Every point on a digital
AFM image has an X, Y, and Z coordinate. In our measurements, centers
of consecutive nucleosomes in a fiber are marked by consecutive computer
mouse clicks which record both X, Y position and height (Z) of the top
of the nucleosomal image. Hence it is very easy to make a large
quantity of measurements very quickly of nucleosome center-to-center distances,
angles formed by the intersection of lines connecting the centers of three
successive nucleosomes, and the heights of nucleosomes in the fiber.
From these kinds of measurements, we have constructed histograms which
often can have more than 2000 points per histogram; this establishes the
strength of SFM as a quantitative tool.