What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

The Double Helix Shape: DNA's Structure and Function

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to the Double Helix Shape

The double helix is the three-dimensional conformation adopted by deoxyribonucleic acid (DNA) in its most common biological state. It consists of two polynucleotide strands wound around each other in a right-handed spiral, with the sugar-phosphate backbones forming the exterior and the nitrogenous bases stacked in the interior. This architecture is not merely aesthetic; it is the structural solution that permits stable storage of genetic information, faithful replication, and regulated access to that information during transcription.

What is a Double Helix?

A helix is a curve in three-dimensional space that turns around an axis at a constant angle. When two such helices coil around the same axis, they form a double helix. In DNA, the two strands are antiparallel—one runs in the 5′ to 3′ direction, the other in the 3′ to 5′ direction—and are held together by hydrogen bonds between complementary bases. The entire structure repeats every 10 base pairs along its length, completing one full turn of the helix every 3.4 nanometers. The diameter of the helix is a remarkably constant 2.0 nanometers, a value dictated by the geometry of base pairing.

The double helix is often compared to a twisted ladder, but this analogy fails to capture an essential feature: the two strands are not identical. They are complementary. Where one strand has an adenine, the other has a thymine; where one has a guanine, the other has a cytosine. This complementarity is the basis of both information storage and replication. For a more detailed treatment of how the double helix differs from other nucleic acid conformations, see Single vs Double Helix.

Why Shape Matters

The shape of DNA is not incidental. Every functional interaction with the genome—whether by polymerases, transcription factors, or histone proteins—depends on the three-dimensional presentation of chemical groups. The double helix creates two grooves of unequal width, the major and minor grooves, which expose different edges of the base pairs to solvent and protein partners. These grooves are the primary reading surfaces for sequence-specific DNA-binding proteins. The shape also determines how tightly DNA can be packed: the helical pitch and diameter dictate the geometry of nucleosome formation, the fundamental unit of chromatin.

Furthermore, the double helix is a dynamic structure. It can bend, twist, and undergo local transitions to alternative conformations. These deformations are biologically exploited during replication and transcription, where the helix must be locally unwound. Understanding the double helix shape is therefore prerequisite to understanding virtually all of molecular biology.

Historical Discovery of the Double Helix

The elucidation of the DNA double helix in 1953 was the culmination of decades of biochemical and biophysical investigation. It required the convergence of chemical knowledge about DNA composition, X-ray diffraction data of exceptional quality, and model-building informed by stereochemical principles.

X-ray Diffraction Evidence

The critical experimental evidence came from Rosalind Franklin and Raymond Gosling at King's College London. Franklin obtained X-ray diffraction patterns of DNA fibers that were significantly better than any previously recorded. The famous "Photo 51," taken in May 1952, showed a clear cross-shaped pattern of reflections characteristic of a helical molecule. The positions of the diffraction spots allowed Franklin to calculate key parameters: the helical pitch was 3.4 nanometers, the repeat distance along the helix axis was 0.34 nanometers per base, and the diameter was approximately 2.0 nanometers.

Franklin also distinguished two forms of DNA: the A-form, which occurs at lower relative humidity (around 75%), and the B-form, which occurs at higher humidity (around 92%). The B-form, which is the physiologically relevant conformation, produced the diffraction pattern that unambiguously indicated a helix. Franklin's data also revealed that the sugar-phosphate backbone must be on the outside of the molecule, with the bases on the inside, because the hydrophobic bases would otherwise not be shielded from water.

The Watson-Crick Model

James Watson and Francis Crick at the University of Cambridge integrated Franklin's diffraction data with Chargaff's rules—the observation that in any DNA molecule, the amount of adenine equals thymine and the amount of guanine equals cytosine—and with known bond lengths and angles from organic chemistry. Their model, published in Nature in April 1953, proposed a right-handed double helix with antiparallel strands, specific base pairing (A with T, G with C), and a sugar-phosphate backbone on the exterior.

The model explained Chargaff's rules mechanistically: adenine forms two hydrogen bonds with thymine, while guanine forms three with cytosine. This pairing scheme also explained why the helix diameter is constant: a purine (A or G) always pairs with a pyrimidine (T or C), keeping the base pair width uniform. The Watson-Crick model was not immediately accepted—some researchers doubted the central role of hydrogen bonding—but it was rapidly confirmed by subsequent work, including the determination of the first high-resolution crystal structures of DNA oligonucleotides in the 1980s. For a concise definition of the key terms, see Double Helix Definition.

Molecular Components of the Double Helix

Nucleotide Structure

DNA is a polymer of nucleotides, each composed of three parts: a phosphate group, a five-carbon sugar (2′-deoxyribose), and a nitrogenous base. The sugar is called 2′-deoxyribose because it lacks a hydroxyl group at the 2′ carbon, a distinction from ribose in RNA. The phosphate group is attached to the 5′ carbon of the sugar, and the base is attached to the 1′ carbon via a β-N-glycosidic bond.

There are four bases in DNA, divided into two classes. The purines—adenine (A) and guanine (G)—have a fused two-ring structure. The pyrimidines—cytosine (C) and thymine (T)—have a single six-membered ring. The bases are planar molecules, and their orientation relative to the sugar is either anti or syn. In B-DNA, all bases adopt the anti conformation, which positions the hydrogen-bonding edges outward.

Nucleotides are linked by phosphodiester bonds between the 3′ hydroxyl of one sugar and the 5′ phosphate of the next. This creates a sugar-phosphate backbone with an intrinsic directionality: the 5′ end has a free phosphate, and the 3′ end has a free hydroxyl. The backbone is highly negatively charged due to the ionized phosphate groups, which are neutralized by counterions such as Mg²⁺ and by basic residues of DNA-binding proteins.

Base Pairing Rules

The double helix is held together by hydrogen bonds between bases on opposite strands. The rules are strict: adenine pairs with thymine, and guanine pairs with cytosine. A–T pairs form two hydrogen bonds (between the N1 of adenine and N3 of thymine, and between the N6 amino group of adenine and the O4 of thymine). G–C pairs form three hydrogen bonds (between the O6 and N1 of guanine with the N4 and N3 of cytosine, respectively, plus a third between the N2 amino group of guanine and the O2 of cytosine).

The energetic difference between two and three hydrogen bonds has consequences: A–T-rich regions of DNA are easier to melt (separate into single strands) than G–C-rich regions. This is exploited in polymerase chain reaction (PCR), where the melting temperature of a primer is calculated based on its GC content. A typical PCR annealing step is performed at 55–65°C, with the exact temperature determined by the formula Tₘ = 4(GC) + 2(AT) for short primers.

The base pairs are not perpendicular to the helix axis; they are tilted by about 6° relative to the axis. This tilt, along with the propeller twist between the two bases in a pair, contributes to the overall geometry of the helix. The base pairs are also stacked on top of each other with an average separation of 0.34 nm, which is the rise per base pair along the helix axis.

Three-Dimensional Architecture of the Helix

Helical Parameters

B-DNA, the standard form under physiological conditions (approximately 150 mM salt, pH 7, 37°C), has well-defined geometric parameters. The helix is right-handed, meaning that if you look down the axis, the strands turn clockwise as they move away from you. There are 10.5 base pairs per turn, giving a helical pitch of 3.57 nm (the distance along the axis for one complete turn). The rise per base pair is 0.34 nm, and the diameter is 2.0 nm.

The two strands are not symmetrically arranged around the axis. Instead, the sugar-phosphate backbones are closer together on one side of the helix, creating a narrow minor groove, and farther apart on the other side, creating a wide major groove. The major groove is 2.2 nm wide and 0.85 nm deep, while the minor groove is 1.2 nm wide and 0.75 nm deep. These dimensions vary slightly with sequence, as certain base steps (e.g., AA/TT) narrow the minor groove due to increased propeller twist.

The helix axis is not perfectly straight; it can bend, particularly at A-tracts (runs of 4–6 adenine residues). This intrinsic curvature is important for DNA wrapping around histone proteins in chromatin. The flexibility of DNA is quantified by its persistence length, which is approximately 50 nm (about 150 base pairs). Over distances shorter than the persistence length, DNA behaves as a rigid rod; over longer distances, it behaves as a flexible polymer.

Major and Minor Grooves

The grooves are the most functionally significant features of the double helix. Because the base pairs are not centered on the helix axis, the edges of the bases are exposed in the grooves. The major groove exposes the N7 and C6 positions of purines and the C4 and C5 positions of pyrimidines. The minor groove exposes the N3 of purines and the O2 of pyrimidines.

The patterns of hydrogen bond donors and acceptors in the grooves are unique for each base pair. In the major groove, an A–T pair presents a different pattern than a G–C pair, allowing sequence-specific DNA-binding proteins to "read" the sequence without unwinding the helix. For example, the transcription factor p53 binds to a consensus sequence of 20 base pairs by making contacts primarily in the major groove. The minor groove is narrower and generally less information-rich, but it is the binding site for many small molecules, including the antibiotic netropsin, which binds preferentially to A–T-rich regions.

The grooves also influence DNA-drug interactions. The chemotherapeutic agent cisplatin forms intrastrand crosslinks by binding to the N7 positions of adjacent guanines, which are accessible in the major groove. Understanding groove geometry is therefore essential for rational drug design. For a deeper comparison of the two grooves and their functional roles, see Double Helix Structure.

Stability of the Double Helix

The double helix is a stable structure under physiological conditions, but its stability is the result of a delicate balance of opposing forces. The primary stabilizing forces are hydrogen bonding between bases, base stacking interactions, and the hydrophobic effect. The primary destabilizing force is the electrostatic repulsion between negatively charged phosphate groups.

Hydrogen Bonding

Hydrogen bonds between complementary bases contribute to the specificity of base pairing but contribute relatively little to overall stability. Each hydrogen bond contributes approximately 1–2 kcal/mol of favorable free energy. A 10-base-pair duplex with 50% GC content would have roughly 25 hydrogen bonds, contributing about 25–50 kcal/mol. However, hydrogen bonds also form between bases and water, so the net contribution of inter-base hydrogen bonds to stability is modest.

The specificity of hydrogen bonding is what ensures correct base pairing. A mismatched pair, such as A–C or G–T, cannot form the same number of hydrogen bonds with the correct geometry. The DNA polymerase proofreading machinery exploits this: when a misincorporated nucleotide is detected, the polymerase pauses and the error is corrected by a 3′→5′ exonuclease activity. The error rate of DNA replication in E. coli is approximately 10⁻⁹ per base pair, a fidelity that depends on both base-pairing specificity and proofreading.

Base Stacking and Hydrophobic Forces

The dominant stabilizing force in the double helix is base stacking. The planar aromatic bases stack on top of each other in the interior of the helix, with their planes approximately parallel. Stacking is driven by van der Waals interactions between the π-electron systems of adjacent bases and by the hydrophobic effect: the bases are nonpolar and are excluded from water, so burying them in the helix interior is thermodynamically favorable.

The stacking free energy varies with sequence. G–C stacks are generally more stable than A–T stacks, partly because the larger dipole of guanine enhances van der Waals contacts. The stacking energy of a single base step is approximately 2–4 kcal/mol, which is larger than the contribution of hydrogen bonds. This is why DNA melting (denaturation) is primarily a function of base stacking, not hydrogen bond breaking.

The hydrophobic effect also drives the exclusion of water from the interior of the helix. The sugar-phosphate backbone is hydrophilic and faces the solvent, while the bases are buried. This arrangement is entropically favorable because it releases ordered water molecules from the hydrophobic surfaces of the bases. The overall stability of the double helix is therefore enthalpy-driven at low temperatures but becomes entropy-driven at higher temperatures, where the hydrophobic effect dominates.

The electrostatic repulsion between phosphate groups is a destabilizing force. In physiological salt concentrations (approximately 150 mM NaCl), this repulsion is screened by counterions. Divalent cations such as Mg²⁺ are particularly effective at stabilizing the helix because they bind to the phosphate groups and neutralize their charge. In the absence of salt, DNA denatures at much lower temperatures; the melting temperature of a 50% GC DNA in pure water is approximately 20°C lower than in 150 mM NaCl.

Alternative DNA Shapes and Conformations

The double helix is not a static, invariant structure. Depending on sequence, hydration, and ionic conditions, DNA can adopt several alternative conformations. These non-B forms are biologically relevant, as they can influence gene expression, replication, and recombination.

A-DNA and Z-DNA

A-DNA is a right-handed helix that forms under conditions of low humidity (75% relative humidity) or in DNA–RNA hybrids. It is wider and flatter than B-DNA, with 11 base pairs per turn and a rise of 0.26 nm per base pair. The base pairs in A-DNA are tilted by about 20° relative to the helix axis, and the sugar pucker is C3′-endo rather than the C2′-endo found in B-DNA. A-DNA has a deep major groove and a shallow minor groove. Although A-DNA is not the predominant form in vivo, RNA duplexes and DNA–RNA hybrids adopt A-form geometry, which is important for transcription.

Z-DNA is a left-handed helix, the most dramatic departure from B-DNA. It forms in alternating purine-pyrimidine sequences, particularly (GC)n repeats, under conditions of high salt or negative supercoiling. Z-DNA has 12 base pairs per turn, a rise of 0.38 nm, and a zigzag backbone that gives it its name. The guanine residues adopt the syn conformation, while cytosines remain anti. Z-DNA is recognized by specific proteins, including the RNA editing enzyme ADAR1, which contains a Z-DNA binding domain. The biological function of Z-DNA is still debated, but it is thought to play a role in transcription regulation and in the response to viral infection.

Cruciforms and Triplex DNA

Inverted repeat sequences can form cruciform structures, in which the two strands of the double helix separate and re-pair intramolecularly to form hairpin loops. Cruciforms are stabilized by negative supercoiling, which is generated by transcription and replication. They have been implicated in the formation of DNA breaks and in the regulation of gene expression at certain loci.

Triplex DNA (H-DNA) forms when a third strand binds in the major groove of a duplex, forming Hoogsteen base pairs. This requires a homopurine-homopyrimidine sequence, such as (GAA)n or (CTT)n. Triplex formation is pH-dependent, as the third strand requires protonation of cytosine to form C+·G–C triplets. H-DNA can form under physiological conditions in supercoiled plasmids and has been found at specific loci in the human genome, including the promoter of the c-myc oncogene. Triplex-forming oligonucleotides are being explored as tools for genome editing and gene regulation.

The existence of these alternative conformations underscores a key point: the double helix shape is a dynamic equilibrium, not a fixed structure. For a broader discussion of how these forms compare, see Double Helix Series.

Methods Used to Study the Double Helix

X-ray Crystallography

X-ray crystallography remains the gold standard for determining DNA structure at atomic resolution. The method requires growing crystals of DNA oligonucleotides, typically 6–20 base pairs in length, and then exposing them to a beam of X-rays. The diffraction pattern is recorded and used to reconstruct the electron density map, from which the positions of individual atoms are determined.

The first high-resolution crystal structure of a DNA duplex was solved by Richard Dickerson and Horace Drew in 1981, using the dodecamer sequence CGCGAATTCGCG. This structure, known as the "Dickerson-Drew dodecamer," confirmed the B-form geometry and revealed the sequence-dependent variations in groove width and base pair parameters. Since then, thousands of DNA structures have been deposited in the Protein Data Bank, including structures of DNA bound to proteins, drugs, and modified bases.

Crystallography requires that the DNA be in a crystalline lattice, which can introduce packing artifacts. However, the method provides the most detailed information about hydrogen bonding, base stacking, and backbone conformation. Modern crystallography can achieve resolutions of 1.0–2.0 Å, sufficient to visualize individual water molecules in the hydration shell.

NMR and AFM

Nuclear magnetic resonance (NMR) spectroscopy is complementary to crystallography. It is performed on DNA in solution, avoiding the need for crystals. NMR provides information about the local conformation of each nucleotide, including sugar pucker, glycosidic bond angle, and base pair geometry. The method is limited to relatively small DNA molecules (up to about 30 base pairs) due to spectral overlap, but it can capture dynamic information, such as the rates of base pair opening and sugar repuckering.

Atomic force microscopy (AFM) is a single-molecule technique that images DNA on a surface. AFM can visualize individual DNA molecules at nanometer resolution, revealing their overall shape, bends, and loops. It is particularly useful for studying DNA–protein interactions, such as the wrapping of DNA around nucleosomes or the looping of DNA by transcription factors. AFM can also measure the mechanical properties of DNA, including its stiffness and response to stretching.

Other methods include circular dichroism (CD) spectroscopy, which distinguishes between B-, A-, and Z-DNA based on their characteristic spectra, and fluorescence resonance energy transfer (FRET), which measures distances between labeled positions on the DNA. Each method has its strengths and limitations, and a complete picture of DNA structure often requires combining several approaches.

Functional Implications of the Double Helix

Replication and Transcription

The double helix is perfectly suited to its two primary functions: storing information and transmitting it. The antiparallel, complementary strands mean that each strand can serve as a template for the synthesis of its partner. During replication, the enzyme helicase unwinds the double helix at the origin of replication, creating a replication fork. The enzyme DNA polymerase then synthesizes new strands complementary to each template, reading the template in the 3′ to 5′ direction and synthesizing the new strand in the 5′ to 3′ direction.

The antiparallel arrangement creates a challenge: one strand (the leading strand) is synthesized continuously, while the other (the lagging strand) is synthesized in short Okazaki fragments that are later ligated. The double helix shape also necessitates the action of topoisomerases, which relieve the torsional stress generated by unwinding. In E. coli, DNA gyrase introduces negative supercoils ahead of the replication fork, facilitating strand separation.

Transcription similarly requires local unwinding of the double helix. RNA polymerase binds to the promoter, melts approximately 13 base pairs of DNA to form the transcription bubble, and then translocates along the template strand, synthesizing RNA. The unwound region is short-lived; the DNA re-anneals behind the polymerase. The double helix therefore must be stable enough to maintain the genome but flexible enough to allow transient unwinding.

Chromatin Packaging

The double helix must also be compacted to fit inside the cell. A human cell contains approximately 2 meters of DNA, which must fit into a nucleus of about 6 micrometers in diameter. This is achieved through hierarchical packaging. The first level is the nucleosome, in which 147 base pairs of DNA wrap around an octamer of histone proteins (two each of H2A, H2B, H3, and H4) in 1.65 left-handed superhelical turns.

The wrapping of DNA around the nucleosome requires bending the double helix. This is facilitated by the intrinsic flexibility of DNA and by sequence-dependent features: A–T-rich sequences are more easily bent into the minor groove, while G–C-rich sequences are more easily bent into the major groove. The nucleosome positions are therefore influenced by the underlying DNA sequence, with certain sequences, such as the 5S rRNA gene, having strong positioning signals.

Higher-order packaging involves the folding of nucleosome arrays into a 30-nanometer fiber, which is then further compacted into loops and domains. The double helix shape is preserved throughout these levels of compaction; the DNA is never fully unwound except during replication and transcription. The shape of the double helix is thus not only a chemical fact but also a functional requirement for the organization of the genome.

Common Misconceptions and Pitfalls

Antiparallel Strands

A frequent error is to draw or imagine the two strands of the double helix as parallel. They are antiparallel: one strand runs 5′ to 3′ in one direction, and the other runs 5′ to 3′ in the opposite direction. This is not a trivial detail; it is essential for base pairing geometry and for the mechanism of DNA synthesis. DNA polymerases can only synthesize DNA in the 5′ to 3′ direction, and the antiparallel arrangement means that the two new strands at a replication fork are synthesized by different mechanisms (continuous versus discontinuous).

A related misconception is that the two strands are identical. They are complementary, not identical. If one strand has the sequence 5′-ATGC-3′, the other has 5′-GCAT-3′ (reading in the same 5′ to 3′ direction). This complementarity is the basis of the genetic code and of all molecular biology techniques that use hybridization, such as Southern blotting and PCR.

Major vs Minor Groove Functions

Students often confuse the major and minor grooves, either by their dimensions or by their functions. The major groove is wider (2.2 nm) and shallower (0.85 nm) than the minor groove (1.2 nm wide, 0.75 nm deep). The major groove is the primary site of sequence-specific protein binding because it presents a richer pattern of hydrogen bond donors and acceptors. The minor groove is narrower and is the site of binding for many small molecules and for proteins that recognize A–T-rich sequences.

A common error is to think that the grooves are merely structural features with no functional significance. In fact, the grooves are the "reading surface" of DNA. Transcription factors such as the TATA-binding protein (TBP) bind in the minor groove, while most sequence-specific factors, such as the lac repressor, bind in the major groove. The grooves also influence DNA bending: minor groove compression is associated with bending toward the major groove, and vice versa.

Another pitfall is the assumption that the double helix is always right-handed. While B-DNA and A-DNA are right-handed, Z-DNA is left-handed. The handedness is not a trivial property; it affects the accessibility of the grooves and the binding of proteins. The transition from B-DNA to Z-DNA is a major conformational change that can be induced by negative supercoiling, and it has been observed in vivo at specific genomic loci.

Frequently Asked Questions

Is double helix a shape?

Yes, the double helix is a three-dimensional shape. It is the conformation adopted by two antiparallel polynucleotide strands that wind around a common axis. The shape is characterized by a constant diameter, a regular helical pitch, and the presence of major and minor grooves. The double helix is not a molecule itself; it is the structure that DNA adopts under physiological conditions.

What does the double helix shape look like?

The double helix looks like a twisted ladder or a spiral staircase. The sugar-phosphate backbones form the two rails of the ladder, and the base pairs form the rungs. The entire structure is twisted into a right-handed spiral, with one complete turn every 10.5 base pairs. The two grooves—the major and minor—run along the length of the helix, spiraling around it.

Why is DNA a double helix?

DNA is a double helix because this structure satisfies several simultaneous requirements. The double-stranded arrangement allows for complementary base pairing, which is essential for accurate replication. The helical twist allows the base pairs to stack efficiently in the interior, maximizing van der Waals contacts and hydrophobic stabilization. The grooves provide access for proteins to read the sequence. No other conformation satisfies all these constraints as effectively.

Who discovered the double helix shape of DNA?

James Watson and Francis Crick proposed the double helix model in 1953, based on X-ray diffraction data collected by Rosalind Franklin and Raymond Gosling, as well as on Chargaff's rules and chemical knowledge. Franklin's X-ray diffraction patterns, particularly Photo 51, were critical for determining the helical parameters. Watson, Crick, and Maurice Wilkins shared the Nobel Prize in Physiology or Medicine in 1962; Franklin had died in 1958 and was not included.

What are the grooves in the double helix?

The grooves are the two spiral indentations that run along the length of the double helix. The major groove is wider and shallower; the minor groove is narrower and deeper. They arise because the base pairs are not centered on the helix axis. The grooves expose the edges of the bases, allowing proteins to read the DNA sequence without unwinding the helix. The major groove is the primary site of sequence-specific protein binding.

Is the double helix left-handed or right-handed?

The most common form of DNA, B-DNA, is right-handed. This means that if you look down the helix axis, the strands turn clockwise as they move away from you. A-DNA is also right-handed. However, Z-DNA is left-handed, with a zigzag backbone. The handedness of the helix affects the geometry of the grooves and the interactions with proteins.

What holds the two strands of the double helix together?

The two strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (three hydrogen bonds). The strands are also stabilized by base stacking interactions, in which the planar aromatic bases stack on top of each other in the helix interior, and by the hydrophobic effect, which excludes water from the interior. The overall stability is a balance of these forces against the electrostatic repulsion of the phosphate backbones.

Key Takeaways

The double helix is the three-dimensional structure of DNA, consisting of two antiparallel, complementary strands wound around a common axis in a right-handed spiral.
The structure was elucidated in 1953 by Watson and Crick, building on X-ray diffraction data from Rosalind Franklin and on Chargaff's base-pairing rules.
DNA is composed of nucleotides, each containing a phosphate group, a 2′-deoxyribose sugar, and one of four nitrogenous bases: adenine, thymine, guanine, or cytosine.
Base pairing is specific: A pairs with T (two hydrogen bonds), and G pairs with C (three hydrogen bonds). This specificity is the basis of replication and information transfer.
The double helix is stabilized primarily by base stacking and hydrophobic effects, with hydrogen bonding providing specificity rather than the dominant energetic contribution.
B-DNA, the physiological form, has 10.5 base pairs per turn, a diameter of 2.0 nm, and distinct major and minor grooves that serve as protein-binding surfaces.
Alternative conformations, including A-DNA and left-handed Z-DNA, exist and are biologically relevant under specific conditions or at specific sequences.
The double helix is dynamic: it bends, unwinds, and undergoes local transitions to enable replication, transcription, and packaging into chromatin.