Question

why do the chromosomes line up along the metaphase plate????

Answer 1

I'm pretty sure nobody exactly knows this yet. There is some evidence the chromosomes ocillate slightly along the metaphase plane during the metaphase and that this is becasue the microtubules dragging the chromosomes gets depolymerated by microtubule depolymerases. But my knowledge on the subject is very limited.

Answer 2

There is no real certainty but most authors agree that the cromossomes align on the equatorial plate because centrossomes use the microtubules to grab the centromeros (don't know the translation) through the formation of the asteres, formed during prophase, and center the cromossomes on the equatorial plate