5-2.1. Introduction
Cell membrane is a sheet like assembly of amphipathic molecules that separate cells from their environment. These physical structures allow only the controlled exchange of materials among the different parts of a cell and with its immediate surroundings. DNA is an anionic polymer, larger molecular weight, hydrophilic and sensitive to nuclease degradation in biological matrices.  They cannot easily cross the physical barrier of membrane and enter the cells unless assisted.

Various charged chemical compounds can be used to facilitate DNA transfer directly to the cell. These synthetic compoundsare introduced near the vicinity of recipient cells thereby disturbing the cell membranes, widening the pore size and allowing the passage of the DNA into the cell.
An ideal chemical used for DNA transfer should have the ability to-

• Protect DNA against nuclease degradation.
• Transport DNA to the target cells.
• Facilitate transport of DNA across the plasma membrane.
• Promote the import of DNA into the nucleus.

The commonly used methods of chemical transfection use the following,

• Calcium phosphate
• DEAE dextran
• Cationic Lipid
• Other polymers - poly-L-lysine (PLL), polyphosphoester, chitosan, dendrimers