A DNA sequence (sometimes genetic sequence) is a succession of letters representing the primary structure of a real or hypothetical DNAmolecule or strand, The possible letters are A, C, G, and T, representing the four nucleotide subunits of a DNA strand (adenine, cytosine, guanine, thymine), and typically these are printed abutting one another without gaps, as in the sequence AAAGTCTGAC. This coded sequence is sometimes referred to as genetic information. 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 (see DNA), and either coding or noncoding. DNA sequences can also contain "junk DNA".
Within organisms, geneticinformation generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.
An important area within molecular genetics is the use of molecular information to determine the patterns of descent, and therefore the correct scientific classification of organisms: this is called molecular systematics.
The foundational discipline is population genetics which studies the distribution of and change in allele frequencies of genes under the influence of the four evolutionary forces: natural selection, genetic drift, mutation and migration.
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