When does DNA replication occur in a cell and why ...

It happens when a cell divides to copy the DNA´s information and transfer it to a living creature´s offspring.

In eukaryotic organisms (organisms that have internal membranes for separation of organelles) DNA replication occurs in the cell nucleus during the "S phase" of the cell cycle.

In animals, After the DNA replication the nucleus membrane in collapses and a chromosomes are separated (in bacterium this chromosomes separation occurs without nucleus membrane collapsing). Also in this stage ("M phase") all the double organelles of the cell and the cytoplasm are being divided and two daughter cell (identical cells) are created.

 # The DNA replication, (and also the replication of all the other organelles) is necessary in order to create a duplicate cell, with all the necessary information and processes of a normal cell.
From each cell you replicate, you get two identical cells - and so on.

DNA replication is the process of copying a double-stranded DNA molecule,and this occurs before cell division. This process is important in all known life forms.  In a cell, DNA replication is absolutely essential to maintain the continuity and constancy of genetic material.  Hence it must must happen before cell division. 

why does dna replicate before the cell divides?

So each of the new cells have the same original cell´s genetic information

where does DNA replication occur?

where does DNA replication occur