DNA routinely folds into shapes that look nothing like the iconic double helix. Under the right conditions, stretches of the genome twist into left-handed spirals, stack into four-stranded towers, loop back on themselves into hairpins, or knot into three-stranded bundles. These alternative shapes, collectively called non-B DNA, are not rare oddities or laboratory artifacts. They form transiently in living cells, influenced by factors as ordinary as salt concentration and temperature, and they play active roles in how genes get switched on or off, how cells divide, and how diseases take hold.
What Counts as Non-B DNA
The “B” in B-DNA refers to the classic Watson-Crick right-handed double helix, the form most people picture when they think of DNA. Non-B DNA is an umbrella term for every other conformation the molecule can adopt. The list is surprisingly long. G-quadruplexes form in guanine-rich sequences, Z-DNA forms in stretches where purines and pyrimidines alternate, triplex (or H-DNA) structures arise from mirror-repeat sequences rich in purines on one strand, cruciforms pop out of inverted repeats, hairpins fold from sequences that can pair with themselves, and i-motifs assemble in cytosine-rich regions under acidic conditions.1Trends in Genetics. DNA Secondary Structure: Beyond the Double Helix – Section: The ubiquity, in vivo formation, and functions of non-B DNA R-loops, where an RNA strand invades and displaces one DNA strand, round out the major categories. Each structure has its own preferred sequence context and its own set of environmental triggers.
A critical point is that these structures are transient. DNA does not permanently lock into a quadruplex or a cruciform. The molecule flickers between conformations depending on what is happening inside the cell at that moment. Potassium ions stabilize G-quadruplexes, magnesium favors triplex DNA, and even temperature shifts can change whether a stretch of adenine-rich DNA bends or stays straight.1Trends in Genetics. DNA Secondary Structure: Beyond the Double Helix – Section: The ubiquity, in vivo formation, and functions of non-B DNA This dynamism is part of what makes these structures biologically interesting: they act as molecular switches rather than permanent fixtures.
G-Quadruplexes and the Four-Stranded Towers
Among all non-B DNA shapes, G-quadruplexes (G4s) have received the most research attention. They form when four guanine bases arrange themselves in a flat square called a G-quartet, held together by a special type of hydrogen bonding called Hoogsteen pairing. Multiple quartets then stack on top of each other, stabilized by a metal ion (usually potassium) sitting in the channel between layers.2Biochemistry and Biophysics Reports. Effects of deficient of the Hoogsteen base-pairs on the G-quadruplex stabilization and binding mode of a cationic porphyrin The result is a compact, stable tower of DNA that is structurally very different from anything Watson and Crick described.
What makes G4s fascinating is where they show up. They cluster in gene promoters, the regions of DNA that control whether a gene is active. Research on liposarcoma cells and keratinocytes found that when a promoter sequence folded into a G4, the associated gene was consistently transcribed at high levels. The same promoter sequences in different cell types either folded or did not fold into G4s, and the folding state tracked with whether the gene was active. Transcription factors AP-1 and SP1, which bind to DNA to help initiate gene reading, were found to associate with their G4-folded promoter sequences, suggesting that the shape of the DNA and the proteins that read it work as a team.3Nature Communications. Promoter G-quadruplexes and transcription factors cooperate to shape the cell type-specific transcriptome This cooperation has led researchers to describe G4s as a kind of epigenetic regulator, a layer of gene control that sits on top of the DNA sequence itself rather than within it.4PubMed Central. The Interplay between G-quadruplex and Transcription
Z-DNA and the Left-Handed Twist
Most DNA winds to the right. Z-DNA winds to the left. It was one of the first alternative conformations recognized, and it forms preferentially in sequences where cytosine and guanine alternate. For years after its discovery, skeptics argued that Z-DNA was just a lab curiosity that required extreme salt concentrations to exist. That turned out to be wrong. When DNA in cells is negatively supercoiled, meaning it is underwound by the cellular machinery, Z-DNA can form at physiological conditions without any exotic chemistry.
Experiments using antibodies that specifically recognize the left-handed conformation demonstrated this directly. Negatively supercoiled plasmids containing alternating CG sequences bound Z-DNA-specific antibodies, confirming that the left-handed shape forms under the kind of torsional stress DNA routinely experiences inside cells.5PubMed Central. Formation of Z-DNA in negatively supercoiled plasmids is sensitive to small changes in salt concentration within the physiological range Even more striking, Z-DNA was found to form at physiological supercoiling densities in sequences that include CA/TG dinucleotide repeats, without needing high salt or chemical modifications.6PubMed Central. Negatively supercoiled plasmids contain left-handed Z-DNA segments as detected by specific antibody binding The formation was sensitive to small changes in salt concentration within the normal physiological range, suggesting that cells could in principle toggle regions of DNA between B-form and Z-form with relatively subtle shifts in their internal environment.7Cell. Negative supercoiling induces a conformational change in alternating CG sequences
Triplex DNA and Its Mutagenic Side
Triplex DNA, also called H-DNA, forms when a third strand of DNA winds into the major groove of a standard double helix and binds through Hoogsteen-type hydrogen bonds. This requires sequences where one strand is rich in purines and the other in pyrimidines, arranged as mirror repeats. The third strand folds back from the same molecule, making it an intramolecular structure.
Triplexes have a dark side. They are inherently mutagenic and promote genetic recombination, meaning they can shuffle, break, or rearrange the DNA around them.8PubMed Central. DNA triple helices: biological consequences and therapeutic potential When researchers looked for triplex-forming sequences across dozens of sequenced strains of E. coli, they found pronounced genetic instability in the regions surrounding those sequences. Comparing closely related bacterial strains revealed that the genetic neighborhoods of triplex motifs were unusually variable, suggesting that the structures actively destabilize the genome around them.9Nucleic Acids Research. Intrastrand triplex repeats in bacteria: a source of genomic instability In cancer genomes, elevated frequencies of double-strand breaks cluster at H-DNA motifs, and non-B DNA motifs are overrepresented in regions of somatic copy number alterations and chromosomal breakage.10Trends in Genetics. DNA Secondary Structure: Beyond the Double Helix – Section: Non-B DNA motifs affect mutation rate and facilitate genome instability
Cruciforms, Hairpins, and the Problem of Repeat Expansions
Cruciforms form when an inverted repeat sequence (a stretch of DNA followed by its reverse complement) unwinds and each strand folds back to pair with itself, creating a four-way junction that looks like a cross. This process, called cruciform extrusion, has been observed in real time using single-molecule techniques that mechanically manipulate individual DNA molecules.11PubMed Central. Real-time detection of cruciform extrusion by single-molecule DNA nanomanipulation Hairpins are the simpler relative: one arm of what would be a cruciform, a single strand folding back on itself to form a stem and a loop.
Hairpins become especially consequential in the context of trinucleotide repeat disorders. Expansions of short repeating sequences like CAG or CTG are the cause of at least 17 degenerative human diseases, including Huntington’s disease.12Nature Communications. Conformational and migrational dynamics of slipped-strand DNA three-way junctions containing trinucleotide repeats The leading theory is that during DNA replication or repair, the repeat region slips out of alignment and forms hairpin structures. These hairpins trick the cellular machinery into copying extra repeats, causing the expansion to grow with each generation.
Single-molecule fluorescence experiments on CAG repeats have captured this slippage in action. Hairpins formed by 15 CAG repeats were found to flicker between multiple conformations, sliding forward and backward by one repeat unit at a time.13PubMed Central. Dynamics of strand slippage in DNA hairpins formed by CAG repeats: roles of sequence parity and trinucleotide interrupts Each slip changes the size of the loop at the hairpin’s tip and the alignment of the stem. This constant sliding helps explain why repeat regions are so unstable: the hairpin itself is a moving target, always finding new ways to mispair.
i-Motifs and R-Loops
While G-quadruplexes form on guanine-rich strands, the complementary cytosine-rich strand can fold into its own alternative structure called an i-motif. In an i-motif, pairs of cytosines interlock through bonds that require one of the cytosines to be protonated, meaning it has picked up an extra hydrogen ion. This makes i-motifs exquisitely sensitive to pH. Their stability drops sharply as pH rises or as salt concentration increases, and pH is the dominant factor controlling whether they form.14PubMed. Assessing the influence of pH and cationic strength on i-motif DNA structure Unusually for nucleic acid structures, i-motifs are actually destabilized by increasing salt concentration, a quirk that sets them apart from virtually every other DNA fold.15Nucleic Acids Research. High-throughput measurement and prediction of the i-motif DNA stability landscape
R-loops are a different beast entirely. They are three-stranded structures where an RNA molecule threads into the double helix and pairs with one DNA strand, pushing the other DNA strand out as a single-stranded loop. R-loops form frequently across genomes and serve legitimate biological purposes: they help regulate gene expression, play roles in DNA replication initiation, and influence chemical modifications on both DNA and histone proteins. But they are also dangerous. Unresolved R-loops can cause DNA damage and genome instability, making them a double-edged sword the cell has to manage carefully.16PubMed Central. The balancing act of R-loop biology: The good, the bad, and the ugly
Connections to Neurodegenerative Disease
One of the most striking disease links involves a hexanucleotide repeat expansion in the C9orf72 gene. Expansions of the sequence GGGGCC in this gene are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).17PubMed Central. G-Quadruplexes Formation by the C9orf72 Nucleotide Repeat Expansion d(GGGGCC)n and Conformation Regulation by Fangchinoline Both the DNA repeat and the RNA transcribed from it fold into G-quadruplex structures. The RNA quadruplexes aggregate into intracellular clumps called RNA foci, which sequester important RNA-binding proteins and prevent them from doing their jobs. Beyond that protein-sequestering toxicity, there is evidence that these G-quadruplexes can bind and activate cellular heme, a molecule involved in oxygen transport and chemical reactions, potentially triggering oxidative damage inside neurons.18PLoS ONE. G-Quadruplex Structures Formed by Expanded Hexanucleotide Repeat RNA and DNA from the Neurodegenerative Disease-Linked C9orf72 Gene Efficiently Sequester and Activate Heme
The link between non-B DNA and disease extends into cancer as well. Researchers have used high-throughput assays to test how non-B DNA structures affect promoter activity in cancer-associated genes including CMYC, KRAS, BCL2, and VEGF.19PubMed Central. High-throughput characterization of the role of non-B DNA motifs on promoter function These are genes whose overactivation drives tumor growth, and the structural conformations their promoters adopt appear to influence how aggressively they are transcribed.
G-Quadruplexes in Viral Genomes
The presence of G4-forming sequences is not limited to human DNA. Comprehensive bioinformatics analyses have found statistically significant distributions of these sequences across the genomes of nearly all human viruses, with high conservation rates suggesting the structures play functional roles in viral replication and evolution.20PubMed Central. Viral G-quadruplexes: New frontiers in virus pathogenesis and antiviral therapy G4-forming sequences have been identified in pathogens responsible for serious diseases including HIV-1 and Hepatitis B and C. They tend to sit in regulatory regions of the viral genome, where they can influence critical stages of the viral life cycle.21PubMed Central. G-Quadruplexes in the Viral Genome: Unlocking Targets for Therapeutic Interventions and Antiviral Strategies
This opens up a therapeutic angle. If viral G4s are important for the virus to replicate, then small molecules that lock those G4s into their folded state could jam the viral machinery. Work on Kaposi’s sarcoma-associated herpesvirus (KSHV) demonstrated exactly this. G-quadruplex-stabilizing compounds activated dormant origins of DNA replication in the virus and caused replication forks to stall, particularly in the terminal repeat region rich in G4-forming sequences. Over time, treatment with the stabilizing compound PhenDC3 led to a loss of viral episomes in infected cells.22Nucleic Acids Research. G-quadruplex-interacting compounds alter latent DNA replication and episomal persistence of KSHV In plain terms, the compound forced the viral DNA into shapes that the replication machinery could not handle, and the virus lost its foothold in the cell.
Drugs That Target DNA Shape
The idea of targeting non-B DNA structures with drugs has progressed beyond proof-of-concept experiments. The small molecule CX-5461, originally developed as a compound that stabilizes G-quadruplexes, is in clinical trials for advanced cancers with BRCA1/2 deficiencies. Binding assays showed that CX-5461 selectively stabilizes G4 structures over standard double-stranded DNA, raising the melting temperature of G4s by more than 15°C while barely affecting the double helix.23Nature Communications. CX-5461 is a DNA G-quadruplex stabilizer with selective lethality in BRCA1/2 deficient tumours – Section: CX-5461 is a G-quadruplex stabilizer in the human genome Molecular dynamics simulations confirmed that CX-5461 binds much more tightly to human telomeric, c-KIT1, and c-Myc G-quadruplexes than to a standard DNA duplex.24PubMed. Molecular Dynamics Study on the Binding of an Anticancer DNA G-Quadruplex Stabilizer, CX-5461, to Human Telomeric, c-KIT1, and c-Myc G-Quadruplexes and a DNA Duplex
Recent work has pushed CX-5461 further still. When exposed to light, the compound’s chemistry shifts from simply stabilizing the G4 to actively oxidizing it, generating reactive oxygen species that damage the DNA and trigger stress responses in cancer cells. In mouse models of colon cancer and melanoma, this photoactivation approach extended survival and provoked immune responses against the tumor, including infiltration of CD8+ T cells into the tumor microenvironment.25Nucleic Acids Research. G-quadruplex DNA reprograms the photochemistry of CX-5461 toward radical-driven anticancer activity The fact that a drug’s mechanism depends on the shape of the DNA it binds, rather than the sequence, represents a genuinely different approach to cancer therapy.
How Researchers Detect These Structures
Proving that non-B DNA forms inside living cells, rather than just in test tubes, required new tools. For G-quadruplexes, one significant advance was the development of a nanobody called SG4, derived from camelid antibodies, that was selected specifically to recognize the G4 structure formed by the human Myc gene promoter. The nanobody can be expressed directly inside cells and used to image endogenous G4 structures in live cells, as well as to map G4 positions on fixed chromatin.26PubMed Central. DNA G-Quadruplex Recognition In Vitro and in Live Cells by a Structure-Specific Nanobody For R-loops, immunoprecipitation-based methods using the S9.6 antibody, which recognizes DNA-RNA hybrids, allow researchers to pull down R-loop structures and sequence the DNA involved, generating genome-wide R-loop maps.27PubMed Central. High-resolution, strand-specific R-loop mapping via S9.6-based DNA-RNA immunoprecipitation and high-throughput sequencing
On the computational side, the growing catalog of experimentally verified non-B DNA sites has enabled the development of algorithms that scan genomes for sequences likely to adopt alternative conformations. These tools combine sequence-motif recognition, thermodynamic modeling, and machine learning to predict where non-B structures might form, even in genomic regions that have not yet been tested experimentally.28Nucleic Acids Research. Non-B DNA structures and their contributions to genetic diversity, aging, and disease – Section: Non-B DNA categories and their detection One large-scale computational study applied these tools to the promoter regions of over 1,100 cellular genomes across 28 taxonomic groups, mapping the distribution of non-B DNA-forming motifs across the tree of life.29PubMed. Computational analysis on the dissemination of non-B DNA structural motifs in promoter regions of 1180 cellular genomes Single-molecule biophysics techniques, such as combined fluorescence and magnetic tweezers, have also allowed researchers to watch individual G-quadruplexes fold and unfold in real time, measuring the forces and rates involved in the structural transitions.30Nucleic Acids Research. Mechanical unfolding of human telomere G-quadruplex DNA probed by integrated fluorescence and magnetic tweezers spectroscopy
Building Machines from DNA Shapes
The pH sensitivity of i-motifs and the ion sensitivity of G-quadruplexes have caught the attention of engineers. Because these structures fold and unfold in response to specific chemical signals, they can serve as moving parts in nanoscale devices. Researchers have used i-motif-driven conformational changes to build molecular motors that, when coordinated, bend micrometer-sized cantilevers. The same principles have been applied to create smart surfaces that switch between water-attracting and water-repelling states, nanopores that open and close in response to pH changes, and molecular logic gates.31Accounts of Chemical Research. DNA Nanotechnology Based on i‑Motif Structures
A more recent application combines i-motif and G-quadruplex folding into a single hybrid structure that functions as a two-input AND logic gate. Because tumor microenvironments tend to be both slightly acidic (favoring i-motif folding) and enriched in potassium (favoring G4 folding), the hybrid device only activates when both conditions are met simultaneously. Researchers used this design to build a DNA nanotweezer coupled to an aptamer that detects ATP, demonstrating sensitive detection of endogenous ATP in cancer cell lysates.32Analysis & Sensing. Engineering Logic‐Gated i‐Motif/G‐Quadruplex (iG4) Hybrid Structures for DNA Nanotweezer‐Based Aptasensing The vision here is that DNA nanodevices could eventually recognize and respond to specific tissue microenvironments inside the body, providing targeted diagnostics or drug delivery.
Non-B DNA as a Driver of Evolution
The tension between function and instability runs through everything about non-B DNA. These structures regulate genes and participate in essential cellular processes, yet they also elevate mutation rates and promote genome rearrangements. That tension turns out to be evolutionarily productive. Non-B DNA structures appear across the tree of life, from bacteria to humans, and recent analyses treat them as functional genomic elements subject to natural selection. They influence the behavior of transposable elements, the “jumping genes” that move around genomes, and they play roles in specifying centromeres, the chromosome regions essential for proper cell division.33Nucleic Acids Research. Non-B DNA structures and their contributions to genetic diversity, aging, and disease The mutagenicity that makes non-B DNA dangerous at the level of an individual cell may, over evolutionary time, be exactly what allows genomes to innovate.