A host cell normally does not take up a foreign DNA until it has been made competent to do so. This is because:
1. DNA is a hydrophilic molecule
2. DNA is a very large molecule
3. there are no receptors for DNA on the cell membrane
4. DNA is an inert molecule
(1) Cell membrane is made of a phospholipid bilayer and allows only lipid soluble substances to diffuse across it.