From owner-biophysics@net.bio.net Thu Jan 21 22:00:00 1999 From: "Iris Delmonte" Newsgroups: bionet.biophysics Subject: DNA Structure: Puzzle Number 19 Date: Fri, 22 Jan 1999 12:24:56 -0000 Lines: 39 X-Newsreader: Microsoft Outlook Express 4.72.3110.5 X-MimeOLE: Produced By Microsoft MimeOLE V4.72.3110.3 NNTP-Posting-Host: 195.7.227.244 Message-ID: <36a86863.0@news.netdirect.net.uk> X-Trace: 22 Jan 1999 12:00:35 GMT, 195.7.227.244 Path: biosci!agate!newsfeed.berkeley.edu!newsfeed.wirehub.nl!peer.news.nildram.co.uk!news-peer.netdirect.net.uk!news.netdirect.net.uk!195.7.227.244 The Microprobe Experiment of Mou et al. Mou et al. (Ref.45) measured the height of duplex DNA when densely packed on cationic lipid bilayers on a mica surface. They found the height to be some 2+ nm. This result reminds us of that of James & Mazia (Puzzle 3, Ref.4) who also measured the film height of duplex DNA under compression in 1953 and found a value of 2.1 - 2.2 nm. Isolated duplexes, not under compression, using AFM & STM mostly have a maximum measured height of some 1.3 nm, or so, and this has led Muir et al. to suggest that "...duplex DNA may have an elliptical cross section when in contact with mica." (Puzzle 18, Ref. 44) Wilkins and co-workers had suggested that DNA had an elliptical section in 1953 (Puzzle 12, Ref.20), and the results of James & Mazia, 2.1 - 2.2 nm x 1.2 nm, agree with this. Even in very recent work (46, page 542) the apparent height of two-dimensional sheets of DNA is reported as 1.35 - 1.45 nm using AFM. Consequently, the work of Wilkins et al., James & Mazia, Muir et al., and many others, would all suggest an elliptical cross section for dupex DNA of approximately 2.1 - 2.2 nm x 1.2 - 1.4 nm. 45 J. Mou et al.; FEBS Lett Vol 371 (1995) 279 46 Design & self-assembly of two-dimensional DNA crystals; E. Winfree et al.; NATURE Vol 394 (1998) 539-544 ---------------------------------------------------------------- Clive Delmonte .