Genetic Code - Research Article from Macmillan Science Library: Genetics

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Genetic Code

The sequence of nucleotides in DNA determines the sequence of amino acids found in all proteins. Since there are only four nucleotide "letters" in the DNA alphabet (A, C, G, T, which stand for adenine, cytosine, guanine, and thymine), but there are 20 different amino acids in the protein alphabet, it is clear that more than one nucleotide must be used to specify an amino acid. Even two nucleotides read at a time would not give sufficient combinations (4 × 4 = 16) to encode all 20 amino acids plus start and stop signals. Therefore it would require a minimum of three DNA nucleotides to "spell out" one amino acid, and indeed this is the number that is actually used. RNA also uses a four letter alphabet when it reads and transcribes DNA instructions during protein synthesis, but its set of nucleotides is somewhat different, substituting U (uracil) for T (thymine).

Figure 1. A messenger RNA is translated in triplets, beginning with the first AUG encountered by the ribosome. Translation stops at a stop codon, one of which is UAA. Figure...

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This section contains 1,616 words
(approx. 6 pages at 300 words per page)
Buy the Genetic Code Encyclopedia Article
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Macmillan Science Library: Genetics
Genetic Code from Macmillan Science Library: Genetics. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.
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