Answer
A
Work Step by Step
Internal regulatory proteins regulate internal cellular events. These proteins let the cell cycle continue only if certain cellular conditions are met. There is a regulatory protein that allows cell division to proceed into anaphase only when spindle fibers have connected to the chromosomes. This attachment is necessary so that the sister chromatids can be pulled to opposite ends of the cell, ensuring that each daughter cell arising from this cell division will have a complete set of chromosomes.
