How can bacteria cells be made more receptive to rDNA molecules?

Answer 1

Assuming rDNA means recombinant (you could just say plasmid and make life easier :P)

You can wash cells in water + glycerol prior to electroporation. The washes help remove salts and the glycerol acts as an osmo-stabilizer.

For chemical transformation, you wash cells in CaCl2 and something else (I haven't done a heat shock in years).

Some bacteria can become naturally competent and the conditins vary for each species. B. subtilis depends on density, growth state and some media components. Neisseria species can be transformed by like adding plasmid to a colony, I believe it's that simple.

edit: those are things that can be done on the bacteria side of things. there are other changes on the DNA side that can be done, as well, to make transformation work better.