OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

During the third stage of mitosis, what occurs?

1. Chromosomes are replicated

2. Chromosomes line up at the cell's center

3. Two new cells are formed

4. The DNA forms long strings

The correct answer is: Chromosomes line up at the cell's center

During the third stage of mitosis, known as anaphase, the key process that occurs is the separation and movement of sister chromatids toward opposite poles of the cell. This movement follows the alignment of chromosomes at the cell’s center, which occurs in the previous stage, metaphase. The correct answer addresses the specific action taking place during this phase of mitosis where the chromosomes are lined up along the metaphase plate, preparing for separation. The other choices refer to different processes within the mitosis cycle. For instance, chromosome replication occurs during the S phase of interphase, not during mitosis itself. The formation of two new cells is characteristic of cytokinesis, which happens after mitosis has completed. Lastly, the DNA forming long strings occurs during the early stages of cell division when the DNA is not yet condensed into individual chromosomes. Thus, the option that highlights the alignment of chromosomes at the cell's center is crucial for the proper progression of mitosis.