The Analysis of Chromosome Organization by Experimental Manipulation

Chromomeric pattern of maize pachytene chromosomes after trypsin treatment

The bivalent chromomeres of maize were analyzed by means of enzymatic treatment with trypsin. The aim was to verify if the effect of swelling observed in human meiotic chromosomes also occurs in these bivalents, and if this morphological change allows a greater visual resolution of the chromomeric pattern. Bivalents treated with trypsin showed a structure with sequential and linear distribution of small rings. Comparisons between bivalent morphology, treated and non-treated, indicated a collinearity among the sequences of rings and the chromomeres. The results obtained were considered useful to analyze and to make a chromomere map. This treatment can also provide cytological evidence that may help to understand the pairing process. It was concluded that the ring-shape was caused by trypsin-induced "disjoining" just in the chromomeric but not in the interchromomeric regions.

Probing chromatin with the scanning force microscope

With the scanning force microscope (SFM), one can image the topography of biological material adsorbed at air-solid or liquid-solid interfaces with up to nanometer resolution. In principle, fixation, contrast enhancement, and labeling are not required. We have adapted specimen preparation techniques of conventional electron microscopy for visualizing chromatin ultrastructures in the SFM. A beaded substructure of the nucleoprotein filament was obtained after hypotonic lysis of chicken erythrocytes and air drying. The beads-on-a-string morphology of the basic nucleosomal assembly was well delineated. The nucleosomes appeared as round protrusions with an apparent height of 4-6 nm. The histogram of center-to-center distances between adjacent nucleosome cores along the filament axis had a peak at approximately 30 nm. Reversible changes in the three-dimensional structure were observed upon exposure of air-dried samples of metaphase chromosomes to solutions of different ionic strengths.