The machinery and the enzymes in DNA replication

dna replication

After the discovery of Double helical structure of DNA by Watson and Crick, they immediately proposed the DNA replication process.

What is semiconservative DNA replication?

In the process of replication, each DNA strand first decoil itself and a new new strand is synthesized parallel to them. This process of newly synthesized strand along with parent strand is known as semiconservative DNA replication.

DNA Replication Mechanism

The replication of DNA requires set of enzymes.

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DNA polymerase

The main enzyme of replication is DNA- dependent DNA polymerase. It has ability to polymerize around 2000 base pairs per second. Along with high efficiency this enzyme also has high accuracy. If any mistake would happen during replication process it would lead to mutation.

Types of DNA polymerase

In prokaryotes DNA polymerase are mainly of three types and all are exonuclease enzymes:

  • DNA Polymerase I: It is also known as Kornberg enzyme. It adds around 1000 base pairs per minute.
  • DNA Polymerase II: it adds around 50 nucleotides per minute.
  • DNA Polymerase III: It adds around 2000 base pairs per second. Very high efficiency!!

In Eukaryotes, DNA polymerases are of 5 types, these are DNA polymerase α, β, γ, δ and epsilon.

How DNA replicates?

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DNA replication completes in following steps:

1. Origin of Replication

The particular region from where DNA replication starts is known as origin of replication. The requirement of origin of replication is needed during recombinant DNA procedure where a vector is required. Vector can be a plasmid or circular DNA which provides origin of replication.

Prokaryotes have single origin of replication whereas eukaryotes have thousands of origin of replication.

2. Activation of deoxyribonucleotides

There are four types of Deoxyribonucleotides:- dAMP, dGMP, dTMP and dCMP. They all are activated by phosphate, energy and enzyme phosphorylase into triphosphate state.

Deoxyribonucleotide triphosphate enzyme

This enzyme serves dual purpose. First it acts as substrate and second it provides energy for polymerisation reaction because of terminal phosphate in its structure. same as in case of ATP.

3. Unwinding of Helix


ATP dependent helicase enzyme unwinds the double helical parental molecule.

Unwinding of DNA molecule results in the formation of Y- shaped structure which is known as Replication Fork. And this Y-shaped structure is stable by the help of special proteins called single strand binding proteins. (SSBP).


Because of unwinding of DNA strand a supercoiling structure gets developed on the end of DNA, this tension is released by topoisomerase.

In prokaryotes this function is done by DNA gyrase.

4. Formation of primer strand

Primer: Primer is a short strand of DNA, which is incapable of initiating DNA synthesis. i.e. it is unable to deposit the first nucleotide in a daughter strand. Primer strand provide a stepping stone to start errorless replication. Once replication is completed, the primer strand is removed automatically through enzymatic actions.


Primase is an enzyme which synthesized primer strand of RNA.

5. Elongation of new strand

The DNA polymerases catalyse the DNA polymerisation only in one direction. i.e. 5′—-> 3′. Due to which few complications are created at the replicating fork.

What is Leading daughter strand?

The continuous strand on one of the template strand with polarity 5′—-> 3′ is known as continuous or leading daughter strand. Means the new continuous strand have polarity of 3′—-5′ direction.

What is Lagging daughter strand?

On the other side of template strand having polarity 5′—> 3′ direction, the replication is discontinuous in the form of short fragments known as okazaki fragments.

DNA Ligase

The discontinuous fragments are latter joined by DNA Ligase enzyme.

In Eukaryotes the replication takes place at S-Phase of cell cycle. A failure in cell division DNA replication results into polyploidy.

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