Making first contacts between the spindle and the chromosomes in HeLa cells.

To guarantee an ordered bipartition of the genetic material during mitosis, the chromosomes must be incorporated into the mitotic spindle. In HeLa cells, this process starts early in prophase when the nuclear envelope is still nearly complete, but only a few small holes in the double membranes offer access to the chromosomes for individual microtubules growing out from the poles. Inside the nuclear domain, these microtubules make contact with the kinetochore/centromere complexes which can be found in the vicinity of the hole. These complexes seem to be distributed at random during early prophase until early prometaphase. Therefore, the chromosomes become incorporated in a sequential order. No accumulation of the complexes in the nucleus near the centrosomes can be recognized. The individual microtubules attach tangentially to the kinetochores. This contact can already take place before the kinetochore is fully developed.