DNA replication must occur before mitosis starts for a simple, but crucial reason: to ensure that each daughter cell receives a complete and identical copy of the genetic material. Here's why:

* Mitosis is cell division: The whole purpose of mitosis is to create two new cells from one original cell.

* Genetic material is in DNA: The instructions for building and maintaining an organism are encoded in DNA.

* Each cell needs a full set of instructions: For the new cells to function properly and develop correctly, they need a complete copy of the DNA.

Here's a breakdown:

1. Before mitosis: The original cell has one copy of its DNA, contained within chromosomes.

2. DNA replication: The DNA is copied exactly, resulting in two identical sets of chromosomes.

3. Mitosis: The cell divides, with each daughter cell receiving one complete set of chromosomes, ensuring both cells have all the genetic information they need to function independently.

Without DNA replication, mitosis would result in daughter cells with only half the genetic material, leading to:

* Incomplete or malfunctioning cells: The new cells would lack essential instructions for development and function.

* Cellular death: Cells with incomplete genetic information may not be able to survive.

In essence, DNA replication is a necessary prerequisite for mitosis. It ensures that the essential genetic blueprint is faithfully passed on to each new generation of cells.