CHROMOSOME FRAGMENTS POSSESSING ONLY ONE KINETOCHORE CAN CONGRESS TO THE SPINDLE EQUATOR

We used laser microsurgery to cut between the two sister kinetochores on bioriented prometaphase chromosomes to produce two chromosome fragm ents containing one kinetochore (CF1K). Each of these CF1Ks then alway s moved toward the spindle pole to which their kinetochores were attac hed before initiating the poleward and away-from-the-pole oscillatory motions characteristic of monooriented chromosomes, CF1Ks then either: (a) remained closely associated with this pole until anaphase (50%), (b) moved (i.e., congressed) to the spindle equator (38%), where they usually (13/19 cells) remained stably positioned throughout the ensuin g anaphase, or (c) reoriented and moved to the other pole (12%), Behav ior of congressing CF1Ks was indistinguishable from that of congressin g chromosomes containing two sister kinetochores. Three-dimensional el ectron microscopic tomographic reconstructions of CF1Ks stably positio ned on the spindle equator during anaphase revealed that the single ki netochore was highly stretched and/or fragmented and that numerous mic rotubules derived from the opposing spindle poles terminated in its st ructure. These observations reveal that a single kinetochore is capabl e of simultaneously supporting the function of two sister kinetochores during chromosome congression and imply that vertebrate kinetochores consist of multiple domains whose motility states can be regulated ind ependently.