Advanced Placement (AP) Biology Practice Exam

Which stage of mitosis is characterized by chromatin condensation and spindle formation?

1. Prometaphase

2. Prophase

3. Anaphase

4. Telophase

The correct answer is: Prophase

The stage of mitosis characterized by chromatin condensation and spindle formation is prophase. During this phase, the chromatin fibers in the nucleus begin to condense into visible chromosomes, making it easier to observe them under a microscope. Each chromosome now consists of two sister chromatids, which are joined at a region known as the centromere. In addition to chromatin condensation, prophase is also when the mitotic spindle begins to form. The spindle apparatus is crucial for the separation of chromosomes during cell division. Microtubules, which are part of the cytoskeleton, start to extend from the centrosomes (the organizing centers for these microtubules) and will eventually attach to the chromosomes at their kinetochores. While prometaphase also involves spindle formation and further condensation of chromosomes, it is distinguished by the preparation for chromosome alignment along the metaphase plate. Anaphase is the stage where the sister chromatids are pulled apart and move toward opposite poles, while telophase is marked by the reformation of the nuclear envelope and decondensation of chromosomes. Thus, prophase is specifically the stage where both chromatin condensation and spindle formation prominently occur.