Spindle fibers attach to kinetochores of chromosomes during

The morphology of the chromosomes is studied during

The centromere splits during

Statement $P$ : Small disc-shaped structures called kinetochores are present on the surface of the centromeres.
Statement $Q$ : Spindle fibers attach to the centromeres of the chromosomes.

In the given figure,what do $a$ and $b$ represent respectively?

The complete disintegration of the nuclear envelope in a cell cycle marks the