DNA replication takes place in areas where there's DNA.
In eukaryotic cells the DNA can be found in the nucleus mainly. So DNA replication takes place in the nucleus during the S phase of the cell cycle.
Also in eukaryotic cells there are mitochondria and chloroplasts (plants) and these have circular DNA and they also get replicated (according to their own mechanism).
Prokaryotic cells do not contain a nucleus. They do contain DNA in the cytoplasm and thus the DNA replication will take place here.
Hope I helped. :)
