From owner-biophysics@net.bio.net Mon Nov 30 22:00:00 1998 From: "clive delmonte" Newsgroups: bionet.biophysics,bionet.molec-model,bionet.xtallography Subject: DNA Structure: Puzzle Number 3 Date: Tue, 1 Dec 1998 10:33:37 -0000 Lines: 28 X-Newsreader: Microsoft Outlook Express 4.71.1712.3 X-MimeOLE: Produced By Microsoft MimeOLE V4.71.1712.3 NNTP-Posting-Host: 195.7.225.160 Message-ID: <36644332.0@news.netdirect.net.uk> X-Trace: 1 Dec 1998 19:27:46 GMT, 195.7.225.160 Path: biosci!news.stanford.edu!newsfeed.berkeley.edu!btnet-peer!btnet!news.freedom2surf.net!peer.news.nildram.co.uk!news-peer.netdirect.net.uk!news.netdirect.net.uk!195.7.225.160 Xref: biosci bionet.biophysics:4609 bionet.molec-model:2335 bionet.xtallography:4501 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 ------------------------------------------------------------------ Clive Delmonte .