Nucleic helical twist

Tang, R. S., and Draper, D. E. (1994). On the use of phasing experiments to measure helical repeat and bulge loop-associated twist in RNA. Nucleic Acids Res. 22(5), 835—841. 

The Helical Nature of Nucleic Acids. The double helix of nucleic acids has a diameter of 20 A. Dimensions such as these can be made visible in modern electron microscopes. In the photograph of a deoxyribonucleic acid preparation of bacteriophages (Fig. 28), even the twisted structure of the DNA can be distinguished. The entire, multiply looped thread is 500,000 A (= 50 /u) long, which corresponds to a molecular weight of 130 million. 

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