Cell Nucleus

A nucleus is a relatively large, usually spherical structure that directs the activities of the cell. It is enclosed in a double-layered nuclear envelope, which consists of an inner and an outer lipid bilayer membrane. These two membranes have a narrow space between them but are joined at places that surround relatively large openings called nuclear pores. These pores are not mere perforations, but channels consisting of more than 100 different types of proteins. Nuclear pores allow certain dissolved substances to move between the nucleus and the cytoplasm (fig. 3.20), most notably molecules of messenger RNA that carry genetic information.

The nucleus contains a fluid (nucleoplasm) in which other structures float. These structures include the following:

1. Nucleolus. A nucleolus (nu-kleco-lus) ("little nucleus") is a small, dense body largely composed of RNA and protein. It has no surrounding membrane and is formed in specialized regions of certain chromosomes. It is the site of ribosome production. Once ribosomes form, they migrate through the nuclear pores to the cytoplasm. A cell may have more than one nucleolus. The nuclei of cells that synthesize large amounts of protein, such as those of glands, may contain especially large nucleoli.

Human Flagella Cell

Light micrograph of human sperm cells (1,000x). Flagella form the tails of these cells.

Microtubules

Light micrograph of human sperm cells (1,000x). Flagella form the tails of these cells.

2. Chromatin. Chromatin consists of loosely coiled fibers in the nuclear fluid. When cell division begins, these fibers become more tightly coiled to form rodlike chromosomes. Chromatin fibers are composed of continuous DNA molecules wrapped around clusters of eight molecules of proteins called histones, giving the appearance of beads on a string. The DNA molecules contain the information for synthesis of proteins.

Table 3.2 summarizes the structures and functions of organelles.

H How are the nuclear contents separated from the cytoplasm?

What is the function of the nucleolus?

H What is chromatin?

Cells die in different ways. Apoptosis (apöo-tocsus) is one form of cell death in which the cell manufactures an enzyme that cuts up DNA not protected by histones. This is an active process because a new substance is made. Apoptosis is important in shaping the embryo, in maintaining organ form during growth, and in developing the immune system and the brain.

Necrosis is a type of cell death that is a passive response to severe injury. Typically proteins lose their characteristic shapes, and the cell membrane deteriorates as the cell swells and bursts. Unlike apoptosis, necrosis causes great inflammation.

Microtubules

Microfilaments And Microtubules

Microfilaments

Figure 3.18

Microfilaments

Figure 3.18

A transmission electron micrograph of microfilaments and microtubules within the cytoplasm (35,000x).

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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