Replication of DNA

One of the most important properties of DNA is that it can make exact copies of itself. This process is called replication, and is the very basis of life. The two strands of a DNA double helix are united by hydrogen bonds between the purine and pyrimidine base pairs. When the hydrogen bonds break the two trands separate and unwind.

DNA is found mostly in the chromosomes. When the chromosome divides into two during metosis (actually during interphase) the two daughter chromosomes have identical DNA double helices. Now, all the cells in the body are ultimately derived from the zygote by repeated division. DNA fulfils the requirement of a genetical material: the ability to replicate.

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 neoconservative.

3. DNA replication requires a DNA template, a primer, and deoxyribonucleoside triphosphates (dATP, dGTP, dTTP and dCTP), Mg++, DNA unwinding protein, super helix relaxing protein, a modiried RNA Polymerase to synthesize the RNA primer, the products of dnaA, dnaB, dnaC-D, dnaE and dnaG genes and polynucleotide ligas, a joining enzyme.

4. Replication starts at a specific point called the origin.

5. According to one model replication starts with a ‘nick’ or 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 catalyzed 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 strands and is synthesized the 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 place 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 stand and the other Okazaki fragments, or both strands may synthesize Okazaki fragments.

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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