The structure of a paramecium that holds a "reserve copy" of all the cell's genes is the micro nucleus.
The micro nucelus is one of the two kinds of nuclei the paramecium has. The other is the large ellipsoidal nucleus called a macronucleus. Every paramecium must have at least one small nucleus called a micronucleus. The micronucleus is a storage site for the germline genetic material of the organism.