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.
Nucleus can either refer to a part of an atom, or a part of a cell. In biology, the nucleus is the information center of a eukaryotic cell, and contains DNA to instruct the cell's life processes.In an atom, the nucleus is the central body of protons and neutrons.n biology, the nucleus is the information center of a eukaryotic cell, and contains DNA to instruct the cell's life processes. In an atom, the nucleus is the central body of protons and neutrons.