Arrange the given steps involved in the formation of recombinant human insulin in correct sequence.
(A) Linkage of separated A and B polypeptide chain by disulfide bonds
(B) Inserting genes of A and B chains into E. coli plasmid
(C) Production and extraction of chain A and B from E. coli
(D) Isolation of desired DNA sequence for A and B chain from the human gene
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (D), (B), (C), (A)
(D) Isolation of desired DNA sequence for A and B chain from the human gene — this comes first. The DNA for the two chains must be ready before anything else can be done.
(B) Inserting genes of A and B chains into E. coli plasmid — this comes second. The DNA for each chain is put into a plasmid, which is then taken up by E. coli.
(C) Production and extraction of chain A and B from E. coli — this comes third. The bacteria make the two chains, which are produced separately. They are then taken out.
(A) Linkage of separated A and B polypeptide chain by disulfide bonds — this comes last. Only then are the two chains joined to form mature human insulin.
Note: Step (D) says that the DNA sequences were isolated from the human gene. NCERT says that the company prepared two DNA sequences corresponding to chains A and B of human insulin. This does not change the order of the steps, because the DNA has to be ready first in either case.
NCERT Extract:
Genetically Engineered Insulin – “… In 1983, Eli Lilly an American company prepared two DNA sequences corresponding to A and B, chains of human insulin and introduced them in plasmids of E. coli to produce insulin chains. Chains A and B were produced separately, extracted and combined by creating disulfide bonds to form human insulin.”