Which of the following techniques is most commonly used to separate $DNA$ molecules by size?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In recombinant $DNA$ technology, lysozyme is used for disrupting bacterial cells while cellulase is for plant cells.
Reason $R$: Isolation of genetic material needs disruption of cells.
In the light of the above statements, choose the most appropriate answer from the options given below:

Describe the role of $CaCl_2$ in the preparation of competent cells.

The process of separation and purification of the expressed protein before marketing is called .........

The optimum temperature for polymerisation in $PCR$ is $....$ while the enzyme responsible for the mentioned step can tolerate temperatures up to $....$. Select the correct option according to the blanks.

Which of the following is $NOT$ included in the downstream processing of recombinant $DNA$ technology?