THE LIVING WORLD
Unit two. The Living Cell
4.5. The Nucleus: The Cell's Control Center
Comparing the animal and plant cells on the previous pages, you cannot help but notice that many parts of the cells are remarkably similar. In paramecia, petunias, and primates, cell organelles look similar and carry out similar functions (see table 4.2 on pages 88-89).
If you were to journey far into the interior of one of your cells, you would eventually reach the center of the cell. There you would find, cradled within a network of fine filaments like a ball in a basket, the nucleus (figure 4.8). The nucleus is the command and control center of the cell, directing all of its activities. It is also the genetic library where the hereditary information is stored.
Figure 4.8. The nucleus.
The nucleus is composed of a double membrane, called a nuclear envelope, enclosing a fluid-filled interior containing the chromosomes. In cross section, the individual nuclear pores are seen to extend through the two membrane layers of the envelope. The pore is lined with protein, which acts to control access through the pore.
The surface of the nucleus is bounded by a special kind of membrane called the nuclear envelope. The nuclear envelope is actually two membranes, one outside the other, like a sweater over a shirt. The nuclear envelope acts as a barrier between the nucleus and the cytoplasm, but substances need to pass through the envelope. The exchange of materials occurs through openings scattered over the surface of this envelope. Called nuclear pores, these openings form when the two membrane layers of the nuclear envelope pinch together. A nuclear pore is not an empty opening, however; rather, it has many proteins embedded within it that permit proteins and RNA to pass into and out of the nucleus.
In both prokaryotes and eukaryotes, all hereditary information specifying cell structure and function is encoded in DNA. However, unlike the circular prokaryotic DNA, the DNA of eukaryotes is divided into several segments and associated with protein, forming chromosomes. The proteins in the chromosome permit the DNA to wind tightly and condense during cell division. Under a light microscope, these condensed chromosomes are readily seen in dividing cells as densely staining rods. After cell division, eukaryotic chromosomes uncoil and fully extend into threadlike strands called chromatin that can no longer be distinguished individually with a light microscope within the nucleoplasm. Once uncoiled, the chromatin is available for protein synthesis. RNA copies of genes are made from the DNA in the nucleus. The RNA molecules leave the nucleus through the nuclear pores and enter the cytoplasm where proteins are synthesized.
To make its many proteins, the cell employs a special structure called a ribosome, a kind of platform on which the proteins are built. Ribosomes read the RNA copy of a gene and use that information to direct the construction of a protein. Ribosomes are made up of several special forms of RNA called ribosomal RNA, or rRNA, bound up within a complex of several dozen different proteins.
You will notice in figure 4.8 that one region of the nucleus appears darker than the rest; this darker region is called the nucleolus. There a cluster of several hundred genes encode rRNA where the ribosome subunits assemble. These subunits leave the nucleus through the nuclear pores and enter the cytoplasm, where final assembly of ribosomes takes place.
Key Learning Outcome 4.5. The nucleus is the command center of the cell, issuing instructions that control cell activities. It also stores the cell's hereditary information.