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

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When does DNA replication occur in the cell cycle?

During metaphase
After mitosis
Before mitosis
During cytokinesis

The correct answer is: Before mitosis

DNA replication occurs before mitosis because it is essential for ensuring that each daughter cell receives an identical set of chromosomes. Prior to the cell entering the mitotic phase, the cell is in the synthesis phase (S phase) of interphase, during which the DNA is duplicated. This replication process allows for the creation of two copies of each chromosome, which are necessary for proper distribution during cell division. Metaphase and cytokinesis are both stages related to the process of mitosis itself, where the actual separation of chromosomes takes place and the cell physically divides, respectively. If replication were to occur during or after mitosis, it would not guarantee that the new cells formed would have the correct genetic information. Therefore, DNA replication must take place before the cell undergoes division to maintain genetic fidelity.