Gene Transfer Methods - The ultimate goal of every genetic engineering experiment can be achieved only when the gene of interest is successfully introduced into the bacteria or cell. DNA is a negatively charged molecule and the membrane is also negatively charged.

Hence due to repulsion force of like charges they never come in contact with each other.

If there is no contact, then the transfer of the DNA into the cell becomes impossible.

This negative charge repulsion can be removed by using chemicals or by using some physical forces such as high voltage.