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Outline of Replication
 

Outline of Replication
1. Replication takes place during 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 and dCTP), MgH, DNA unwinding protein, super helix relaxing protein, a modified RNA polymerase to synthesize the RNA primer, the product of dnaA, dnaB, dnaC-D, dnaE and dnaG genes and polynucleotide ligase, a joining enzyme

4. Replication starts at a specific point called the origin

5. According to one model replication starts with a 'nick' 0: incision made by an incision enzyme (endonuclease)

6. The two strands of the DNA double helix unwind with the help of a DNA unwinding protein (also called the DNA binding protein) which binds to single DNA strands.

7. The unwinding of the strands imposes strain which is relieved by the action of a super helix relaxing protein.

8. Initiation of DNA synthesis requires an 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 primer and the main DNA strand is synthesized on the DNA template. This strands is complementary to the DNA strand and is synthesized by DNA polymerase III.

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

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'-7-3' direction. Thus one strand is synthesized forwards and the other backwards.

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).

 
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