Why is DNA replication semi-conservative?

DNA is in a double helix structure, meaning there are two strands. When we need DNA to replicate, for example when a cell divides during growth, the strands undergo unwinding. This process is done by an enzyme called a helicase enzyme.This allows the bases that make up DNA to become exposed so free bases in the cell can bind following the Watson-Crick base pairing rules: A-T and G-C. DNA polymerase is the enzyme that catalyses this reaction.So, when the strands have finished unwinding and pairing to new bases, you get two strands made up of one old strand and one new strand which is why it is semi conservative.