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Concept
Nucleic acid notation
Résumé
The nucleic acid notation currently in use was first formalized by the International Union of Pure and Applied Chemistry (IUPAC) in 1970. This universally accepted notation uses the Roman characters G, C, A, and T, to represent the four nucleotides commonly found in deoxyribonucleic acids (DNA).
Given the rapidly expanding role for genetic sequencing, synthesis, and analysis in biology, some researchers have developed alternate notations to further support the analysis and manipulation of genetic data. These notations generally exploit size, shape, and symmetry to accomplish these objectives.
Degenerate base symbols in biochemistry are an IUPAC representation for a position on a DNA sequence that can have multiple possible alternatives. These should not be confused with non-canonical bases because each particular sequence will have in fact one of the regular bases. These are used to encode the consensus sequence of a population of aligned sequences and are used for example in phylogenetic analysis to summarise into one multiple sequences or for BLAST searches, even though IUPAC degenerate symbols are masked (as they are not coded).
Under the commonly used IUPAC system, nucleobases are represented by the first letters of their chemical names: guanine, cytosine, adenine, and thymine. This shorthand also includes eleven "ambiguity" characters associated with every possible combination of the four DNA bases. The ambiguity characters were designed to encode positional variations in order to report DNA sequencing errors, consensus sequences, or single-nucleotide polymorphisms. The IUPAC notation, including ambiguity characters and suggested mnemonics, is shown in Table 1.
Despite its broad and nearly universal acceptance, the IUPAC system has a number of limitations, which stem from its reliance on the Roman alphabet. The poor legibility of upper-case Roman characters, which are generally used when displaying genetic data, may be chief among these limitations. The value of external projections in distinguishing letters has been well documented.
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