title: Accuracy of AFM measurements of the contour length of DNA fragments adsorbed on mica in air and in aqueous buffer.
abstract: The measurement by atomic force microscope of the contour length of DNA fragments adsorbed on mica has been made as accurate as possible by revisiting the different steps of image acquisition and processing. In air, the DNA helical rise was estimated at 2.97 +/- 0.15 A per base pair (bp) (mean +/- standard deviation) by imaging a 648-bp DNA fragment and 2.95 +/- 0.14 A per bp for a 115-bp fragment. This confirms earlier observations suggesting that drying DNA fragments on mica in the presence of nickel induces limited conformational changes. At this point the exact nature of these conformational changes remains unknown. Simple hypotheses are the transconformation of stretches of the DNA molecules to the A-form of the double helix or alteration of the helix structure at the points of contact between DNA and mica. By contrast, in aqueous buffer, the measured helical rise was 3.14 +/- 0.15 A per bp for the 648-bp fragment and 3.17 +/- 0.13 A per bp for the 1115-bp fragment. Thus, measured helical rises do not depend on the fragment length and are significantly shorter than the 3.38 A per bp measured by crystallography, but close to the 3.18 A per bp found in NMR studies. These findings are discussed with respect to discrepancies in earlier results published in the literature.
authors.affiliation: RGFCP, Faculté de Médecine, Université de la Méditerranée, Marseille, France.
authors.forename: Albert
authors.initials: A
authors.lastname: Sanchez-Sevilla
authors.forename: Jean
authors.initials: J
authors.lastname: Thimonier
authors.forename: Monique
authors.initials: M
authors.lastname: Marilley
authors.forename: José
authors.initials: J
authors.lastname: Rocca-Serra
authors.forename: Jacques
authors.initials: J
authors.lastname: Barbet
mesh_terms: D000388:Air; D000538:Aluminum Silicates; D002021:Buffers; D004247:DNA / Q000737:chemistry*; D015246:Deoxyribonuclease EcoRI / Q000378:metabolism; D015252:Deoxyribonucleases, Type II Site-Specific / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D018625:Microscopy, Atomic Force / Q000379:methods*; D009690:Nucleic Acid Conformation; D010957:Plasmids / Q000235:genetics / Q000378:metabolism
chemical_list: D000538:Aluminum Silicates; D002021:Buffers; D004247:DNA; D015246:Deoxyribonuclease EcoRI; D015252:Deoxyribonucleases, Type II Site-Specific; C029792:GTCGAC-specific type II deoxyribonucleases; C011934:mica