Nucleic Acid Structures

Core Idea

Nucleic acid structural biology determines the three-dimensional architecture of DNA and RNA molecules and their complexes with proteins, revealing how structure encodes function beyond the primary sequence. DNA adopts distinct helical forms — B-form (the canonical right-handed Watson-Crick duplex), A-form (wider, shorter, adopted by RNA duplexes and DNA-RNA hybrids), and Z-form (left-handed, formed by alternating purine-pyrimidine sequences under high salt) — each with characteristic geometric parameters (rise, twist, groove dimensions) that determine protein recognition and biological activity. RNA structural biology is richer still: single-stranded RNA folds into complex three-dimensional architectures through a hierarchy of structural organization (secondary structure motifs like stems, loops, bulges, and pseudoknots; tertiary structure through long-range interactions, metal coordination, and ribose zippers) that enable catalytic (ribozymes), regulatory (riboswitches), and structural (ribosome) functions. Structural methods (crystallography, cryo-EM, NMR) have revealed the atomic details of major protein-nucleic acid complexes including the ribosome, nucleosome, CRISPR-Cas systems, and transcription factors bound to DNA.

Explainer

Structural biology has historically been protein-centric — the vast majority of structures in the PDB are proteins or protein-ligand complexes. But nucleic acids are structural molecules in their own right, and some of the most important structures in biology are nucleic acids or nucleic acid-protein complexes. Understanding nucleic acid structure requires different principles from protein structure: the backbone is a phosphodiester chain rather than a peptide chain, the monomers (nucleotides) are larger and more structurally homogeneous than amino acids, and the dominant organizing principle is base pairing rather than hydrophobic collapse.

DNA structure is defined by the iconic double helix, but the helix exists in multiple forms depending on sequence, hydration, and the presence of bound proteins. B-DNA — the form discovered by Watson and Crick and predominant under physiological conditions — is a right-handed helix with 10 base pairs per turn, a rise of 3.4 Angstroms per base pair, and a characteristic pattern of a wide major groove (where most transcription factors read the sequence through hydrogen bonding to base edges) and a narrow minor groove (important for minor-groove-binding drugs and AT-rich recognition). A-DNA is a wider, more compact right-handed helix (11 bp/turn, 2.6 A rise) adopted under dehydrating conditions and by all RNA duplexes and DNA-RNA hybrids. Z-DNA is a left-handed helix formed by alternating purine-pyrimidine sequences (especially d(CG) repeats) under high-salt conditions; its biological role remains debated but it has been linked to transcription regulation and immune sensing (the ZBP1 protein recognizes Z-form nucleic acids). These forms are not mere crystallographic curiosities — local transitions between B and A or B and Z forms occur in vivo and affect protein binding, replication, and recombination.

RNA structural biology is dramatically more complex than DNA. While DNA is primarily a double helix that stores information, RNA is a structurally versatile polymer that folds into three-dimensional shapes rivaling proteins in complexity. The key to RNA's structural repertoire is its single-stranded nature and the 2'-OH group on ribose. Single-stranded regions fold back on themselves to form secondary structure — stems (Watson-Crick duplexes), hairpin loops (single-stranded loops capping stems), internal loops, bulges, and multi-way junctions. These secondary structure elements then pack against each other through tertiary interactions: pseudoknots (where nucleotides in a loop pair with a distant region, threading the chain through itself), A-minor motifs (adenines docking into the minor groove of a helix), tetraloop-receptor interactions, ribose zippers, and metal-ion-mediated contacts (Mg2+ ions are essential for RNA tertiary structure). This hierarchical folding produces functional structures — ribozymes that catalyze chemical reactions (the ribosome's peptidyl transferase is an RNA enzyme), riboswitches that sense metabolites and regulate gene expression through conformational change, and structural scaffolds that organize protein assembly (ribosomal RNA provides the architectural framework for the ribosome).

The structural biology of protein-nucleic acid complexes has been revolutionized by cryo-EM, which can image large complexes that resist crystallization. The ribosome structures (Nobel Prize 2009 to Ramakrishnan, Steitz, and Yonath) revealed that the peptidyl transferase center is entirely RNA — confirming the ribosome as a ribozyme and supporting the RNA world hypothesis. Nucleosome structures showed how 147 bp of DNA wrap around the histone octamer in 1.65 left-handed superhelical turns, with sequence-dependent bending and specific histone-DNA contacts that influence gene regulation. CRISPR-Cas structures revealed the molecular basis of RNA-guided DNA recognition and cleavage, directly enabling the rational engineering of genome-editing tools. In each case, the three-dimensional structure provided mechanistic insights that biochemistry alone could not — understanding how a transcription factor distinguishes its target sequence, how the ribosome maintains reading frame, or how Cas9 discriminates on-target from off-target sites requires seeing the atomic architecture of these molecular machines.