Question

How is DNA replication semi-conservative

Answer 1

DNA is double stranded, DNA splits into single strands and the DNA polymerase uses the one of the original DNA single strand to code the other strand making one double stranded DNA, the same happens with the other half of the original DNA molecule.

each double stranded DNA had one strand (half) that came from the original strand

Answer 2

Semiconservative replication would produce two copies that each contained one of the original strands and one new strand. Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.