50cc-b49f-a954688cd659.png"/></p>
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A schematic shows a horizontal row of 15 nucleotides above a row of four amino acids </p>
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