At the beginning of nuclear division two wheel-shaped protein structures called centrioles position themselves at opposite ends of the cell forming cell poles. Long protein fibers called microtubules extend from these centrioles in all possible directions forming what is known as a spindle.
During this process microtubules grow and shorten to form a cytoskeleton in which the genetic material is organized. They also play a key role in dividing genetic material during mitotic and meiotic cell divisions.
The microtubules originate from the centrosome, a ring-shaped structure within the cell that contains a type of microtubule protein called gamma tubulin (see figure). This protein seeding process begins as the cell divides and continues until anaphase is reached.
As the chromosomes are positioned along the mitotic spindle's midline they must undergo a series of motions to position their centromere on the spindle equator, a process known as congression. During this time the kinetochore microtubules that attach the chromosome to one of the spindle poles must elongate and those that attach the chromosome to the other spindle pole must shorten.
This is a remarkable biochemical checkpoint that inhibits the onset of anaphase until all chromosomes are properly attached to the spindle. It occurs in conjunction with the binding of kinetochores to MTs, a mechanism that is not completely understood but is likely to be based on tension and MT attachment.