Stanley Cohen and Herbert Boyer constructed the first recombinant DNA using a native plasmid of Salmonella typhimurium. Arrange the following steps of their work in the correct sequence:
A. The recombinant DNA was transferred into Escherichia coli, a bacterium closely related to Salmonella.
B. A piece of DNA carrying an antibiotic resistance gene was cut out from a plasmid using restriction enzymes.
C. The recombinant DNA replicated using the DNA polymerase enzyme of the new host and made multiple copies.
D. The cut piece of DNA was linked with the plasmid DNA using the enzyme DNA ligase.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B, D, A, C
Cohen and Boyer made the first recombinant DNA in 1972. The order follows the work itself: the gene has to be cut out before it can be joined, the joined DNA has to be ready before it can be put into a host, and copies are made only inside the host.
B. A piece of DNA carrying an antibiotic resistance gene was cut out from a plasmid using restriction enzymes. — First. The antibiotic resistance gene was isolated by cutting it out with the ‘molecular scissors’, the restriction enzymes.
D. The cut piece of DNA was linked with the plasmid DNA using the enzyme DNA ligase. — Second. DNA ligase joins the ends of the cut DNA molecules. The new circular, autonomously replicating DNA made in vitro is the recombinant DNA.
A. The recombinant DNA was transferred into Escherichia coli, a bacterium closely related to Salmonella. — Third. The plasmid acts as a vector and carries the joined gene into the new host.
C. The recombinant DNA replicated using the DNA polymerase enzyme of the new host and made multiple copies. — Fourth. Copying needs the host’s DNA polymerase, so it can happen only after the DNA is inside E. coli. This was called cloning of the antibiotic resistance gene in E. coli.
NCERT Extract:
Principles of Biotechnology – “… Stanley Cohen and Herbert Boyer accomplished this in 1972 by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance. The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’– restriction enzymes. The cut piece of DNA was then linked with the plasmid DNA. … The linking of antibiotic resistance gene with the plasmid vector became possible with the enzyme DNA ligase, which acts on cut DNA molecules and joins their ends. … When this DNA is transferred into Escherichia coli, a bacterium closely related to Salmonella, it could replicate using the new host’s DNA polymerase enzyme and make multiple copies.”