During replication, the double stranded helix of DNA is unwinded and unzipped by helicase. Polymerase cannot just start adding nucleotide to the strand so DNA primase synthesizes a short strand of nucleotide called primers, then the enzyme polymerase can now start the additional of free nucleotide. The enzyme polymerase adds free complementary nucleotide to the leading strand in 3' to 5' and the same is done for the lagging strand. The new daughter complementary strand formed on the leading strand is identical to the parent lagging strand and the new daughter complementary strand formed on the lagging strand is identical to the parent leading strand, so a pair of identical daughter double stranded DNA or two identical daughter double stranded DNA are formed.
