Final answer:
During mitosis, the DNA in the nucleus needs to be replicated so that each new cell formed after cell division receives a complete set of genetic material.
Step-by-step explanation:
During mitosis, the DNA in the nucleus needs to be replicated so that each new cell formed after cell division receives a complete set of genetic material. The process of DNA replication ensures that each daughter cell will have the same DNA as the parent cell. This replication occurs during the S phase of the cell cycle. The replicated DNA forms chromosomes, which then divide and separate into two complete sets during mitosis.