DNA Sequence and Structure under the Prism of Group Theory and Algebraic Surfaces

Taking a DNA sequence, a word with letters/bases A, T, G and C, as the relation between the generators of an infinite group [Formula: see text] , one can discriminate between two important families: (i) the cardinality structure for conjugacy classes of subgroups of [Formula: see text] is that of a free group on one to four bases, and the DNA word, viewed as a substitution sequence, is aperiodic; (ii) the cardinality structure for conjugacy classes of subgroups of [Formula: see text] is not that of a free group, the sequence is generally not aperiodic and topological properties of [Formula: see text] have to be determined differently. The two cases rely on DNA conformations such as A-DNA, B-DNA, Z-DNA, G-quadruplexes, etc. We found a few salient results: Z-DNA, when involved in transcription, replication and regulation in a healthy situation, implies (i). The sequence of telomeric repeats comprising three distinct bases most of the time satisfies (i). For two-base sequences in the free case (i) or non-free case (ii), the topology of [Formula: see text] may be found in terms of the [Formula: see text] character variety of [Formula: see text] and the attached algebraic surfaces. The linking of two unknotted curves—the Hopf link—may occur in the topology of [Formula: see text] in cases of biological importance, in telomeres, G-quadruplexes, hairpins and junctions, a feature that we already found in the context of models of topological quantum computing. For three- and four-base sequences, other knotting configurations are noticed and a building block of the topology is the four-punctured sphere. Our methods have the potential to discriminate between potential diseases associated to the sequences.