How the First Recombinant DNA Was Made (Cohen and Boyer)
Biology · Biotechnology: Principles and Processes · NEET
The first recombinant DNA was made in 1972 by Stanley Cohen and Herbert Boyer. They cut out an antibiotic resistance gene from a native plasmid of Salmonella typhimurium using restriction enzymes, joined it to another plasmid with DNA ligase, and put this new circular DNA into E. coli where it copied itself. Memory hook: "Cohen and Boyer, 1972 — Salmonella plasmid + resistance gene → E. coli." NEET loves this exact date, these two names, and the source bacterium as a one-mark fact.
The 1972 Cohen and Boyer experiment: a resistance gene is cut from a Salmonella typhimurium plasmid by a restriction enzyme, joined to a vector plasmid by DNA ligase to form recombinant DNA in vitro, then transferred into E. coli where it replicates (cloning). Salmonella is the source; E. coli is the host.
Your doubts, answered
Who actually made the first recombinant DNA?
Stanley Cohen and Herbert Boyer made the first recombinant DNA in 1972. NCERT names both scientists together, so in a match-the-following or fill-in-the-blank question you must remember both names, not just one.
Which bacterium's plasmid was used?
The plasmid came from Salmonella typhimurium. This is a native plasmid, meaning a natural, autonomously replicating, circular, extra-chromosomal DNA. NEET has directly asked which organism's plasmid was used for the first rDNA, so lock this name in.
Which gene did they actually transfer?
They isolated an antibiotic resistance gene. This gene was cut out of the Salmonella plasmid using restriction enzymes and then linked to plasmid vector DNA. The antibiotic resistance gene was useful because it could later be used to check if the DNA entered the host.
Why was E. coli used and not Salmonella again?
After making the recombinant DNA, it was transferred into E. coli, a bacterium closely related to Salmonella. Inside E. coli the recombinant DNA replicated using the host's own DNA polymerase and made many copies. This step of making many copies is called cloning of the antibiotic resistance gene. So Salmonella was the SOURCE of the plasmid, and E. coli was the HOST where it was multiplied.
What was the role of DNA ligase here?
DNA ligase joined the cut ends of the two DNA pieces. Restriction enzymes did the cutting, but only ligase could seal the antibiotic resistance gene onto the plasmid vector to make one continuous circular molecule. Without ligase there is no recombinant DNA. NEET 2016 tested exactly this: the enzyme that joins is ligase, not EcoRI or a polymerase.
⚠️ The NEET trap ✗ The first recombinant DNA was made using a plasmid of Escherichia coli. ✓ The plasmid was taken from Salmonella typhimurium; E. coli was only the host where the recombinant DNA was later multiplied (cloned). 🧠 Salmonella = SOURCE of the plasmid. E. coli = HOST for cloning. NTA swaps these two roles to trap you.
Real NEET questions
2020
Match the organism with its use in biotechnology: (a) Bacillus (b) Thermus aquaticus (c) Agrobacterium tumefaciens (d) Salmonella typhimurium ; (i) Cloning vector (ii) Construction of first rDNA (iii) DNA polymerase (iv) Cry proteins
A · (a)-(iii); (b)-(ii); (c)-(iv); (d)-(i)
B · (a)-(iii); (b)-(iv); (c)-(i); (d)-(ii)
C · (a)-(ii); (b)-(iv); (c)-(iii); (d)-(i)
D · (a)-(iv); (b)-(iii); (c)-(i); (d)-(ii) ✓
Solution: Bacillus (thuringiensis) gives Cry proteins (iv); Thermus aquaticus gives the thermostable DNA polymerase used in PCR (iii); Agrobacterium tumefaciens provides the Ti-plasmid cloning vector (i); and the first recombinant DNA was constructed using a native plasmid of Salmonella typhimurium by Cohen and Boyer (ii). This gives (a)-iv, (b)-iii, (c)-i, (d)-ii = option D.
2016
A foreign DNA and plasmid cut by the same restriction endonuclease can be joined to form a recombinant plasmid using
A · Eco RI
B · Taq polymerase
C · Polymerase III
D · Ligase ✓
Solution: When a foreign DNA and a plasmid are cut with the same restriction endonuclease, the fragments carry identical complementary sticky ends. DNA ligase acts on these cut DNA molecules and joins their ends to form a recombinant plasmid. EcoRI cuts (does not join), and the polymerases synthesise DNA rather than seal the join, so the answer is ligase.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1972. This is a common one-mark fact, so memorise the year along with the names Cohen and Boyer.
What is a native plasmid?
A native plasmid is a natural plasmid found inside a bacterium. It is a circular, extra-chromosomal DNA that can replicate on its own (autonomously). The Salmonella typhimurium plasmid used by Cohen and Boyer was such a native plasmid.
Was the recombinant DNA made inside a cell or in a test tube?
It was made in vitro, meaning outside the cell in a test tube. The cutting and joining were done in vitro, and only after the recombinant DNA was formed was it transferred into E. coli.
What are the three basic steps this experiment showed?
Identification of DNA with the desirable gene, cutting and joining it into a vector to make recombinant DNA, and transferring it into a host where it multiplies. This is why the next topic is the three basic steps of genetic engineering.
Why is this experiment important for NEET?
It gives fixed, factual answers: the two scientists, the year 1972, the source Salmonella typhimurium, the host E. coli, and the two enzymes (restriction enzyme and ligase). These appear as match-the-following and single-fact MCQs almost every year.
