An enzyme, which cleaves DNA after identifying a specific sequence of DNA is known as a restriction enzyme. The restriction enzymes are regarded as molecular scissors. The restriction enzymes are used by scientists to cleave a single gene from a bigger DNA piece that is, a fragment of foreign DNA and is spliced into a phage genome with the help of DNA ligase and restriction enzyme.
The packaging of recombinant phage DNA is done in vitro and is permitted to infect a bacterial cell. The infected bacteria generate novel phage particles, each comprising a foreign DNA.