DNA

Because cells do . . .??

All cells need the same copy of an organisms DNA.

topoisomerase

A long time ago lots of chemicals were bumping around and making random combinations. Those combinations which happened to be able to make copies of themselves became more prevalent because of those very copies. Once the process was started the progression was inevitable.

The short answer is that DNA replicates because its natural for it to do so.

A deeper question might be, why do the natural laws of the Universe express themselves in this way. Such that the most basic forms of matter, mixed with variation and replication and selection, can result in life, and thought, and in beings like ourselves which can ask this question.

So we could ask, Why does the Universe create life? And we could say, Because it's in its nature to do so. But the question of "why" always waits.

did you read about the protein reactions that are kindled by topoisomerase? My suggestion is to look at the reports that show the interactions and compare them to the myriad of REASONS that topoisomerase was actually formulated , and in a specific case , why it was reacting to "unzip" the DNA /RNA. Its an interesting "flood" of chemicals involved.

DNA replicates as part of cell replication. Cells replicate to create new tissue for growth or repair.

Six months late here, Ros, but isn't that about like saying locomotives move because they are posessed by a locomotive force?

Maybe. But the question was kind of mal-formed, so it resulted in a mal-formed answer

Never ask why.

The question should be "How does DNA replicate". You will never never know the "why", unless of course you are God.

Exactly.