THE NUCLEUS

Within the cells of most organisms (though not of bacteria and Cyano¬bacteria), the largest and one of the most conspicuous structural areas is the membrane-bounded nucleus. The nucleus plays the central role in cellular reproduction, the process by which a single cell divides and forms two new cells, as seen under a compound microscope. It also plays a crucial part, in conjunction with the environment, in determining the way the cell will develop and what form it will exhibit at maturity. And the nu¬cleus directs the chemical activities of the living cell. In short, it is the nucleus that the “instructions” that guide the life processes of the cell as long as it lives.

The cells of bacteria and Cyanobacteria differ from those of all other kinds of organisms in lacking a membrane-¬bounded nucleus (though they do possess genetic material that controls the cell’s activities). Similarly, these two groups lack many of the other subcellular structures found in other organisms. These differ¬ences, as studied under a compound microscope, are so fundamental that bacteria and Cyanobacteria are clas¬sified in a kingdom of their own (the Monera), and their cells are designated as procaryotic (”having a primitive nucleus”), whereas the cells of all other organisms are designated as eucaryotic (”having a true nucleus”).

When viewed under a compound microscope, eucaryotic nucleus have two relatively distinct types of structures, the chromosomes and the nucleoli. These are embedded in a mass of granular-appearing nucleoplasm. The entire nucleus is bounded by a double nuclear membrane, as seen under a compound microscope.

The chromosomes, elongate, threadlike bodies, are clearly visible only when the cell is undergoing division. The chromosomes bear, apparently in lin¬ear arrangement, the basic units of heredity, called genes, which are composed of DNA. Passed from generation to generation, the genes determine the characteristics of cells and act as the units of control in the day-to-day activities of living cells. They are the code units, if you will, for the transmission of hereditary information from parent to offspring.

The hereditary information is written in the sequence of the nu¬cleotide building blocks of the DNA molecules, as studied by high-powered microscopes. This sequence deter¬mines the sequence of amino acids in the proteins (including enzymes) synthesized by the cell. Thus the genes are at the very hub of life; they encode the information necessary for the synthesis of the enzymes regulating the myriad interdependent chemical reactions that determine the characteristics of cells and organisms.

The other prominent structures in the nucleus (which was discovered with the compound microscope), besides the chromo¬somes, are the nucleoli, the dark staining, generally oval bodies that more often than not are clearly visible within the nuclei of nondivid¬ing cells. Nucleoli are, in fact, really specialized parts of the chromo¬somes. They are responsible for and exporting to the cytoplasm the precursors of the particles (ribosomes) on which proteins will be synthesized.

The presence of a nuclear membrane surrounding the nucleus permits maintenance within the nucleus of an environment dif¬ferent from that in the surrounding cytoplasm. Unlike the plasma membrane, the complete nuclear envelope consists of two membranes; it is double. A distinct space is enclosed between the inner and the outer membrane, a characteristic seen under a microscope.

Electron microscope studies indicate that the double-membrane en¬velope is interrupted at intervals by fairly large pores at pun, where the outer and inner membranes are continuous. Nevertheless, the membrane is highly selective. According to perme¬ability experiments, some substances that can cross the cell membrane¬ into the cytoplasm and apparently can readily cross the nuclear brave into the nucleus and are consequently restricted ugh the cyto¬plasm. Yet some macromolecules readily pass through the pores. These seem to be primarily substances produced on the genes that are moving out of the nucleus, proteins moving into the nucleus to be incorporated into nuclear structures or to catalyze chemical reactions in the nucleus, and various substances from the cytoplasm that move into the nucleus and help regulate gene activity. There is thus a care¬fully controlled and highly selective two-way exchange of materials between the nucleus and the cytoplasm through the pores.

The electron microscope has revealed another particularly interest¬ing fact about the nuclear membrane-the continuity, at some points of this double membrane with an extensive cytoplasmic membrane system called the endoplasmic reticulum.