G. Jannink

Polymers in solution

The authors discuss the physical nature of interactions between linear flexible chains in a solvent. They review the results obtained on the structure of polymer solutions, with the help of the observation techniques and conceptual approaches developed since 1965. Comparison between observed and predicted behaviour is made for values of the critical exponents and the universal amplitude.

Forces on chromosomal DNA during anaphase

In the course of anaphase, the chromosomal DNA is submitted to the traction of the spindle. Several physical problems are associated with this action. In particular, the sister chromatids are generally topologically intertwined at the onset of anaphase, and the removal of the intertwinings results from a coupling between the enzymatic action of type II DNA topoisomerases and the force exerted by the spindle. We propose a physical analysis of some of these problems: 1) We compare the maximum force the spindle can produce with the force required to break a DNA molecule, and define the conditions compatible with biological safety during anaphase. 2) We show that the behavior of the sister chromatids in the absence of type II DNA topoisomerases can be described by two distinct models: a chain pullout model accounts for the experimental observations made in the budding yeast, and a model of the mechanical rupture of rubbers accounts for the nondisjunction in standard cases. 3) Using the fluctuation-dissipation theorem, we introduce an effective protein friction associated with the strand-passing activity of type II DNA topoisomerases. We show that this friction can be used to describe the situation in which one chromosome passes entirely through another one. Possible experiments that could test these theoretical analyses are discussed.

Anaphase chromatid motion: involvement of type II DNA topoisomerases

Sister chromatids are topologically intertwined at the onset of anaphase: their segregation during anaphase is known to require strand-passing activity by type II DNA topoisomerase. We propose that the removal of the intertwinings involves at the same time the traction of the mitotic spindle and the activity of topoisomerases. This implies that the velocity of the chromatids is compatible with the kinetic constraints imposed by the enzymatic reaction. We show that the greatest observed velocities (about 0.1 microns s-1) are close to the theoretical upper bound compatible with both the diffusion rate (calculated here within a probabilistic model) and the measured reaction rate of the enzyme.

Contrast in multicomponent systems

The forward scattering of radiation by multicomponent systems is expressed in terms of the chemical potentials and of the collision lengths associated with the components.

Polymers at Interfaces: A Study by Neutron Reflection

The vicinity of total reflection of a polymer solution — air interfaces is investigated using a neutron radiation. Experiments and calculations reveal a significant singular behaviour, from which the exact nature of the polymer concentration profile can be determined.