Read the following paragraph and answer the given questions.
Repeated amplification is achieved by the use of a thermostable DNA polymerase (isolated from a bacterium, Thermus aquaticus), which remain active during the high temperature induced denaturation of double stranded DNA. The separated DNA fragments can be visualised only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation. The separated bands of DNA are cut out from the agarose gel and extracted from the gel piece. The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species of the prokaryotic cell from which they were isolated, e.g., EcoRI comes from Escherichia coli RY 13. In EcoRI, the letter ‘R’ is derived from the name of strain. Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria. 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. Since the DNA is enclosed within the membranes, we have to break the cell open to release DNA, RNA, protein, polysaccharides etc. The RNA can be removed by treatment with ribonuclease whereas proteins can be removed by treatment with protease. Other molecules can be removed by appropriate treatments and purified DNA ultimately precipitates out after the addition of chilled ethanol.
Stanley Cohen and Herbert Boyer constructed the first recombinant DNA by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid of __________________ which was responsible for conferring antibiotic resistance.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Salmonella typhimurium
The passage says that Cohen and Boyer cut out a piece of DNA from a plasmid that carried antibiotic resistance, but it does not name the bacterium. The book names it: the plasmid was a native plasmid of Salmonella typhimurium, and they linked the antibiotic resistance gene to it in 1972.
Option (1) Thermus aquaticus — Incorrect. It is the source of the thermostable Taq polymerase used in PCR.
Option (2) Agrobacterium tumefaciens — Incorrect. Its Ti plasmid is used as a cloning vector for plants, but it was not used in the first recombinant DNA.
Option (3) Escherichia coli — Incorrect. E. coli is the usual host for cloning and the source of EcoRI, but it was not the source of this plasmid.
Option (4) Salmonella typhimurium — Correct. Its native plasmid carried the antibiotic resistance gene.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Principles of Biotechnology – “… Let us now focus on the first instance of the construction of an artificial recombinant DNA molecule. The construction of the first recombinant DNA emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid (autonomously replicating circular extra-chromosomal DNA) of Salmonella typhimurium. 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.”