DNA replication occurs in the nucleus of eukaryotic cells and in the nucleoid region of prokaryotic cells. In eukaryotes, the nucleus houses the chromosomes and is where the replication machinery operates during the S phase of the cell cycle. In prokaryotes, which lack a membrane-bound nucleus, replication occurs in the cytoplasm within the nucleoid region where their circular chromosome resides. The process is coordinated by origins of replication and replisomes, ensuring the genome is copied once per cell cycle. Key points:

• Eukaryotes: DNA replication in the nucleus during S phase (part of the cell cycle, after G1 and before mitosis).

• Prokaryotes: DNA replication in the nucleoid region of the cytoplasm, initiated at origins of replication, with replication proceeding bidirectionally from a single origin in many bacteria.

• The replication machinery includes helicases, primases, DNA polymerases, ligases, and other factors that organize at origins to unwind DNA and synthesize new strands (leading and lagging strands).

If you’d like, I can tailor this to a specific organism (e.g., human cells vs. E. coli) and include diagrams or a brief step-by-step timeline of initiation, elongation, and termination.