double helix
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| double helix | |
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| Name | double helix |
| Caption | Artistic representation of the B-DNA form, showing major and minor grooves. |
| Width | 220 |
double helix is the iconic structural form of deoxyribonucleic acid (DNA), the molecule that carries the genetic instructions for life. This elegant, twisted-ladder shape was first described in detail by James Watson and Francis Crick in 1953, based in part on the X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins. The structure's complementary base pairing immediately suggested a mechanism for genetic replication, revolutionizing the fields of molecular biology and genetics.
Structure and geometry
The canonical B-DNA form features two long polynucleotide strands coiled around a common axis, forming a right-handed helix with a diameter of approximately 20 ångströms. The sugar-phosphate backbones run antiparallel, with one strand oriented 5' to 3' and the other 3' to 5', creating major and minor grooves that are critical for protein recognition. Key parameters include a helical rise of about 3.4 ångströms per base pair and a complete turn every 10-10.5 base pairs, as characterized by researchers like Wilhelm His and later refined through techniques such as X-ray crystallography and nuclear magnetic resonance spectroscopy. The precise geometry allows for specific interactions with proteins like histones in chromatin and enzymes such as RNA polymerase during transcription.
Discovery and history
The elucidation of the structure was a pivotal moment in 20th-century science, culminating in the 1953 publication in the journal Nature by James Watson and Francis Crick of the University of Cambridge. Their model was heavily informed by "Photo 51," an X-ray diffraction image obtained by Rosalind Franklin at King's College London, work overseen by John Randall and shared without her full knowledge by Maurice Wilkins. Earlier foundational work came from Erwin Chargaff, who established the base pairing rules, and Linus Pauling, who proposed an incorrect triple-helix model for DNA. The discovery was recognized with the 1962 Nobel Prize in Physiology or Medicine awarded to Watson, Crick, and Wilkins.
Biological significance
The structure provides the physical basis for the storage and transmission of genetic information in organisms from bacteria like Escherichia coli to complex eukaryotes. Its complementary base pairing enables semi-conservative replication, a process demonstrated by the Meselson-Stahl experiment and carried out by the DNA polymerase complex. The double helix is packaged into chromosomes within the nucleus, with its sequence serving as the template for messenger RNA synthesis, linking it directly to the Central Dogma of Molecular Biology. This architecture is fundamental to processes studied at institutions like the Cold Spring Harbor Laboratory and the European Molecular Biology Laboratory.
Chemical composition and bonding
Each strand is a polymer of nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). The strands are held together by hydrogen bonds between complementary base pairs—A with T (forming two bonds) and G with C (forming three bonds)—a specificity first noted by Erwin Chargaff. Covalent phosphodiester bonds link the sugar and phosphate groups to form the backbone, while van der Waals forces and base stacking interactions between adjacent aromatic rings provide additional stability to the helical structure.
Alternative DNA structures
Under specific conditions, DNA can adopt conformations other than the standard B-DNA. A-DNA, a shorter, wider right-handed helix, is favored in low-humidity environments and was observed in early X-ray studies by Rosalind Franklin. Z-DNA is a left-handed helix with a zigzag backbone, associated with regions of alternating purine-pyrimidine sequences and implicated in gene regulation, as researched by Alexander Rich at the Massachusetts Institute of Technology. Other non-canonical forms include G-quadruplex structures, which can form in guanine-rich sequences, and triplex DNA, investigated in contexts like the Human Genome Project.
In popular culture and symbolism
The double helix has transcended science to become a ubiquitous cultural icon, representing life, heredity, and the essence of biology itself. It features prominently in the logo of the Wellcome Trust and the former Celera Genomics corporation. The structure has been rendered in large-scale art installations, such as the sculpture "DNA" in London, and serves as a central visual motif in the opening credits of the television series The X-Files. It is frequently invoked in discussions of identity and ethics in works like the film Gattaca and in the branding of consumer genetic testing services like 23andMe.