Nucleus

Almost all cells contain a single nucleus, the largest of the membrane-bound cell organelles. A few specialized cells, for example, skeletal-muscle cells, contain multiple nuclei, while the mature red blood cell has none. The primary function of the nucleus is the storage and the transmission of genetic information to the next generation of cells. This information coded in molecules of DNA is also used to synthesize the proteins that determine the structure and function of the cell (Chapter 5).

Surrounding the nucleus is a barrier, the nuclear envelope, composed of two membranes. At regular intervals along the surface of the nuclear envelope, the two membranes are joined to each other, forming the rims of circular openings known as nuclear pores (Figure 3-11). Molecules of RNA that determine the structure of proteins synthesized in the cytoplasm move between the nucleus and cytoplasm through these nuclear pores. Proteins that modulate the expression of various genes in DNA move into the nucleus through these pores. The movement of very large molecules, such as RNA and proteins, is selective—that is, restricted to specific macromolecules. An energy-dependent process that alters the diameter of the pore in response to specific signals is involved in the transfer process.

Within the nucleus, DNA, in association with proteins, forms a fine network of threads known as chromatin; the threads are coiled to a greater or lesser degree, producing the variations in the density of the nuclear contents seen in electron micrographs (Figure 3-11). At the time of cell division, the chromatin threads become tightly condensed, forming rodlike bodies known as chromosomes.

The most prominent structure in the nucleus is the nucleolus, a densely staining filamentous region without a membrane. It is associated with specific regions of DNA that contain the genes for forming the particular RNA found in cytoplasmic organelles called ribo-somes (see next page). It is in the nucleolus that this RNA and the protein components of ribosomal sub-units are assembled; these subunits are then transferred through the nuclear pores to the cytoplasm, where they combine to form functional ribosomes.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Structure Organelles

Structure: Largest organelle. Round or oval body located near the cell center. Surrounded by a nuclear envelope composed of two membranes. Envelope contains nuclear pores through which messenger molecules pass between the nucleus and the cytoplasm. No membrane-bound organelles are present in the nucleus, which contains coiled strands of DNA known as chromatin. These condense to form chromosomes at the time of cell division.

Structure: Densely stained filamentous structure within the nucleus. Consists of proteins associated with DNA in regions where information concerning ribosomal proteins is being expressed.

Nucleus

Structure: Largest organelle. Round or oval body located near the cell center. Surrounded by a nuclear envelope composed of two membranes. Envelope contains nuclear pores through which messenger molecules pass between the nucleus and the cytoplasm. No membrane-bound organelles are present in the nucleus, which contains coiled strands of DNA known as chromatin. These condense to form chromosomes at the time of cell division.

Nucleolus

Structure: Densely stained filamentous structure within the nucleus. Consists of proteins associated with DNA in regions where information concerning ribosomal proteins is being expressed.

Function: Site of ribosomal RNA synthesis. Assembles RNA and protein components of ribosomal subunits, which then move to the cytoplasm through nuclear pores.

Function: Stores and transmits genetic information in the form of DNA. Genetic information passes from the nucleus to the cytoplasm, where amino acids are assembled into proteins.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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