ATOMIC-FORCE MICROSCOPY OF MAMMALIAN SPERM CHROMATIN

We have used the atomic force microscope (AFM) to image the surfaces o f intact bull, mouse and rat sperm chromatin and partially decondensed mouse sperm chromatin attached to coverglass. High resolution AFM ima ging was performed in air and saline using uncoated, unfixed and unsta ined chromatin. Images of the surfaces of intact chromatin from all th ree species and of an AFM-dissected bull sperm nucleus have revealed t hat the DNA is organized into large nodular subunits, which vary in di ameter between 50 and 100 nm. Other images of partially decondensed mo use sperm chromatin show that the nodules are arranged along thick fib ers that loop out away from the nucleus upon decondensation. These fib ers appear to stretch or unravel, generating narrow smooth fibers with thicknesses equivalent to a single DNA-protamine complex. High resolu tion AFM images of the nodular subunits suggest that they are discrete , ellipsoid-shaped DNA packaging units possibly only one level of pack aging above the protamine-DNA complex.