Structural multitude of nucleic acids serves basis for its multiple merits and applications. During structural transitions, significant to perform respective cellular functions, these DNA forms can vary from the single stranded to multi-stranded species. Hence, beyond the image of a monotonous DNA double-helix, there is now increasing interest in other polymorphic/ multi-stranded forms, the roles they may play in vivo and their potential use in therapeutics. Distinct guanine-rich nucleic acid sequences readily form a structurally diverse four-stranded architecture called G-quadruplexes. In addition to their presence at physical ends of chromosomes called telomeres, occurrence of these structural motifs in the upstream promoter regions of a number of genes, oncogenes and near transcription start sites, highlights that G-quadruplexes are involved in regulation of gene expression. Cancer cells typically possess shorter telomeres and have telomerase activity greatly exceeding that of normal cells. These differences create an opportunity to use anticancer therapies targeting telomerase and telomeres. The ability of small molecules to interact with and presumably stabilize Gquadruplex structures as a means of inhibiting telomerase has been a major drug design effort. Ligands, capable of interacting with four-stranded G-quadruplex have been generated. The discovery of proteins including transcription factors, recognizing G-quadruplexes, and conferring stabilization or unfolding them in biological systems, again makes G-quadruplexes, biologically pertinent structures. This review is an attempt to summarize the rapidly evolving literature exploring the amazing polymorphism of G-quadruplexes, and understanding their structure-specific-recognition and biological relevance, keeping in mind that G-tetraplexes are not only important drug targets, but may also act as gene regulatory elements. A pertinent detail of the challenges towards the rational design of structure-specific novel drugs has also been discussed.
Keywords: G-quadruplexes, quadruplex-binding proteins, quadruplex-interacting ligands, structural polymorphism, structure-specific recognition, telomerase inhibitors, human genome, X-ray diffraction, DNA fibers, allomorphs, conformational transitions, microsatellites, telomeres, oncogenes, eukaryotic genes