Single chromatin fibers were assembled directly in the flow cell of an optical tweezers setup. A single λ phage DNA molecule, suspended between two polystyrene beads, was exposed to a Xenopus laevis egg extract, leading to chromatin assembly with concomitant apparent shortening of the DNA molecule. Assembly was force-dependent and could not take place at forces exceeding 10 pN. The assembled single chromatin fiber was subjected to stretching by controlled movement of one of the beads with the force generated in the molecule continuously monitored with the second bead trapped in the optical trap. The force displayed discrete, sudden drops upon fiber stretching, reflecting discrete opening events in fiber structure. These opening events were quantized at increments in fiber length of ∼65 nm and are attributed to unwrapping of the DNA from around individual histone octamers. Repeated stretching and relaxing of the fiber in the absence of egg extract showed that the loss of histone octamers was irreversible. The forces measured for individual nucleosome disruptions are in the range of 20–40 pN, comparable to forces reported for RNA- and DNA-polymerases.

A simple one-dimensional model is presented for the motion of a Brownian particle. It is shown how the collisions between a Brownian particle and its surrounding molecules lead to the Langevin equation, the power spectrum of the stochastic force, and the equipartition of kinetic energy.

We have developed a platform for cell analysis based on immunomagnetic selection and magnetic alignment of cells in combination with an epi-illumination tracking and detection system. Whole blood was labeled with ferromagnetic nanoparticles and fluorescent probes, and placed in a magnetic field in a chamber. Cells labeled with ferromagnetic nanoparticles moved upward and aligned along ferromagnetic lines deposited by lithographic techniques on an optically transparent surface of the chamber. An epi-illumination system using a 635 nm laser diode as a light source scanned the lines and measured signals obtained from the aligned cells. The cell counts per unit of blood volume obtained with the system correlated well with those obtained from the counts from a standard hematology analyzer and flow cytometer. The cell analysis platform is significantly less complex and more sensitive than current cell analysis equipment and provides additional functionality through its ability to subject the cells to repeated and varied analyses while they remain in a natural environment (i.e., whole blood).

The studies described in this manuscript have been aimed to obtain a better insight into the cytotoxic interaction between human Natural Killer (NK) cells and K562 target cells1. In order to obtain information about crucial processes leading to target cell death, the processes taking place in the target cell during a killer cell attack were studied using different optical techniques. Furthermore, flow cytometric methods for determination of the phenotype and the cytotoxic activity of killer cells were developed.

A stand-alone atomic force microscope (AFM) has been developed, which features a large scan area and which allows operation under liquid. This system was combined with a confocal laser scanning microscope (CLSM). Information about cell structures, obtained by CLSM, can be complemented with images of the cell surface obtained with the AFM. This is illustrated by studying the pseudopodia of cells from a human cell line (K562-cells, predecessor of erythroblasts) and the cytoskeleton of monkey kidney cells (in air and under liquid), both stained with F-actin-specific fluorescent probes. Images of the cytoskeleton during the cytotoxic interaction between a natural killer and a K562 target cell are presented. Our results show that combination of these techniques can provide new information about cells and cellular structures.

A small ring-shaped vacuum chamber has been constructed and connected to the piezotube used for scanning samples in the atomic force microscope (AFM). Samples made up of any material, up to 50 mm in diameter, can be firmly attached onto the piezotube without causing damage to the sample. A 50-ℓ beer container forms a buffer between vacuum pump and chamber. With this supply of vacuum, the AFM can be operated for a 4–8 h period without turning on the vacuum pump again. Samples can be changed within 30 s. The scan frequency when using microscope slides is limited to 40 Hz due to resonance effects of the microscope slides.

The design and implementation of a tial induced by interactions between nat- real-time signal processing system for ural killer (NK) cells and their target cells slit-scan flow cytometry is described. The are presented. system is used to measure the separate scatter and fluorescence peak heights of 2 adherent cells. Preliminary measure- ments of changes in the membrane poten-