The nucleus contains a blueprint for all cell structures and activities, encoded in the DNA of the chromosomes. It also contains the molecular machinery to replicate its DNA and to synthesize and process the three types of RNA : ribosomal (rRNA), messenger (mRNA), and transfer (tRNA). Mitochondria have a small DNA genome and produce RNAs to be used in this organelle, but the genome is so small that it is not sufficient even for the mitochondrion itself. On the other hand, the nucleus does not produce proteins; the numerous protein molecules needed for the activities of the nucleus are imported from the cytoplasm.
The nucleus frequently appears as a rounded or elongated structure, usually in the center of the cell (Figure 3-1). Its main components are the nuclear envelope, chromatin (Figures 3-2 and 3-3), nucleolus, and nuclear matrix. The size and morphological features of nuclei in a specific normal tissue tend to be uniform. In contrast, the nuclei in cancer cells have an irregular shape, variable size, and atypical chromatin patterns.
Liver cells (hepatocytes). Several dark-stained nuclei are shown. Note the apparent nuclear membrane consisting mainly of a superficial condensation of chromatin. Several nucleoli are seen inside the nuclei, suggesting intense protein synthesis. One hepatocyte contains two nuclei. Pararosanilineâ€“toluidine blue (PT) stain. Medium magnification.
Schematic representation of a cell nucleus. The nuclear envelope is composed of two membranes of the endoplasmic reticulum, enclosing a perinuclear cisterna. Where the two membranes fuse, they form nuclear pores. Ribosomes are attached to the outer nuclear membrane. Heterochromatin clumps are associated with the nuclear lamina, whereas the euchromatin (EC) appears dispersed in the interior of the nucleus. In the nucleolus, note the associated chromatin, heterochromatin (Hc), the pars granulosa (G), and the pars fibrosa (F).
Three-dimensional representation of a cell nucleus showing the distribution of the nuclear pores, the heterochromatin (dark regions), the euchromatin (light regions), and a nucleolus. Note that there is no chromatin closing the pores. The number of nuclear pores varies greatly from cell to cell.