B-form DNA
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| B-form DNA | |
|---|---|
| Name | B-form DNA |
| Caption | Canonical right-handed double helix conformation |
| Discovered | 1953 (structure elucidation) |
| Major features | Right-handed helix, ~10 bp per turn, major and minor grooves |
| Handedness | Right-handed |
| Pitch | ~34 Å |
| Rise per base pair | ~3.4 Å |
| Groove major | wide and deep |
| Groove minor | narrow and shallow |
B-form DNA is the canonical right-handed double helix conformation of deoxyribonucleic acid characterized by specific helical parameters and groove geometry. It serves as the predominant structural form under physiological conditions in cells of Homo sapiens, Escherichia coli and many other organisms, and underpins molecular recognition by proteins, enzymes, and small molecules. B-form DNA was central to the work of James Watson, Francis Crick, Rosalind Franklin, and Maurice Wilkins in the mid-20th century and remains a foundational model in structural biology, biochemistry, and molecular genetics.
Structure
The B-form helix is a right-handed duplex with approximately 10 base pairs per turn, a helical pitch near 34 ångström, and a base-pair rise of about 3.4 ångström, parameters that distinguish it from A-form and Z-form helices studied by Linus Pauling and others. Bases lie nearly perpendicular to the helix axis producing a deep, wide major groove and a narrow, shallow minor groove exploited by DNA-binding factors such as Lac repressor, TATA-binding protein, and transcription factors like p53. Backbone geometry features C2'-endo sugar puckering and anti glycosidic bond angles—attributes compared and contrasted in structural analyses by groups at Protein Data Bank, European Molecular Biology Laboratory, and Max Planck Institute laboratories. Crystal and fiber models reveal repeating phosphate–deoxyribose chains whose negative charges are neutralized by cations studied in work at Brookhaven National Laboratory and Lawrence Berkeley National Laboratory.
Physical and chemical properties
Electrostatic repulsion among phosphate groups in the B helix is mitigated by monovalent cations such as Sodium and Potassium and divalent ions like Magnesium ion, which influence stability and melting behavior measured in experiments by teams at Cold Spring Harbor Laboratory and Howard Hughes Medical Institute. UV absorbance at 260 nm, circular dichroism spectra recorded at facilities including National Institutes of Health cores, and thermal denaturation profiles are diagnostic of the B conformation versus alternative forms examined by researchers at University of Cambridge and Massachusetts Institute of Technology. Chemical modifications—methylation catalyzed by enzymes such as DNA methyltransferase 1—and lesions induced by agents investigated at National Cancer Institute alter local geometry and base stacking, affecting protein recognition by complexes like DNA polymerase I and repair assemblies including MutS.
Biological significance and occurrence
B-form geometry underlies replication by replisome assemblies characterized in studies at European Molecular Biology Laboratory and Cold Spring Harbor Laboratory; helicases such as MCM complex and polymerases including DNA polymerase III interact with B-form duplex regions. Transcription initiation by RNA polymerase II and chromatin organization by nucleosome particles containing Histone H3 and Histone H4 juxtapose B-form DNA with protein interfaces mapped by consortia such as ENCODE Project and Human Genome Project. Viral genomes from Bacteriophage lambda and many eukaryotic chromosomes adopt B-like local structures except where sequence-driven bends, repeats like Alu element insertions, or bound factors induce alternative conformations; pathology linked to triplet expansions in diseases studied at National Institute of Neurological Disorders and Stroke often perturbs B-form stability.
Formation and transitions (A-, Z- and other forms)
Transitions among canonical forms are sequence- and environment-dependent: dehydration, high salt, or crystallization favor the A-form documented by Robert Corey and Linus Pauling-era studies, whereas alternating purine–pyrimidine sequences rich in Guanine and Cytosine under negative supercoiling can adopt left-handed Z-form discovered in work associated with Alexander Rich. Proteins such as Z-DNA binding protein 1 recognize Z-form tracts and influence signaling, while transient Hoogsteen base pairs and triple-helical structures noted in studies at Stanford University and University of Oxford exemplify non-B conformers. Superhelical stress imposed by topoisomerases like DNA gyrase and Topoisomerase II promotes local B-to-Z or B-to-A transitions, and chemical conditions employed in laboratory protocols at National Institute of Standards and Technology reproducibly shift equilibria among forms.
Detection and experimental characterization
B-form DNA is identified via X-ray fiber diffraction pioneered in the 1950s at King's College London, circular dichroism measured at spectroscopy cores at Imperial College London, and NMR spectroscopy studies performed at centers such as Bruker and National Magnetic Resonance Facility. Cryo-electron microscopy facilities at European Synchrotron Radiation Facility and Rutherford Appleton Laboratory provide visualization of B-like duplex regions within macromolecular complexes. Footprinting assays developed by Allan Maxam and Walter Gilbert (chemical) and by DNase I experiments popularized at Salk Institute map groove accessibility, while single-molecule techniques from groups at Caltech and University of Chicago—including optical tweezers and magnetic tweezers—quantify mechanical parameters like persistence length and torsional stiffness.
Historical discovery and nomenclature
The structural model of the right-handed double helix widely called the canonical form was articulated by James Watson and Francis Crick following interpretation of X-ray diffraction patterns generated by Rosalind Franklin and Maurice Wilkins at King's College London and contextualized by chemical knowledge from Erwin Chargaff regarding base pairing stoichiometry. The formal nomenclature distinguishing A-, B- and Z-forms emerged from comparative studies throughout the 20th century at institutions including Cold Spring Harbor Laboratory and Brookhaven National Laboratory, and was standardized in classification schemes used by the International Union of Crystallography and the Protein Data Bank.