The possible letters are A, C, G, and T, representing the four nucleotide subunits of a DNA strand - adenine, cytosine, guanine, thymine bases covalently linked to phospho-backbone. In the typical case, the sequences are printed abutting one another without gaps, as in the sequence AAAGTCTGAC, going from 5' to 3' from left to right. A succession of any number of nucleotides greater than four is liable to be called a sequence. With regard to its biological function, which may depend on context, a sequence may be sense or anti-sense, and either coding or noncoding. DNA sequences can also contain "junk DNA."
Sequences can be derived from the biological raw material through a process called DNA sequencing.
In some special cases, letters besides A, T, C, and G are present in a sequence. These letters represent ambiguity. Of all the molecules sampled, there is more than one kind of nucleotide at that position. The rules of the International Union of Pure and Applied Chemistry (IUPAC) are as follows:
A = adenine C = cytosine G = guanine T = thymine R = G A (purine) Y = T C (pyrimidine) K = G T (keto) M = A C (amino) S = G C (strong bonds) W = A T (weak bonds) B = G T C (all but A) D = G A T (all but C) H = A C T (all but G) V = G C A (all but T) N = A G C T (any)
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