From the given combination of steps in $PCR$,identify the enzyme-dependent step.

The components of a bioreactor are:
$I.$ An agitator system
$II.$ An oxygen delivery system
$III.$ Foam control system
$IV.$ Temperature control system
$V.$ $pH$ control system
$VI.$ Sampling ports to withdraw cultures periodically
Choose the correct option.

Choose the correct statement:
$P -$ Endonucleases cut the $DNA$ at a specific site inside the $DNA$.
$Q -$ $A$ stirred-tank reactor is usually square in shape.
$R -$ In order to force bacteria to take up the plasmid,the bacterial cells must first be made 'competent' to take up $DNA$ by treating them with a divalent cation like $Ca^{2+}$.

In Gene cloning:

When was the first extra $DNA$ fragment successfully inoculated into $E. coli$ using a plasmid?

Explain how the synthesis of a gene of interest is performed using $PCR$.