During the process of isolation of $DNA$,chilled ethanol is added to

Consider the following statements:
$I.$ In microinjection method,foreign $DNA$ is directly injected into the nucleus of an animal cell or plant cell by using microneedles or micropipettes.
$II.$ Microinjection method is used in oocytes,eggs,and embryos.
$III.$ Electroporation is the formation of temporary pores in the plasma membrane of a host cell by using lysozyme or calcium chloride.
$IV.$ In chemical-mediated gene transfer method,certain chemicals such as $CO_{2}$ help foreign $DNA$ to enter the host cell.
Which of the statements given above are correct?

Which method is used to introduce $r-DNA$ directly into the nucleus of an animal cell?

How many types of primers are required for the amplification of a gene in $PCR$?

In gel electrophoresis,the separated $DNA$ fragments can be visualized by:

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 $Ca^{2+}$.