Helical structures continue to inspire, prompted by examples such as DNA double-helix and alpha-helix in proteins. Most synthetic polymers also crystallize as helices, which relieves steric clashes by twisting, while keeping the molecules straight for their ordered packing. In columnar liquid crystals, which often display useful optoelectronic properties, overall helical chirality can be induced by inclusion of chiral chemical groups or dopants; these bias molecular twist to either left or right, analogous to a magnetic field aligning the spins in a paramagnet. In this work, however, we show that liquid-crystalline columns with long-range helical order can form by spontaneous self-assembly of straight- or bent-rod molecules without inclusion of any chiral moiety. A complex lattice with Fddd symmetry and 8 columns per unit cell (4 right-, 4 left-handed) characterizes this “antiferrochiral” structure. In selected compounds it allows close packing of their fluorescent groups reducing their bandgap and giving them promising light-emitting properties. A 3D‐ordered liquid crystal phase of regular right and left helically twisted columns self‐assembles from straight‐ and bent‐rod molecules. Here calculations indicate that, among four alternative models, the observed complex Fddd structure provides the lowest packing energy for twisted ribbons.

The role of alternate DNA conformations such as Z-DNA in the regulation of transcription is currently underappreciated. These structures are encoded by sequences called flipons, many of which are enriched in promoter and enhancer regions. Through a change in their conformation, flipons provide a tunable mechanism to mechanically reset promoters for the next round of transcription. They act as actuators that capture and release energy to ensure that the turnover of the proteins at promoters is optimized to cell state. Likewise, the single-stranded DNA formed as flipons cycle facilitates the docking of RNAs that are able to microcode promoter conformations and canalize the pervasive transcription commonly observed in metazoan genomes. The strand-specific nature of the interaction between RNA and DNA likely accounts for the known asymmetry of epigenetic marks present on the histone tetramers that pair to form nucleosomes. The role of these supercoil-dependent processes in promoter choice and transcriptional interference is reviewed. The evolutionary implications are examined: the resilience and canalization of flipon-dependent gene regulation is contrasted with the rapid adaptation enabled by the spread of flipon repeats throughout the genome. Overall, the current findings underscore the important role of flipons in modulating the readout of genetic information and how little we know about their biology.

Early X-ray diffraction patterns from oriented fibres indicated that DNA must have a simple, repetitious structure and encouraged some researchers, who were already convinced that DNA was the genetic material, to undertake more detailed diffraction analyses and speculative modelling. The pioneering experimental work by Wilkins in the Wheatstone Laboratory at King's College London in the late 1940s first inspired, and then was overtaken by, the conjectural modelling of Watson and Crick in the Cavendish Laboratory at Cambridge. Why this was allowed to happen is still something of a puzzle. Here, I explore the puzzle and expose a peculiar flaw in the details of the original Watson-Crick model that was left for Wilkins to resolve.

The folding angle distribution of stretched and negatively supercoiled DNA double-helix is investigated based on a previously proposed model of double-stranded biopolymers [H. Zhou et al., Phys. Rev. Lett.82, 4560 (1999)]. It is shown that pulling can cause transition of a negatively supercoiled DNA double-helix from the right-handed B-form to a left-handed configuration which resembles DNA Z-form in some important aspects. The energetics of this possible transition is calculated and a comparison with recent experimental observations is qualitatively discussed.

In early May 1950, Bern chemistry professor Rudolf Signer traveled to a meeting of the Faraday Society in London with a few grams of DNA to report on his success in the isolation of nucleic acids from calf thymus glands. After the meeting, he distributed his DNA samples to interested parties amongst those present. One of the recipients was Maurice Wilkins, who worked intensively with nucleic acids at King's College in London. The outstanding quality of Signer's DNA – unique at that time – enabled Maurice Wilkins' colleague Rosalind Franklin to make the famous X-ray fiber diagrams that were a decisive pre-requisite for the discovery of the DNA double helix by James Watson and Francis Crick in the year 1953. Rudolf Signer, however, had already measured the physical characteristics of native DNA in the late thirties. In an oft-quoted work which he published in Nature in 1938, he described the thymonucleic acid as a long, thread-like molecule with a molecular weight of 500,000 to 1,000,000, in which the base rings lie in planes perpendicular to the long axis of the molecule. Signer's achievements and contributions to DNA research have, however, been forgotten even in Switzerland.

Nucleic acids, molecules essential for all life, can adopt many alternative structures besides the well-known right-handed double helix, some of which have been reported to exist and function in vivo. One of the most appropriate methods for structural studies of nucleic acids is circular dichroism spectroscopy, utilizing structure-induced chirality due to the asymmetric winding of absorbing nucleobases. Using electronic CD and absorption spectroscopies in combination with melting experiments, we analyzed a conformational equilibrium between DNA double helix and two alternative conformations of nucleic acids, cytosine i-motifs and guanine quadruplexes, as a function of the primary structure of model G/C-rich sequences, containing blocks of G and C runs in particular DNA strands. This paper is a part of special issue dedicated to 70th anniversary of the Biophysical Institute of the Czech Academy of Sciences, where circular dichroism spectroscopy of nucleic acids has been used successfully and impactfully for many years.

A linker unit was designed and synthesized that can serve both as a hairpin turn in a DNA duplex and anchor point for an aromatic helical foldamer mimicking the shape and surface properties of B-DNA. Methods were developed to synthesize natural/non-natural chimeric molecules combining foldamer and DNA segments. The ability of the linker to position the foldamer helix and the duplex DNA so that their rims and grooves are in register, despite their completely different chemical nature, was demonstrated using single crystal X-ray diffraction, circular dichroism and molecular models. Bio-layer interferometry confirmed that artificial hairpin DNA duplexes keep their ability to bind to DNA binding proteins. The chimeric molecules may pave the way to competitive inhibitors of protein-DNA interactions involving sequence-selective DNA-binding proteins.

The potentially Z-DNA-forming sequence d(GTGTACAC) crystallizes as A-DNA. (GT)n/(CA)n sequences have stimulated much interest because of their frequent occurrence in eukaryotic DNA and their potential for forming the left-handed Z-DNA structure. We here report the X-ray crystal structure of a self-complementary octadeoxynucleotide, d(GTGTACAC), at 2.5 A resolution. The molecule adopts a right-handed double-helical conformation belonging to the A-DNA family. In this alternating purine-pyrimidine DNA minihelix the roll and twist angles show alternations qualitatively consistent with Calladine's rules. The average tilt angle of 9.3 degrees is between the values found in A-DNA (19 degrees) and B-DNA (-6 degrees) fibers. It is envisaged that such intermediate conformations may render diversity to genomic DNA. The base-pair tilt angles and the base-pair displacements from the helix axis are found to be correlated for the known A-DNA double-helical fragments. Published in Journal of molecular biology (1987)

structural conformations of the DNA double helix, along with B-DNA and Z-DNA. The A-form helix has a right-handed twist with 11 base pairs per full turn This glossary of cellular and molecular biology is a list of definitions of terms and concepts commonly used in the study of cell biology, molecular biology, and related disciplines, including genetics, biochemistry, and microbiology. It is split across two articles: This page, Glossary of cellular and molecular biology (0–L), lists terms beginning with numbers and with the letters A through L. G A-DNA One of three main biologically active structural conformations of the DNA double helix, along with B-DNA and Z-DNA. The A-form helix has a right-handed twist with 11 base pairs per full turn, only slightly more compact than B-DNA, but its bases are sharply tilted with respect to the helical axis. It is often favored in dehydrated conditions and within sequences of consecutive purine nucleotides (e.g. GAAGGGGA); it is also the primary conformation adopted by double-stranded RNA and RNA-DNA hybrids. B-DNA The "standard" or classical structural conformation of the DNA double helix in vivo, thought to represent an average of the various distinct conformations assumed by very long DNA molecules under physiological conditions. The B-form double helix has a right-handed twist with a diameter of 23.7 ångströms and a pitch of 35.7 ångströms or about 10.5 base pairs per full turn, such that each nucleotide pair is rotated 36° around the helical axis with respect to its neighboring pairs. See also A-DNA and Z-DNA. double helix The shape most commonly assumed by double-stranded nucleic acid molecules, resembling a ladder that has been twisted upon its long axis, with the rungs of the ladder consisting of paired nucleobases. This secondary structure is the most energetically stable conformation of the double-stranded forms of both DNA and RNA under most naturally occurring conditions, arising as a consequence of the primary structure of the phosphodiester backbone and the stacking of the nucleotides bonded to it. In B-DNA, the most common DNA variant found in nature, the double helix has a right-handed twist with about 10 base pairs per full turn, and the molecular geometry results in an alternating pattern of "grooves" of differing widths (a major groove and a minor groove) between the parallel backbones.

SEM images of DNA double helix and nucleosomes observed by ultrahigh-resolution scanning electron microscopy. We observed DNA double helix and nucleosomes in the chromatin of chicken erythrocytes by ultrahigh-resolution scanning electron microscopy. Specimens were prepared according to a modified microspreading technique in combination with the carbon plate method and observed without metal coating. A part of the DNA fibers without nucleosomes showed "left-handed" double-strands with twisting appearances and regular periodicities of the helix. Linker DNA between the nucleosomes showed "right-handed" appearances. Most of the nucleosome particles appeared as prolate ellipsoidal shapes of various sizes. DNA appeared to enter and exit the nucleosome particles on opposite sides winding around the histone core. Published in Journal of electron microscopy (1991)

two positions in the DNA helix. The helix itself does not move. Model 4. The DNA helix is made up of a … Atoms Surface of DNA Double Helices Surfaces for RNA Double Helix and Single Helix Transfer RNA Two-thirds … five base pairs having a right-handed twist followed by five with a left-handed twist, and so on, indefinitely

When Watson and Crick (WC) constructed their famous 3D model for DNA, they had to decide whether to make the double helix left or right handed.

When Watson and Crick (WC) constructed their famous 3D model for DNA, they had to decide whether to make the double helix left or right handed.