Why do cells use both DNA and RNA in protein synthesis?

Ok here is what I know that happens. It is said that our cells are like a big building that has the planes for its own construction in the wardrobe of any room, this is the cell core, in this planes there are the instructions for anything, for eating, for breathing, for reproducing and for dying. A hormone arribes to the cell membrane, this hormone is the messager, it carries some instruction for the cell, the cell then starts consulting its dna looking for the specific part of that instruction, when it find it it copy the specific part to another molecule, the RNA (messager rna), this rna goes to the ribosome, where it copy again to another rna molecule (rna trascription) the ribosome starts to read the rna and to produce the specific aminoacids that are codified, this aminoacids are then assembled to form proteins, this proteins are then assembled to form bigger structures.