Explain how recombinant $DNA$ is introduced into a host cell or a living organism.

(N/A) There are several methods for introducing ligated $DNA$ into recipient cells. Recipient cells,after being made competent,can take up $DNA$ present in their surroundings.
If a recombinant $DNA$ molecule carrying a gene for antibiotic resistance (e.g.,ampicillin resistance) is transferred into $E. coli$ cells,the host cells become transformed into ampicillin-resistant cells.
When these transformed cells are spread on agar plates containing ampicillin,only the transformants will grow,while untransformed recipient cells will die.
Because of the ampicillin resistance gene,it is possible to select the transformed cells in the presence of ampicillin. In this context,the ampicillin resistance gene is referred to as a selectable marker.