Spiral staircase

In 1953, James Watson and Francis Crick made their classic proposal for the secondary structure of DNA. According to the Watson-Crick model, DNA under physiological conditions consists of two polynucleotide strands, running in opposite directions and coiled around each other in a double helix like the handrails on a spiral staircase. The two strands are complementary rather than identical and are held together by hydrogen bonds between specific pairs of... 

A screw dislocation creates a fault in a crystal that looks rather like a spiral staircase. The dislocation can be conceptually formed by cutting halfway through a crystal and sliding the regions on each side of the cut parallel to the cut, to create spiraling atom planes (Fig. 3.6). The dislocation line is the central axis of the spiral. 

The DNA double helix has both chains twisting on a common axis. The bases are directed inwards to allow hydrogen bonding, and the sugar and phosphodiester parts of the main chain form the outside portion. The planes of the base pairs are perpendicular to the helix axis, so that the molecule looks like a spiral staircase with the base-pair combinations forming the treads. The helix makes... 

Today my taste for home decorations has morphed from bedroom anatomy posters to wooden African masks that currently surround a spiral staircase near the dining room. These masks are modern works, some from the early part of the 20th century, some contemporary. Portraiture is one of the great universal traditions in art the face as expressed in... 

Defining interaction is something like asking someone to describe a spiral staircase with his hands tied behind his back. The example is much more effective than the verbal description. Let us take an example therefore. 

Fig. 18 Spiral staircase (left), staggered (middle), or tilted and rotated (right) arrangement of 114d in the Col phase. Reproduced by permission of AAAS from [128]...

The VCD calculated for this model polymer is shown in Figure 3, and resembles that observed for the decamer sequence d(CG)5, both in overall spectral appearance and signal magnitude (cf. Section 5). Thus, the simplistic "spiral staircase" model appears to be a good starting point for the qualitative interpretation of the experimental data. 

Figure 2. The "Spiral Staircase" geometry used in the original DECO test calculations...

For increasing chain lengths, low frequency exciton components between 1630 and 1640 cm1, gain in intensity in the calculated absorption strengths. For the "spiral staircase" geometry, the computed VCD intensities of the low frequency exciton components cancel. 

From a chemical perspective, the double-helix produced by two intertwining strands of oligomeric DNA is a fascinating and unique molecular structure. (See Fig. 1 for a structural model of a 12-base pair duplex of B-form DNA.) In it nucleic acid bases are stacked in pairs one on top of the other with a slight twist reminiscent of a spiral staircase [16]. The unique stacking and overlapping of the n- and Tr-electrons of DNA bases may provide a preferred path for electron transfer. Similarly, the exceptional closeness of the stacked bases may have important consequences for charge motion in DNA duplexes. Additionally, the... 

Figure 8-11. (a) Spiral staircase (and its shadow) in Cambridge, England,... 

The base pairs that join the two strands are stacked like a spiral staircase in the interior of the molecule. 

C. DNA chains are composed of nucleosides joined by 3, 5-phosphodiester bonds. Each nucleoside consists of a nitrogenous base linked to deoxyribose. Two DNA chains, oriented in opposite directions, base-pair with each other and are twisted to form a double helix. The bases are stacked on top of each other, forming a spiral staircase in the center of the molecule, and the sugar-phosphate backbone, in which the phosphates are negatively charged, is wrapped around the outside. 

An object that is not superimposable with its mirror image is said to be chiral (from the Greek word cheir, meaning hand ) an object that is superimposable with its mirror image is said to be achiral. Examples of familiar objects that are chiral are a screw, a propeller, a foot, an ear, and a spiral staircase examples of common objects that are achiral are a plain cup with no decoration, a pair of eyeglasses, and a sock. 

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