Describe various enzymes involved for $DNA$Replication.
 

The process of replication required a set of catalysts (enzymes) as given below:

$DNA$ Dependent $DNA$Polymerase : It is the main enzyme which uses a $DNA$ template to catalyse the polymerisation of deoxynucleotides.

These enzymes are highly efficient enzyme as they have to catalyse polymerisation of a large number of nucleotides in a very short time.

E. coli that has only $4.6 \times 10^{6}$ bp (Compare it with human whose diploid content is $6.6 \times 10^{9} \mathrm{bp}$ ) completes the process of replication within 38 minutes; that means the average rate of polymerisation has to be approximately 2000 bp per second.

Not only do these polymerases have to be fast, but they also have to catalyse the reaction with high degree of accuracy.

Any mistake during replication would result into mutations.

Furthermore energetically replication is a very expensive process.

Deoxyribonucleoside triphosphates serve dual purposes.

In addition to acting as substrates they provide energy for polymerisation reaction (the two terminal phosphates in a deoxynucleoside triphosphates are high-energy phosphates, same as in case of $ATP$).

In prokaryotes these are three types of $DNA$ polymerases i.e.$DNA$ polymerase $I$, $II$ and $III$ whereas in eukaryotes five different $DNA$ polymerases have been identified i.e. $DNA$ polymerases $\alpha, \beta, \gamma, \delta$ and $\varepsilon$.

These enzyme also help in removing mismatched nucleotides by a mechanism called proof reading.

Helicase : It unwinds the $DNA$ strand, i.e. separates the two strands from one point, for the formation of replication fork.

Topoisomerase : The unwinding of $DNA$ creates a tension in $DNA$ strands, which gets released by the enzyme topoisomerase.

$ DNA$  Ligase : It facilitates the joining of $ DNA$  strands together by catalysing the formation of phosphodiester bond.

It plays a role in repairing single strand breaks in duplex $ DNA.$ 

In addition to $ DNA$  dependent $ DNA$  polymerases, many additional enzymes are required to complete the process of replication with high degree of accuracy.

For long $ DNA$  molecules since the two strands of $ DNA$  cannot be separated in its entire length (due to very high energy requirement) the replication within a small opening of the $ DNA$ helix. referred to as replication fork.

The $ DNA$  - dependent $ DNA$  polymerases catalyse polymerisation only in one direction that is $5^{\prime} \rightarrow 3^{\prime}$

This creates some additional complications at the replicating fork.

Consequently, on one strand (the template with polarity $\left.3^{\prime} \rightarrow 5^{\prime}\right)$, the replication is continuous.