Why is DNA replication called semi-conservative replication?

Semi-conservative replication refers to replication of DNA in which the newly produced DNA molecules will contain 1 strand from the original DNA and a another strand that had been newly synthesised. In order to understand this further, it is best to look at how DNA replication occurs. We start with a simple DNA molecule, consisted of 2 strands of DNA tightly wound together. If we want to replicate the DNA we will have to do 2 things first the DNA molecule will have to uncoil and secondly we must break the hydrogen bonds between the complementary bases of the 2 DNA strands (remember that complementary bases A-T and C-G hydrogen bond). To be able to break apart the DNA molecule we must use a special enzyme called DNA helicase (it will catalyse the breakdown of the hydrogen bonds). Both strands of DNA molecule will now act as template strands for the formation of 2 new DNA molecules. In this instance free DNA nucleotides (recall what nucleotides are) will complementary base pair with the exposed DNA bases on each of the template strands. Now that the bases have been bound we now need to join the nucleotides together to reform the DNA strand, we use an enzyme called DNA polymerase (look at name going from monomers to polymers). Now that the strands are formed, the 2 strands will coil together once more and we receive the newly synthesised DNA strands. If you look at each new DNA molecule made, you can see that they contain one new and one old strand of DNA as shown by the different colours.