Why does DNA replication need to occur?

DNA is like the instruction manual for building and operating a cell.

DNA replication needs to occur because existing cells divide to produce new cells.

Each cell needs a full instruction manual to operate properly. So the DNA needs to be copied before cell division so that each new cell receives a full set of instructions!

Here is a video which uses an animated tutorial to explain the process of .

I'd also recommend visiting the following sites which give a nice overview of the process of replication and why it needs to happen.

Lew Port - DNA Replication

Lew Port - Cell Division

Hope this helps!