Biosynthesis of DNA or semi-conservative replication of DNA

• It occurs in the nucleus of the cell.
• In this method, half of the original DNA molecule is conserved and the complementary half is newly synthesized by base pairing.

Process of replication

1. Activation of nucleotides: The deoxyribonucleotides monophosphate such as dAMP ( Deoxy adenine monophosphate), dTMP, dGMP, and dCMP occur in nucleoplasm. This nucleotide is activated by the enzyme phosphorylase to form deoxyribose nucleotide triphosphate by uniting with ATP (dATP, dTTP, dGTP, dCTP). This process is called phosphorylation.

2. The unwinding of DNA: Replication of DNA starts at a specific point called the initiation point. The two DNA molecules uncoil and separate due to the leakage of hydrogen bonds between nitrogenous bases by an enzyme helicase. (The enzyme topoisomerase cuts and rejoins one strand of DNA. In bacteria and viruses, only one replication or initiation point is present because of which replication starts and ends at the same point. In Eukaryotes, the DNA molecules are large so it has many initiating points or replicon (over 1000 points).

3. Base pairing: Deoxyribonucleotide triphosphate pairs with appropriate nitrogenous bases of template DNA strand with the enzyme ligase (i.e. Adenine joint with Thymine and Cytosine joint with Guanine).

4. Conservation of triphosphate to mono phosphate: The deoxyribonucleotide triphosphate molecules pair with nitrogenous bases of template strand change into Deoxy ribonucleotide monophosphate and release a large amount of energy in the presence of enzyme pyrophosphatase.

5. Formation of new DNA Chain: the energy released by it is utilized in joining the adjacent nucleotides to form a only nucleotide chain by the enzyme polymerase.