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.
The nucleus is one of the most important organelles in a cell. It is often the largest organelle in animal cells, but this is not always the case. Nuclei contain the genetic material called DNA that is responsible for controlling and directing all cell activities.he Cytoplasm is the gel-like substance that fills the cell outside the nucleus. All of the cell's organelles are held and located within the cytoplasm. There are many different chemicals, nucleic acids, and other substances that the cell needs can be found there also. It supports the cell and gives it shape.