From owner-biophysics@net.bio.net Wed Dec 02 22:00:00 1998 From: "clive delmonte" Newsgroups: bionet.biophysics,bionet.molec-model,bionet.xtallography Subject: DNA Structure: Puzzle Number 6 Date: Thu, 3 Dec 1998 07:37:37 -0000 Lines: 50 X-Newsreader: Microsoft Outlook Express 4.71.1712.3 X-MimeOLE: Produced By Microsoft MimeOLE V4.71.1712.3 NNTP-Posting-Host: 195.7.227.194 Message-ID: <3666bd28.0@news.netdirect.net.uk> X-Trace: 3 Dec 1998 16:32:40 GMT, 195.7.227.194 Path: biosci!news.stanford.edu!su-news-feed2.bbnplanet.com!su-news-hub1.bbnplanet.com!cpk-news-hub1.bbnplanet.com!news.gtei.net!newsfeed.cwix.com!194.72.7.126!btnet-peer!btnet!news.freedom2surf.net!peer.news.nildram.co.uk!news-peer.netdirect.net.uk!news.netdirect.net.uk!195.7.227.194 Xref: biosci bionet.biophysics:4614 bionet.molec-model:2337 bionet.xtallography:4505 Leslie et al.(Puzzle 5, re. 26) reported that a fibre of poly(dI).poly(dC), which gave sharp X-ray diffraction spots shortly after being drawn, was unstable and transformed irreversibly after a few days into poly(dC).poly(dI).poly(dC+). This new, three-stranded molecule also gave sharp spots in its own X-ray diffraction pattern which did not have any of the original spots. Therefore the conversion was complete. Very similar results had been reported for fibres of poly(dA).poly(dT) converting to poly(dT).poly(dA).poly(dT) (9) and for the formation of poly(U).poly(A).poly(U) from poly(U).poly(A) (10). The problem is evident: How and where would the torque arise inside a solid fibre that would permit one double helix to rotate so as to unwind one strand and rewind it onto an adjacent triple helix having a different diameter and rotating at a different angular velocity ? Inside the fibre, individual molecules would have a random axial translation so only very rarely would two adjacent double helices have the same axial starting point down the fibre, and they would be unlikely to be perfectly straight inside the fibre but would probably be entwined around neighbours. How would there be a very high conversion of double to triple stranded molecules when any rotation of adjacent double helices would normally find the free ends of one of them axially displaced down the fibre compared to its neighbour ? 9 Structures for the Polynucleotide Complexes Poly(dA).Poly(dT) & Poly(dT).Poly(dA).Poly(dT); Struther Arnott & E. Selsing; J Mol Biol Vol 88 (1974) 509-521 10 Structures for Poly(U).poly(A).poly(U) Triple Stranded Polynucleotides; Struther Arnott & P.J. Bond; Nature New Biology Vol 244 (1973) 99-101 ------------------------------------------------------------------ Clive Delmonte .