Winding twisted pair

The first teehnique that enhanees eoupling between windings is twisted pair winding. This oeeurs when two or more wires are twisted together and then... 

The circular dichroism of polynucleotides at 275 nm is a linear function of both the helix winding angle and the base pair twist.103 Measurement of CD spectra on large polynucleotides or large molecular aggregates... 

Figure 28-14 A "tailed circle" consisting of an enhancer linked to, but topologically separated from, a gene. One of the DNA strands of this plasmid bears two copies of an SV40 enhancer sequence, one copy inverted with respect to the other. This extra region protrudes from the circle and self-pairs to form a functional enhancer. The main body of the circle contains the p-globin gene, transcription of which is increased by the enhancer. Twisting of the enhancer has no effect on the winding of the strands on the main body of the circle nevertheless, the enhancer efficiently increases P-globin transcription. From Ptashne.355...

Figure 25-23 Schematic representation of configuration of DNA, showing the relationship between the axes of hydrogen-bonded purine and pyrimidine bases and the deoxyribosephosphate strands. There are 10 pairs of bases per complete 360° twist of the chain. The spacing between the strands is such that there is a wide and a narrow helical groove around the molecule. Proteins known as histones coordinate with DNA by winding around the helix, filling one of the other of the grooves. The histone-DNA combination is important in regulating the action of DNA.

Compaction of DNA into the nucleosome involves the winding of 146 base pairs of DNA into ca 1.7 left-handed turns around the histones (Fig. 6). Such a representation is very useful to help visualize what a nucleosome looks like but, unfortunately, presents the erroneous view that DNA is complacently wound onto the histones with little or no structural stress. The reality is quite contrary to what one could divine from this drawing in assembling into the nucleosome, DNA is very severely distorted from its conventional and familiar B-form. First, to twist around the histones, the DNA backbone has to be very severely bent— the turns that it makes likely approach the limit of thermodynamic feasibility. In addition, topological requirements of winding a right-handed double helix into a left-handed superhelix necessitate that the DNA be partially unwound from its conventional 10.5 base pairs per helix turn. 

Scheme I consists of two layer pairs with opposing fiber orientations and relative polarizations, whereas the relative polarization around the cross-section is constant over all quadrants. Thus, it represents two pairs of helical windings one of which contracts while the other expands, resulting in beam twist of similar direction but compensation of beam elongation.

Scheme II consists of a central lengthwise oriented layer surrounded by a layer pair with uniform fiber orientation. With an expansion of this unidirectional helical winding, twist and elongation of the beam are induced in the case of variant a. With an expansion of the lengthwise oriented fibers, elongation of the beam is achieved and complemented with twist via the passive coupling due to the helical winding in the case of variant c. Both mechanisms are employed simultaneously in the case of variant b. The relative polarization around the cross-section remains constant for any of the variants.

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