MOVEMENT AND ORIENTATION

The controlled distribution of chromosomes during meiosis, as in mitosis, depends on the orderly behaviour of chromosomes on the division spindles. This is achieved principally by two series of coupled movements which lead to the development of first metaphase orientation.

Prophase movements

Chromosome movements in the first prophase nucleus are dominated by an interaction between the chromosomes and the centrosome or, in cases where there is no centrosome, by an equivalent spindle-forming centre. At the onset of meiosis the centromeres are often gathered close to that centre, in Rabl orientation. This is a passive form of orientation, indicating that little movement has occurred since the preceding pre-meiotic telophase. Bouquet formation, which commonly replaces Rabl orientation, involves an active movement of chromosome ends in relation to the centrosome or its equivalent.The centrosome itself separates into two daughter centres during first prophase and these take up positions on opposite sides of the nucleus. It is this movement which, in effect, first establishes bipolarity within the cell.

In some species the bouquet forms during leptotene. Here all chromosome ends become associated with the side of the nucleus close to the undivided centrosome with the remainder adopting a looped arrangement within the body of the nucleus. This involves an interaction between chromosome ends and the centrosome, with all ends moving along the inner surface of the nuclear membrane relative to the extranuclear polarizing centre represented by the centrosome or its equivalent.