Firstly, the DNA double helix unzips and unwinds as the hydrogen bond between them break. There is presence of free DNA nucleotides in the nucleus with two phosphate groups which activate them to take part in the reaction. These free nucleotides pair up with the complementary bases of the old DNA strand. The sugar phosphate groups are joined between the neighbouring nucleotides and sugar phosphate backbone is formed. This action is also controlled by DNA polymerase. Thus, two new DNA are formed each with a strand of the old DNA where it is semi conserved.