$DNA$ is a hydrophilic molecule, it cannot pass through cell membranes. In order to force bacteria to take up the plasmids the bacterial cells must be made competent to take up $DNA$. This is done by following way.

Bacterial cell is treated with a specific concentration of a divalent cation such as calcium.

Which increases the efficiency of $DNA$ to enter the bacterium through pores in its cell wall.

$r-DNA$ can then be forced into such cells by incubating the cells with recombinant $DNA$ on ice, followed by placing them briefly at $42^{\circ} \mathrm{C}$, and then putting them back on ice. This enables the bacteria to take up the recombinant $DNA.$

Micro-injection : Recombinant $DNA$ is directly injected into the nucleus of an animal cell.

Particle Bombardements : Cells are bombarded with high velocity micro particles of gold or tungsten coated with $DNA$ in a method known as biolistics or gene gun.

And the last method uses disarmed pathogen vectors, which when allowed to infect the cells transfer the recombinant $DNA$ into the host.

More Information :

Electroporation : For the entry of recombinant $DNA$, making cytoplasmic layer/cell membrane permeable, cells are given quick beats/pulse of high voltage electric current.

Lipofection : Recombinant $DNA$ is covered with fat layer. So cell membrane allows it to pass through Particle Bombardmante