Mechanism of DNA replication is the direct result of DNA double helical structure proposed by Watson and Crick. It is a complex multistep process involving many enzymes.

1. Initiation:

It involves the origin of replication. Before the DNA synthesis begins, both the parental strands must unwind and separate permanently into single stranded state. The synthesis of new daughter strands is initiated at the replication fork. In fact, there are many start sites.

2. Elongation:

The next step involves the addition of new complementary strands. The choice of nucleotides to be added in the new strand is dictated by the sequence of bases on the template strand. New nucleotides are added one by one to the end of growing strand by an enzyme called DNA polymerase. There are four nucleotides, deoxyribrnucleotide triphosphates dGTP, dCTP, dATP, dTTP present in the cytoplasm.

3. Termination:

All the end termination reactions occur. Duplicated DNA molecules are separated from one another.

The purpose of DNA replication is to create two daughter DNA molecules which are identical to the parent molecule

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DNA Replication is Semi-Conservative:

Watson and Crick model suggested that DNA replication is semi-conservative. It implies that half of the DNA is conserved. Only one new strand is synthesized, the other strand is the original DNA strand (template) that is retained. Each parental DNA strand serves as a template for one new complementary strand.

:The new strand is hydrogen bonded to its parental template strand and forms double helix. Each of these strands of the double helix contains one original parental strand and one newly formed strand.

Meselson and Stahl Experiment:

Mathew Meselson and Franklin Stahl proved experimentally that parental strands of a helix are distributed equally between the two daughter molecules. They made use of the heavy isotope 15N as a tag to differentially label the parental strands. E. coli was grown in a medium containing 15N labeled NH4Cl.

In this way both strands of DNA molecules were labeled with radioactive heavy isotope 15N in their purines and pyrimidines. Therefore both strands were heavy or HH DNA. The bacteria were then transferred into a medium containing the common non-radioactive nitrogen 14N, which is a light medium. It was found that after one cell division daughter molecules had one 15N strand the other 14N strand. So this is a hybrid molecule, a heavy light of HL.

After the second cell division, out of four molecules, two DNA molecules contained UN(LL). The other two were hybrid molecules (HL). This proves that during replication, one parent strand is conserved and the other new strand is synthesized. Thus DNA replication is a semi-conservative process.

Enzymes of DNA Replication:

The enzymes which take part in replication are able to copy DNA molecules which may contain millions of bases. They perform this function with utmost accuracy and at high speed, even though DNA molecule is highly compact and is bound with proteins. Maintenance of integrity of genetic information is the main feature of replication.

In E. coli, two main enzymes that take part in polymerization are DNA polymerase I and DNA polymerase III. Of these DNA polymerase III is the main enzyme involved in replication. Polymerization involves addition of new nucleotides to a growing strand.

In addition, there is an enzyme DNA polymerase II which takes part in DNA repair. DNA polymerase IV and DNA polymerase V have also been discovered. DNA polymerases are capable of adding 1000 nucleotides per second. The speed of DNA synthesis is known as processivity.

Polymerization:

Nucleotide monophosphates, which are building blocks of DNA, cannot be added to the growing strand of DNA. The DNA polymerase can act only on deoxyribose