DNA and RNA are built from the same basic blueprint: long chains of small chemical units called nucleotides, each containing a sugar, a phosphate group, and a nitrogen-containing base. That shared architecture lets both molecules store and transmit genetic information, but the structural differences between them are what give each its distinct biological role. DNA’s famous double helix is optimized for long-term storage and faithful copying, while RNA folds into an astonishing variety of three-dimensional shapes that let it do everything from carrying messages to catalyzing chemical reactions. The details of how these molecules are shaped, and how that shape changes under different conditions, turn out to be far more complex and interesting than the iconic twisted-ladder image suggests.
The Backbone That Holds It All Together
Every nucleotide in a nucleic acid chain has the same three-part structure: a five-carbon sugar, a phosphate group, and one of a handful of bases that jut out from the chain like teeth on a comb. The phosphate of one nucleotide links to the sugar of the next, forming a repeating sugar-phosphate backbone that gives the molecule its overall shape. This linkage is called a phosphodiester bond, and it creates a chain with a built-in directionality: one end of the strand is chemically different from the other.
The backbone is not a limp noodle. Computational studies of the rotational angles around those phosphodiester bonds show that the backbone naturally favors certain conformations, and one of the most energetically stable of these corresponds to the helical shape we associate with DNA.1PubMed Central. Backbone conformations in secondary and tertiary structural units of nucleic acids. Constraint in the phosphodiester conformation In other words, the tendency of nucleic acids to twist into a helix is not just a product of base pairing; it is partly built into the backbone itself. For deoxyribonucleotides specifically, a particular sugar shape (called the C2′-endo pucker) further strengthens this preference for helical stacking.2Biopolymers. Backbone conformations in deoxyribonucleic acids. Conformational role of the 2′‐hydroxyl group on the internucleotide phosphodiester conformations
DNA’s Double Helix and What Stabilizes It
The most recognizable feature of DNA is that it consists of two strands wound around each other in opposite directions, forming a right-handed double helix. The bases on one strand pair with the bases on the other through hydrogen bonds: adenine pairs with thymine, and guanine pairs with cytosine. These Watson-Crick base pairs are central to how DNA stores genetic information, because the sequence on one strand automatically dictates the sequence on the other.
But hydrogen bonding between bases is only part of what holds the helix together. An analysis of 30 high-resolution DNA crystal structures found that base stacking, the interaction between the flat surfaces of bases piled on top of one another, contributes at least as much stability as hydrogen bonding. Hydrophobic forces and electrostatic interactions play smaller supporting roles.3PubMed. Stability of DNA duplexes with Watson-Crick base pairs: a predicted model This is why single-stranded DNA also has some tendency to stack its bases even without a partner strand to pair with.
The double helix creates two spiral grooves running along its length: the major groove, which is wider, and the minor groove, which is narrower. Many proteins that read or regulate DNA do so by slotting into these grooves, where they can sense the pattern of bases without prying the strands apart. The exact width and depth of these grooves vary depending on the DNA sequence. In stretches rich in guanine-cytosine pairs, the minor groove tends to be wider and deeper than average, while the major groove becomes relatively shallow.4PubMed Central. Double helix conformation, groove dimensions and ligand binding potential of a G/C stretch in B-DNA These variations help explain why certain proteins bind preferentially to certain sequences: the shape of the groove matters as much as the chemical identity of the bases.
A, B, and Z: DNA Does Not Come in Just One Shape
The classic Watson-Crick double helix, called B-DNA, is the form most common under normal cellular conditions. But DNA can adopt other geometries depending on its environment, its sequence, and how much water surrounds it.
A-DNA is a broader, more compressed helix. Its base pairs are tilted and pushed off the central axis, creating a cavernously deep major groove and a very shallow minor groove. Z-DNA, by contrast, is a left-handed helix, twisting in the opposite direction from B-DNA. In Z-DNA, the base pairs are displaced off the axis in the opposite direction from A-DNA, producing a deep minor groove and a major groove so flat it is barely a groove at all.5Methods in Enzymology. DNA structure from A to Z These alternative forms are not just laboratory curiosities. Z-DNA, for instance, tends to form in stretches of alternating guanine and cytosine under certain conditions and has been linked to gene regulation and immune signaling.
Beyond the classic lettered forms, DNA can also adopt non-standard base pairing. In Hoogsteen pairing, one base rotates so that a different face is used for hydrogen bonding than in the standard Watson-Crick arrangement. Hoogsteen pairs can stabilize parallel-stranded DNA duplexes and triple-helix structures, where a third strand winds into one of the grooves and forms additional hydrogen bonds.6PubMed. Parallel-stranded DNA with Hoogsteen base pairing stabilized by a trans-[Pt(NH3)2]2+ cross-link: characterization and conversion into a homodimer and a triplex DNA rich in guanine can also fold into four-stranded structures called G-quadruplexes, which are common at chromosome ends (telomeres) and in regulatory regions of genes. Research into these structures has shown that even the telomeric sequences can adopt multiple conformations, including reverse Hoogsteen hairpins, depending on the ionic environment.7PubMed Central. Evidence for Reverse Hoogsteen Hairpin Intermediates in the Photocrosslinking of Human Telomeric DNA Sequences
What Makes RNA Structurally Different
RNA and DNA differ in two key chemical ways that have enormous structural consequences. First, RNA’s sugar has an extra hydroxyl group at the 2′ position (it is ribose rather than deoxyribose). This makes the RNA backbone slightly more rigid and favors a different sugar pucker, which in turn means that double-stranded RNA adopts a geometry closer to A-form than to B-form DNA. Second, RNA uses uracil where DNA uses thymine. Both bases pair with adenine in the same way, but thymine has an extra methyl group that helps protect DNA from certain types of damage. The thymine-uracil swap is one of the major chemical differences between the two molecules, and cells go to considerable effort to keep uracil out of DNA: specialized enzymes exist whose sole job is to destroy the uracil-containing building block before it can be mistakenly incorporated into DNA strands.8PubMed Central. Keeping uracil out of DNA: physiological role, structure and catalytic mechanism of dUTPases
The extra hydroxyl group in RNA also makes it less chemically stable than DNA, which is one reason cells use DNA for permanent information storage and RNA for more temporary tasks. But that same hydroxyl group, combined with the fact that RNA is usually single-stranded, gives RNA a structural versatility that DNA lacks. A single RNA strand can fold back on itself, forming internal base-paired stems, loops, bulges, and junctions. These secondary structures then fold further into complex three-dimensional shapes.
RNA’s Remarkable Structural Repertoire
While DNA’s secondary structure is dominated by the double helix, RNA inhabits a world of structural variety. Transfer RNA, ribosomal RNA, messenger RNA, and the many classes of small regulatory RNA all fold into specific shapes suited to their jobs. Among the most interesting structural motifs are pseudoknots: arrangements in which a single-stranded loop region base-pairs with a sequence outside its own stem-loop, creating a knot-like topology. Pseudoknots show up across biology, serving as structural scaffolds and playing roles in viral replication, gene expression control, and toxin inactivation.9Communications Biology. Accurate prediction of RNA secondary structure including pseudoknots through solving minimum-cost flow with learned potentials
Some RNA molecules go further, folding into shapes that can catalyze chemical reactions, a feat once thought to be the exclusive domain of protein enzymes. These catalytic RNAs, called ribozymes, have active sites shaped by precise three-dimensional folding. The hammerhead ribozyme, for instance, relies on tertiary contacts far from its active site to position the correct atoms for cleavage of an RNA strand.10PubMed Central. Tertiary contacts distant from the active site prime a ribozyme for catalysis The discovery that RNA can be both information carrier and enzyme was a landmark in biology, lending weight to the idea that early life may have relied on RNA before DNA and proteins took over most of those functions.
How DNA Gets Packed Into Cells
If you stretched out all the DNA in a single human cell, it would extend roughly two meters. Fitting that into a nucleus about six micrometers across requires extraordinary compaction. The solution is a hierarchy of packaging. DNA first wraps around disc-shaped protein complexes called histones, forming structures called nucleosomes. Each nucleosome achieves roughly a sevenfold linear compaction of the DNA wound around it.11PubMed Central. A brief review of nucleosome structure Strings of nucleosomes then coil and loop into progressively higher-order chromatin structures, ultimately producing the dense chromosomes visible during cell division.
This packaging is not just a space-saving trick. Which genes are accessible depends on how tightly the local chromatin is wound. Loosely packed regions tend to be actively transcribed; tightly packed regions are silenced. The cell regulates this by chemically modifying the histone proteins or the DNA itself, changing local structure to expose or hide genes as needed.
Twisting, Unwinding, and the Topology of a Living Helix
DNA in living cells is not a static molecule. Every time a cell copies its DNA or reads a gene, the two strands must be pulled apart locally. Because the strands are wound around each other, separating them at one point creates torsional stress elsewhere, like twisting a rubber band from one end. This stress takes the form of supercoiling: the double helix itself coils into a higher-order spiral.12PubMed Central. The importance of being supercoiled: how DNA mechanics regulate dynamic processes
During replication, positive supercoils pile up ahead of the replication fork as the machinery unwinds the helix. Left unchecked, this would quickly stall the process. Cells employ enzymes called topoisomerases that cut, rotate, and rejoin the DNA to relieve this tension. The interplay between supercoiling, knotting, and interlinking of daughter DNA strands creates a topological puzzle that cells must solve every time they divide.13Nucleic Acids Research. Closing the DNA replication cycle: from simple circular molecules to supercoiled and knotted DNA catenanes
Transcription, the process of reading a gene to produce an RNA copy, introduces its own structural drama. The enzyme RNA polymerase pries open a small region of the double helix called the transcription bubble, where the two strands are temporarily separated. Single-molecule experiments have revealed that this bubble is not a fixed, rigid opening: its apparent length and width fluctuate on the millisecond timescale, with the DNA strands sampling multiple conformations.14PubMed Central. The Transcription Bubble of the RNA Polymerase–Promoter Open Complex Exhibits Conformational Heterogeneity and Millisecond-Scale Dynamics: Implications for Transcription Start-Site Selection As the polymerase moves along the gene, both the protein and the nucleotides undergo conformational changes with each step of RNA synthesis.15PubMed Central. Elucidation of the Dynamics of Transcription Elongation by RNA Polymerase II using Kinetic Network Models
Chemical Modifications That Reshape RNA
The structure of RNA does not end with its sequence. After an RNA molecule is transcribed, cells can chemically modify individual nucleotides in ways that alter the molecule’s shape and behavior. More than 170 types of RNA modification have been identified, and a growing field called epitranscriptomics studies how these marks regulate gene expression. One of the most common is N6-methyladenosine, or m6A, in which a methyl group is added to adenine. This single modification weakens the standard base pairing between adenine and uracil while stabilizing non-standard pairings, changing the local folding of the RNA and exposing or hiding binding sites for proteins.16Life Science Alliance. Interconnections between m6A RNA modification, RNA structure, and protein–RNA complex assembly
A dedicated database now catalogs over 880,000 RNA modification sites across nine species, documenting how modifications like m6A, pseudouridine, and adenosine-to-inosine editing each alter RNA secondary structure in predictable ways.17PubMed Central. StructRMDB: A database of RNA modification sites that affect RNA secondary structure The implication is that the “structure” of an RNA molecule is not determined by its sequence alone. It is continuously tuned by enzymes that add, remove, and read chemical marks, giving cells a fast, reversible way to control RNA function without changing the underlying genetic code.
When Structure Goes Wrong: DNA Damage
Ultraviolet light, reactive chemicals, and even normal metabolic byproducts can damage DNA in ways that distort its structure. One of the best-studied examples is the thymine dimer, a lesion in which UV light causes two adjacent thymine bases to fuse together. A crystal structure of a DNA strand containing such a dimer showed that the helix bends about 30 degrees toward the major groove and unwinds by roughly 9 degrees at the damage site. The minor groove pinches on one side of the lesion, and both grooves widen on the other side.18PubMed Central. Crystal structure of a DNA decamer containing a cis-syn thymine dimer
While the overall architecture of the helix is preserved, chemical probing confirms that the region near the dimer is locally destabilized, and there are hints that the whole duplex becomes slightly more accessible to the surrounding solution, as if the equilibrium between double-stranded and single-stranded forms shifts subtly.19PubMed Central. Thymine dimer-induced structural changes to the DNA duplex examined with reactive probes Cells detect and repair these distortions using specialized enzymes that essentially recognize the abnormal shape of the helix at the damage site. Understanding these structural changes is central to understanding how repair mechanisms work, and why some types of damage are more mutagenic than others.
How Viruses Exploit Nucleic Acid Structure
Viruses offer a fascinating case study in nucleic acid packaging and structural economy. Viral genomes can be double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, and the way each type is packed into its protein shell (capsid) depends heavily on the structural properties of the nucleic acid. Single-stranded genomes are highly flexible and can co-assemble with capsid proteins cooperatively, with the genome adapting to the internal symmetry of the shell. Double-stranded DNA viruses, being much stiffer, typically take a different approach: a molecular motor pumps the genome into a preformed capsid under considerable pressure.20PubMed Central. Nucleic acid packaging in viruses
The structural properties of the genome may even influence how the capsid assembles in the first place. Modeling studies have shown that the secondary and tertiary structure of single-stranded RNA, with its branched, tree-like folding patterns, can promote more efficient capsid assembly than a simple linear polymer would. The inherent base-paired regions of the RNA create a compact, branched shape that nucleates capsid formation more readily.21eLife. Viral genome structures are optimal for capsid assembly In short, the virus’s protein coat and its genome have co-evolved structurally, each shaping the other.
Seeing the Structures: How Scientists Study Nucleic Acids
Much of what we know about nucleic acid structure comes from a handful of powerful experimental techniques. X-ray crystallography provided the first view of the double helix in the 1950s. The famous “Photo 51” image taken by Rosalind Franklin revealed the characteristic X-shaped diffraction pattern of a helix, and from that pattern researchers extracted the key dimensions: a pitch (one full turn) of 3.4 nanometers and a radius of about 1 nanometer.22American Journal of Physics. Rosalind Franklin’s X-ray photo of DNA as an undergraduate optical diffraction experiment Crystallography remains indispensable for capturing nucleic acids at atomic resolution, though it requires coaxing molecules into an orderly crystal, which is not always possible.
Nuclear magnetic resonance (NMR) spectroscopy offers a complementary view, capturing molecular structures in solution rather than in a crystal. This is especially valuable for RNA, whose flexible, dynamic structures may not survive the crystallization process. More recently, cryo-electron microscopy has emerged as a way to image large nucleic acid complexes without crystals. In one study, cryo-EM and NMR were combined to determine the structure of an HIV-1 RNA dimer at a resolution where major groove features and superhelical twisting were clearly visible.23Structure. Atomic Model of the HIV-1 Dimer Initiation Site Determined by Cryo-EM and NMR Spectroscopy These hybrid approaches, fitting NMR-derived solution structures into cryo-EM maps, are increasingly used to study the assembly and dynamics of large biological machines that include nucleic acids.24PubMed. Structures of biomolecular complexes by combination of NMR and cryoEM methods
DNA as a Building Material
The same base-pairing rules that encode genetic information can be harnessed to build things. In the field of DNA nanotechnology, researchers design synthetic DNA strands whose sequences are chosen so that they self-assemble into predetermined two- and three-dimensional shapes. The most prominent technique, called DNA origami, uses a long single strand as a scaffold and hundreds of shorter “staple” strands that fold it into a target shape, from flat tiles to boxes to curved shells.25PubMed Central. Concepts and Application of DNA Origami and DNA Self-Assembly: A Systematic Review
Recent advances have pushed this field toward modular design, where subunits consisting of a constant core with variable bonding and angle modules can be mixed and matched to produce structures with different curvatures, including flat sheets, hollow spheres, and tubes.26ACS Nano. Economical and Versatile Subunit Design Principles for Self-Assembled DNA Origami Structures Simulation and modeling work has clarified the physical principles behind this self-assembly, showing how cooperativity and nucleation barriers can be tuned to ensure that structures form correctly rather than getting trapped in misfolded states.27PubMed. Models and simulations of structural DNA nanotechnology reveal fundamental principles of self-assembly Potential applications range from targeted drug delivery vehicles to molecular-scale electronic components, all built from a molecule that evolution optimized for a very different purpose.
RNA Structure and the Rise of mRNA Therapeutics
The structural properties of RNA have become medically relevant in a very direct way with the development of mRNA vaccines and therapeutics. Delivering a synthetic mRNA molecule into a cell so that it can produce a desired protein requires solving a structural problem: naked mRNA is quickly degraded and cannot easily cross cell membranes. The solution that enabled the COVID-19 mRNA vaccines involves wrapping the RNA in lipid nanoparticles, tiny fat-based shells whose composition is carefully engineered to protect the cargo, facilitate cellular uptake, and release the mRNA inside the cell.28Nature Reviews Materials. Lipid nanoparticles for mRNA delivery
The mRNA itself is also structurally optimized. Researchers modify the nucleotides (for example, replacing uridine with pseudouridine) to reduce the immune system’s tendency to destroy foreign RNA, and they engineer the sequence to fold in ways that enhance stability and translation efficiency. The structural modifications discussed earlier, like those cataloged in epitranscriptomics databases, are not just curiosities of basic biology: they are now tools that vaccine designers use deliberately to control how long an mRNA lasts and how much protein it produces. The entire field rests on decades of foundational work on how nucleic acid structure determines function.