In 1953, James & Mazia (4) reported that 1 mg of dry calf thymus DNA could
be spread in a monolayer to cover 0.28 sq. m. From this measurement, it is
possible to calculate the diameter of the DNA as it lies across the film
surface.
Calf thymus DNA, with 40% C+G base pairs, has an average mass of 330 daltons
per base unit, and a base repeat distance of 0.334 nm, giving a helical
diameter. measured across the surface film, of 1.2 nm.
James & Mazia measured the film height as 2.16 nm using an ellipsometer. So
there are two independent determinations of the height and width of the
duplex DNA as 1.2 nm x 2.1 nm, that is, DNA would have an elliptical cross
section.
The DNA molecular width in the film of 1.2 nm is close to that of 1.3 nm
found by Lee et al. (Puzzle 1, ref 1) using STM on duplexes not under
lateral compression, and to the value found by applying Stokes' equation
(Puzzle 2, ref. 3) to Franklin's B-DNA photograph.
4 Surface Films of Desoxyribonucleic Acid; T.W. James & D. Mazia;
Biochim. Biophys. Acta Vol 10 (1953) 367 - 370
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Clive Delmonte
