Double spiral arrangement

The double helix, a symmetrical spiral arrangement of two long-chain DNA molecules, is a result of hydrogen bonding between the chains. 

Figure A3.2 The structure of an alpha-helix. Oxygen atoms are shown in red and nitrogens in blue. In this spiral arrangement of the polypeptide chain, the C=0 and N—H bonds are approximately parallel to the helix axis and able to form multiple H-bonds, shown here as double dashed lines.

This is a type of machine used to cut coal at the face. The cutting is done by a rotating drum with a spiral arrangement of picks. Shearers can be single or double ended (i.e. a cutting drum at one or both ends). The machine is mounted on the face conveyor and drops the cut coal onto the conveyor. 

In order to clarify the elasticity of rubber, several investigators have stated that long molecules form spirals, and to be sure the spiral form of the molecules is promoted through the double bonds. By this arrangement, the secondary valences of the double bonds can be satisfied. The elasticity of rubber depends upon the extensibility of such spirals."... 

In 1953, James Watson and Francis Crick proposed that DNA was a double helix that consists of two polynucleotide strands winding about each other like a spiral staircase. The sugar-phosphate backbones are analogous to the outside railings, with the bases arranged like steps along the inside. 

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