Experimental study of the functional morphology of the kinetochore in mitosis.

Chinese hamster cells in culture were blocked in prometaphase by the addition of chloral hydrate. After 2 h the chloral hydrate was removed and the ultrastructural changes in the kinetochore studied during recovery of mitosis. The kinetochore in the blocked cells is a disc measuring 300-350 nm in diameter and 30-35 nm in thickness. Imediately after recovery of mitosis the disc begins to break up and fuse with the inner nucleus of the kinetochore, when it acquires a spherical organization. On combined treatment with chloral hydrate and cobalt nitrate, inducing despiralization of the chromosomal ifbrils, the knetochore in the blocked cells becomes spherical. It is concluded that activation of the kintochore in prometaphase probably involves despiralization of the fibrils composing the disc.