The nucleus of a cell contains chromatin and a nucleolus. It is bounded by a porous nuclear envelope.
A comprensive diagram of a human cell nucleous. (Photo credit: Wikipedia)
Not all organisms have a nucleus in their cells. It is lacking in bacteria and blue-green algae, which are generally called cyanobacteria in more recent literature. However, the nucleus is a common feature in the cells of all animals. It is also present in the cells of protists, fungi, and all the organisms that are classified as plants in the five-kingdom system.
With a light microscope, the nucleus of a cell is clearly visible. It is bounded by a porous membrane called the nuclear envelope. Inside is one or more bodies called nucleoli plus an apparently disorderly mass of material called chromatin, which periodically organizes itself into a discrete number of units when the cell is about to divide into two cells. Half of this material ends up in one of the daughter cells, the other half migrates to its sibling.
With the advent of the electron microscope and improved viewing techniques, our knowledge of the cell nucleus has deepened. It is possible to examine the nuclear envelope more clearly. It is not a single membrane, but two parallel membranes, each of which resembles the cell membrane in its composition. Each has a lipid bilayer with a generous admixture of protein.
It is also possible to see the pores of the nuclear envelope more clearly. According to my biology book (“Biology” by Neil Campbell and others), the two membranes are fused together at the lip of each pore, and an intricate protein structure lines each pore. These proteins serve as sort of watchdogs controlling the passage of materials through the pores. My biology book also speaks of a nuclear lamina, which it defines as “a netlike array of protein filaments that maintain the shape of the nuclear envelope.” This nuclear lamina hugs the inside surface of the nuclear envelope.
When cell division occurs, the nuclear envelope is conspicuous by its absence. It simply disappears from view. Since so much more has been learned about the cell in recent years, I was hoping that some scientist would have found out what happened to it. However, none of my books even bother to mention its disappearance, though one of them says that says that fragments of the nuclear envelope of the parent cell furnish some of the materials for the nuclear envelope of the daughter cells.