Answer:2. Salmonella typhimurium
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. 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. These plasmid DNA act as vectors to
transfer the piece of DNA attached to it.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. This makes a new
combination of circular autonomously replicating DNA created in vitro
and is known as recombinant DNA. 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. The ability to multiply copies of antibiotic resistance gene in
E. coli was called cloning.
•Salmonella typhi is a pathogenic bacterium which causes typhoid fever in human.
•Sustained high fever (39° to 40°C) is indication of typhoid.
•Typhoid fever could be confirmed by Widal Test.
