Here's why:

* Mitosis is the process of cell division where one cell splits into two identical daughter cells. It has four main phases: prophase, metaphase, anaphase, and telophase.

* DNA replication is the process of copying the entire genome (the cell's DNA). This has to occur before mitosis so that each daughter cell receives a complete and identical set of chromosomes.

Think of it like this:

1. DNA replication is like making a copy of a recipe before you start cooking.

2. Mitosis is like using the recipe to cook two identical meals.

You wouldn't start cooking before you had a copy of the recipe, right? Similarly, the cell needs to copy its DNA before it can divide into two new cells.

The process of DNA replication takes place during the S phase of the cell cycle, which occurs between the G1 and G2 phases, and precedes mitosis.