Hidden DNA Knots: G-Quadruplexes Mapped Across the Complete Human Genome

Deep within the double helix, DNA is not always the tidy, spiraling ladder familiar from textbooks. In certain sequences, a single strand of DNA can fold back on itself into a compact, four-stranded structure known as a G-quadruplex, or G4. These structures form in stretches of DNA rich in guanine, one of the four chemical letters of the genetic code, and they have been implicated in processes ranging from gene regulation to genome instability. Now, a team of researchers led by Nikol Chantzi and Ilias Georgakopoulos-Soares at The University of Texas at Austin, together with collaborators at City University of Hong Kong and other institutions, has produced the most comprehensive survey yet of G-quadruplexes across the complete human genome, including the repetitive, hard-to-read regions that earlier studies could not reach. The work, published in Genome Biology, offers a striking new portrait of these unusual DNA structures and suggests they are far more dynamic and functionally significant than previously appreciated.

The challenge that the team set out to overcome was technical rather than conceptual. For decades, scientists have known that G4-forming sequences exist throughout the genome, but the most repetitive parts of human DNA, such as centromeres and the long arrays of ribosomal DNA, resisted assembly with standard short-read sequencing technologies. These regions are composed of long, near-identical repeats that short reads cannot be uniquely placed, leaving gaps in even the most celebrated reference assemblies. The recent arrival of long-read sequencing and the resulting gap-less, telomere-to-telomere human genome assemblies changed the picture entirely. For the first time, researchers could scan the entirety of a complete human genome, from end to end, for sequences with the potential to form G-quadruplexes, and then compare those findings across many individuals.

In the new study, the researchers systematically identified potential G4-forming sequences in the gap-less reference human genome assembly and extended their analysis to 88 haplotypes, or individual genome copies, drawn from people of diverse ancestries. This breadth mattered. By looking across many genomes rather than a single reference, the team could ask not only where G4s sit in the genome but also how variable they are between people, a direct window into their evolutionary and mutational behavior. The scale of the analysis allowed them to move beyond anecdotal observations and build a population-level picture of G4 landscape and instability.

One of the most striking findings concerns where G4s concentrate. The team reports that G4-forming sequences are highly enriched in specific repetitive regions of the genome, including certain centromeric and pericentromeric repeat types, the specialized DNA that surrounds and structures the centromere, the point where chromosomes attach to the segregation machinery during cell division. They also found marked enrichment in ribosomal DNA arrays, the tandem repeats that encode the RNA components of ribosomes, the molecular machines that build proteins in every cell. These are precisely the regions that had been invisible to earlier G4 surveys, which means that a substantial fraction of the human G4 landscape had effectively been missing from the scientific record until now.

Importantly, the researchers did not rely on sequence patterns alone. Computational predictions of G4 formation are useful, but they can be misleading, because not every guanine-rich motif actually folds into a quadruplex under physiological conditions. To address this, the team experimentally validated the most prevalent G4 structures they detected, confirming that the sequences identified in these repetitive regions can indeed form the characteristic four-stranded architecture. This biophysical validation anchors the computational survey in laboratory reality and strengthens the case that the newly mapped G4s are genuine structural features of human DNA rather than artifacts of pattern matching.

The study also examined the chemical modification landscape around G4s, with intriguing results. DNA methylation, the addition of a methyl group to cytosine bases, is a key epigenetic mark that influences whether genes are active or silent. The researchers found that G4-forming regions tend to show lower methylation than would be expected by chance. This observation fits with a growing body of evidence that alternative DNA structures can influence the epigenetic state of the sequences that carry them, potentially affecting how nearby genes are regulated. While the study establishes the correlation across the complete genome, it also raises new questions about cause and effect: whether G4 formation actively discourages methylation, or whether unmethylated regions are simply more permissive to quadruplex folding, remains an open avenue for future work.

Perhaps the most consequential finding relates to genome stability. By comparing G4 loci across the 88 haplotypes, the researchers could measure how often these sequences change from one generation to the next. The answer was unambiguous: G4-forming regions are genomically unstable, showing an excess of insertions and deletions relative to the surrounding genome. This makes mechanistic sense. A four-stranded structure folded out of one DNA strand creates an awkward substrate for the enzymes that copy and repair DNA, and stalled replication forks at stable secondary structures are a classic source of small insertions and deletions. The new data suggest that this mutagenic tendency is a genome-wide property of G4s, measurable at the population scale, and that these structures act as ongoing engines of genetic variation in repetitive DNA.

The team then connected G4s to one of the most dramatic events in the life of a genome: meiosis, the specialized cell division that produces sperm and eggs. During meiosis, programmed double-strand breaks are deliberately introduced at thousands of hotspots throughout the genome to initiate recombination, the shuffling of genetic material between chromosomes. Which sites are cut is largely determined by PRDM9, a protein whose DNA-binding domain varies between individuals and whose genotype therefore reshapes the recombination landscape from person to person. Remarkably, the researchers found that G4-forming motifs are consistently enriched at meiotic double-strand break hotspots across individuals with different PRDM9 genotypes. This consistency suggests that the physical propensity of DNA to fold into quadruplexes may contribute to the targeting or processing of meiotic breaks in a way that transcends the specific PRDM9 variant an individual carries, hinting at a structural layer of regulation overlaid on the well-known genetic one.

Taken together, the findings reframe G-quadruplexes not as curiosities of guanine-rich sequence but as dynamic, widespread, and functionally meaningful elements of the human genome. Their enrichment in centromeric and ribosomal DNA places them at the heart of chromosome architecture and protein synthesis; their reduced methylation ties them to epigenetic regulation; their excess of insertions and deletions marks them as drivers of mutation; and their presence at recombination hotspots links them to the machinery of heredity itself. Each of these threads also points toward disease. Genome instability at repetitive regions is a recurring theme in cancer and in a range of neurological and developmental disorders, and a complete map of where G4s form, and where they mutate, gives researchers a new tool for interpreting disease-associated variation in parts of the genome that were previously unreadable.

The study also illustrates how technological progress reshapes entire fields. Just as the telomere-to-telomere assemblies revealed previously hidden genes and structural variants, they have now revealed a hidden structural genome, one written not in the order of bases alone but in the capacity of those bases to fold. With 88 diverse haplotypes already surveyed, the stage is set for broader population studies that could trace how G4 landscapes differ across ancestries, how they evolve under selection, and how their instability contributes to both human evolution and human disease. For a structure first noticed as an oddity of guanine chemistry, the G-quadruplex has come a long way, and the complete human genome, it turns out, is full of it.

Subject of Research: Genomic landscape and mutational dynamics of G-quadruplex DNA structures in the complete human genome

Article Title: Landscape and mutational dynamics of G-quadruplexes across the complete human genome

Article References: Landscape and mutational dynamics of G-quadruplexes across the complete human genome. (n.d.). https://doi.org/10.1186/s13059-026-04255-z

Image Credits: AI Generated

DOI: 10.1186/s13059-026-04255-z

Keywords: G-quadruplexes, human genome, telomere-to-telomere assembly, centromeres, ribosomal DNA, genomic instability, DNA methylation, meiotic recombination, PRDM9, long-read sequencing, insertions and deletions, genome biology