Hybridoma cells are

Consider the following statements about insulin.
$I.$ Human insulin is made up of $51$ amino acids arranged in two polypeptide chains.
$II.$ The two polypeptide chains are interconnected by two disulphide bridges.
$III.$ In mammals,including humans,insulin is synthesised as a pro-hormone,which contains an extra stretch called the $C$-peptide.
$IV.$ $C$-peptide is not present in the mature insulin.
Which of the statements given above are correct?

How was insulin obtained before the advent of $rDNA$ technology? What were the problems encountered?

Which microorganisms are commonly used to produce vaccines through recombinant $DNA$ technology?

Which of the following is produced using $E. coli$?

The $ADA$ deficiency can be permanently cured by: