Advanced Placement (AP) Biology Practice Exam

What key event occurs during cytokinesis?

1. Formation of chromosomes

2. Pinching of the plasma membrane

3. Separation of chromatids

4. Condensation of chromatin

The correct answer is: Pinching of the plasma membrane

During cytokinesis, the key event that occurs is the pinching of the plasma membrane. This process is crucial for separating the two daughter cells after mitosis has occurred. In animal cells, a contractile ring made of actin filaments forms just beneath the plasma membrane, creating a cleavage furrow that deepens until the membrane is pinched in two, leading to the formation of two distinct daughter cells. In plant cells, a cell plate forms along the center of the cell, ultimately leading to the development of a new cell wall that divides the cell into two. The formation of chromosomes, separation of chromatids, and condensation of chromatin are all critical phases that occur during earlier stages of cell division, specifically during prophase and metaphase of mitosis, rather than during cytokinesis itself. The distinction between these phases is important to understand the overall process of cell division.