Give general information about the origin of the recombinant $DNA$ molecule.

(A) The construction of the first recombinant $DNA$ emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid of $Salmonella$ $typhimurium$.
Stanley Cohen and Herbert Boyer accomplished this in $1972$.
- The cutting of $DNA$ at specific locations became possible with the discovery of molecular scissors,known as restriction enzymes.
The cut piece of $DNA$ was then linked with the plasmid $DNA$. These plasmid $DNA$ molecules act as vectors to transfer the piece of $DNA$ attached to them.
$\Rightarrow$ Just as a mosquito acts as an insect vector to transfer the malarial parasite into the human body,a plasmid can be used as a vector to deliver an alien piece of $DNA$ into the host organism.
The linking of the antibiotic resistance gene with the plasmid vector became possible with the enzyme $DNA$ ligase.
This creates a new combination of circular,autonomously replicating $DNA$ in vitro,which is known as recombinant $DNA$.
When this $DNA$ is transferred into $Escherichia$ $coli$,a bacterium closely related to $Salmonella$,it can replicate using the new host's $DNA$ polymerase enzyme and produce multiple copies.