DNA is negatively charged particle and any charged particle cannot enter across the cell membrane. Once the DNA is induced in the vector cell it needs a host cell so that it can be cloned.  A bacterial cell is made competitive enough to take up the DNA because in normal condition cell does not take out the foreign gene. By treating the bacterial cell with a specific concentration of a divalent cation, such as calcium increases the efficiency with which DNA enters the bacteria through pores in the cell wall. Microinjection is another method in which DNA directly injected into the nucleus of an animal.