An experimental study of Rayleigh-Be'nard convection in helium gas at roughly 5 K is performed in a cell with aspect ratio 1. Data are analysed in a ' hard turbulence ' region (4 x 10' < Ra < 6 x 10l2) in which the F'randtl number remains between 0.65 and 1.5, The main observation is a simple scaling behaviour over this entire range of Ra. However the results are not the same as in previous theories. For example, a classical result gives the dimensionless heat flux, Nu, proportional to R d while experiment gives an index much closer to 5. A new scaling theory is described. This new approach suggests scaling indices very close to the observed ones. The new approach is based upon the assumption that the boundary layer remains in existence even though its Rayleigh number is considerably greater than unity and is, in fact, diverging. A stability analysis of the boundary layer is performed which indicates that the boundary layer may be stabilized by the interaction of buoyancy driven effects and a fluctuating wind.

In a process called "molecular combining," DNA molecules attached at one end to a solid surface were extended and aligned by a receding air-water interface and left to dry on the surface. Molecular combing was observed to extend the length of the bacteriophage lambda DNA molecule to 21.5 +/- 0.5 micrometers (unextended length, 16.2 micrometers). With the combing process, it was possible to (i) extend a chromosomal Escherichia coli DNA fragment (10(6) base pairs) and (ii) detect a minute quantity of DNA (10(3) molecules). These results open the way for a faster physical mapping of the genome and for the detection of small quantities of target DNA from a population of molecules.

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We describe the mechanical separation of the two complementary strands of a single molecule of bacteriophage DNA. The 3 and 5 extremities on one end of the molecule are pulled progressively apart, and this leads to the opening of the double helix. The typical forces along the opening are in the range of 10-15 pN. The separation force signal is shown to be related to the local GC vs. AT content along the molecule. Variations of this content on a typical scale of 100-500 bases are presently detected. Mechanical force at the molecular level is involved in the action of many enzymes. This is the case for the processes of replication or transcription in which enzymes translocate processively with respect to DNA. Such translocation occurs unidirectionally over long segments of DNA, and the enzymatic machinery has to develop a force against a number of impediments: the disruption of complementary base pairs, the possible attachments of the DNA or the enzymes to cellular components, structural proteins that coat DNA and have to be displaced, topological constraint, and viscous friction. The force necessary to stop a transcribing Escherichia coli polymerase recently has been measured (1). In the case of replication (2), the DNA double helix is opened, and two daughter strands are formed. The opening may be associated to the translocation mechanism of the polymerase itself or may be assisted by accessory proteins like helicases or single-strand binding proteins. Moreover, because the strands are intertwined, strand separation is coupled to a local rotation. Topological constraints are resolved by topoisomerases (3, 4).Long before the enzymes associated to replication were known, a simple model had been considered (5) in which the mechanism of unwinding the strands during replication is coupled to rotation of the whole molecule. A molecular configuration with a Y shape had been proposed in which the vertical part is the parent helix, and the two arms are the separated strands that get replicated. As replication proceeds, a ''speedometer cable'' rotation motion was proposed for all three branches of the Y.We have set up an experiment to measure directly the forces involved in the elementary process of mechanical strand separation, with no enzyme present. The experiment presented here is approaching the Levinthal and Crane (5) configuration.Force measurement on single molecules of DNA is an emerging field (6-13). For typical molecular interactions involving biomolecules, the forces involved are small (sub-picoNewton to 10s of picoNewtons [pN ϭ 10 Ϫ12 N]). For this reason, sensitive measuring devices (14-17) such as optical tweezers, soft microneedles, or levers of atomic force microscopes have been used.

Force measurements are performed on single DNA molecules with an optical trapping interferometer that combines subpiconewton force resolution and millisecond time resolution. A molecular construction is prepared for mechanically unzipping several thousand-basepair DNA sequences in an in vitro configuration. The force signals corresponding to opening and closing the double helix at low velocity are studied experimentally and are compared to calculations assuming thermal equilibrium. We address the effect of the stiffness on the basepair sensitivity and consider fluctuations in the force signal. With respect to earlier work performed with soft microneedles, we obtain a very significant increase in basepair sensitivity: presently, sequence features appearing at a scale of 10 basepairs are observed. When measured with the optical trap the unzipping force exhibits characteristic flips between different values at specific positions that are determined by the base sequence. This behavior is attributed to bistabilities in the position of the opening fork; the force flips directly reflect transitions between different states involved in the time-averaging of the molecular system.

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