Mechanism of DNA Replication. Watson and Crick while explaining their model of DNA structure, suggested the semi-conservative mechanism of its replication. The quality and quantity of DNA in parent and daughter cells must be the same.

Replication is one of the most important properties of DNA and forms the very basis of life.

1. Replication takes place during S-phase of the interphase between two mitotic cycles.

2. Replication is a semi-conservative process in which each of the two double helices formed from the parent double strand have one old and one new strand. Repair replication is non-conservative.

3. DNA replication requires a DNA template, a primer, deoxyribonucleoside triphosphates (dATP, dGTP, dTTP, dCTP), Mg⁺⁺, DNA unwinding protein, superhelix relaxing protein, a modified RNA polymerase to synthesize the RNA primer, a joining polynucleotide ligase, enzyme.

4. It starts at a specific point called the origin. Bacterial and viral DNA have single origin whereas eukaryotic DNA has many origins.

Replication starts with an incision made by an ‘incision enzyme’ (endonuclease)

5. According to one model replication starts with a ‘nick’ or incision made by incision enzyme (endonuclease) (Fig. 2.9).

6. The two strands of the DNA double helix unwind with the help of a DNA unwinding protein or helicases enzymes also called the DNA binding protein which binds to single DNA strand.

They become open, however, whole of DNA does not open in one stretch but the point of separation proceeds slowly. It gives the Y-shaped appearance called replication fork.

7. The unwinding of the strands imposes strain which is relieved by the action of a superhelix relaxing protein. There are enzymes called topoisomerases which can break and reseal the one strand of DNA.

8. Initiation of DNA synthesis requires RNA primer. The primer is synthesized by the DNA template close to the origin of replication. The synthesis is catalysed by a special form of RNA Polymerase.

9. Deoxyribose nucleotides are now added to the 3’ end of the RNA prime and the main DNA strand is synthesized on the DNA template. This strand is complementary to the DNA strand and is synthesized by DNA polymerase III.

10. The enzyme DNA polymerase I now degrades that RNA primer and simultaneously catalyses the synthesis of a short DNA segment to replace the primer. The segment is then joined to the main DNA strand by a DNA ligase.

Continuous and discontinuous synthesis of DNA.

11. Replication takes place discontinuously and short pieces called Okazaki fragments are synthesized. One strand may synthesize a continuous strand and the other Okazaki fragments; or both strands may synthesize Okazaki fragments. Both new strands are synthesized in the 5’→ 3’ direction. Thus one strand is synthesized forward and the other backward.

12. The Okazaki pieces are joined by polynucleotide ligase, a joining enzyme, to form continuous strands.

13. Replication may be in one direction (unidirectional) from the point of origin or in both directions (bidirectional).