Final answer:
DNA replication occurs during the S phase of interphase, prior to the onset of mitosis, to ensure each daughter cell has an identical set of genetic information. So the correct option is C.
Step-by-step explanation:
During the cell cycle, DNA is replicated before mitosis begins. This critical process of replication occurs in the Synthesis phase, or S phase, of the cell cycle, during interphase. The purpose of this replication is to ensure that each daughter cell produced after cell division will have an identical copy of the DNA, preserving the cell's genetic information.
Observations through autoradiographs indicate that none of the cells in mitosis were radioactively labeled, confirming that DNA replication does not occur during mitosis but rather beforehand. It's essential for DNA to be copied during interphase so that, during the mitotic phase, which includes mitosis and cytokinesis, each resulting daughter cell contains the same genetic content as the parent cell.
Therefore, the correct answer to the question of when DNA is replicated during the cell cycle is:
C) DNA is replicated before mitosis begins.