Models of an axiomatic system are isomorphic if there is a one-to-one correspondence between their elements that preserves all relations. That is, the models are abstractly the same; only the notation is different. An axiomatic system is categorical if every two models of the system are isomorphic.
The Nucleus. The largest and most clearly visible of the cell's constituents is the nucleus. Within the nucleus, DNA molecules, the cell's genetic machinery, are stored, repaired, transcribed and eventually replicated. Around the outside of the nucleus is an envelope consisting of two layers of membrane.
All cells have a cell membrane and the cytoplasm (liquid medium). A nucleus is found in eukaryotic cells, along with various organelles such as ribosomes, vacuoles and mitochondria.Viruses have neither, and instead have a protein coat and a structure, usually a tail, that can penetrate a living cell.ll cells have a cell membrane and the cytoplasm (liquid medium). A nucleus is found in eukaryotic cells, along with various organelles such as ribosomes, vacuoles and mitochondria.