Advanced Placement (AP) Biology Practice Exam

During which stage of mitosis are chromosomes aligned at the metaphase plate?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

The correct answer is: Metaphase

During the stage of metaphase in mitosis, chromosomes are aligned at the metaphase plate, an imaginary plane equidistant from the two spindle poles. At this point, each chromosome, which consists of two sister chromatids, is attached to spindle fibers originating from opposite spindle poles. This alignment is crucial because it ensures that when the chromosomes are separated during the next stage, anaphase, each daughter cell will receive an identical set of chromosomes. The proper alignment at the metaphase plate also allows for the monitoring of errors before any segregation occurs, acting as a checkpoint that maintains genetic stability in the daughter cells. In contrast, during prophase, chromosomes begin to condense and become visible, but they haven't yet aligned. Anaphase follows metaphase and involves the separation of the sister chromatids, which move toward opposite poles of the cell. Telophase marks the conclusion of mitosis, where the separated chromatids reach the poles and begin to de-condense, forming new nuclear membranes around each set.