HESI A2 SCIENCE Practice Test

During the anaphase of mitosis, the key event is the separation of sister chromatids, which are identical copies of a chromosome that were connected at a region called the centromere. Once the anaphase begins, the protein connections holding the sister chromatids together are cleaved, allowing them to be pulled apart by the spindle fibers. These spindle fibers extend from structures called centrosomes, which are located at the opposite poles of the cell.

As the sister chromatids are separated, they are moved toward opposite poles, ensuring that each new daughter cell will receive an identical set of chromosomes. This movement is crucial for maintaining the correct chromosome number in the resulting daughter cells.

The other choices describe processes that occur at different stages of mitosis. For instance, aligning of homologous chromosomes occurs during metaphase, whereas the reformation of the nuclear envelope takes place in the telophase. The condensation of chromatin into visible chromosomes is a characteristic of prophase. Therefore, the correct answer regarding anaphase is that sister chromatids separate and move towards opposite poles of the cell.