CONCEPTS IN BIOLOGY

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

4. Cell Structure and Function

4.6. Nuclear Components

As stated at the beginning of this chapter, one of the first structures to be identified in eukaryotic cells was the nucleus. If the nucleus is removed from a eukaryotic cell or the cell loses its nucleus, the cell can live only a short time. For example, human red blood cells begin life in bone marrow, where they have nuclei. Before they are released into the bloodstream to carry oxygen and carbon dioxide, they lose their nuclei. As a consequence, red blood cells are able to function only for about 120 days before they disintegrate.

When nuclei were first identified, it was noted that certain dyes stained some parts of the nuclear contents more than others. The parts that stained more heavily were called chromatin, which means colored material. Today, we know that chromatin is composed of long molecules of DNA, along with proteins. Most of the time, the chromatin is arranged as a long, tangled mass of threads in the nucleus. However, during cell division, the chromatin becomes tightly coiled into short, dense structures called chromosomes (chromo = color; some = body). Chromatin and chromosomes are really the same molecules, but they differ in structural arrangement.

In addition to chromosomes, the nucleus may also contain one, two, or several nucleoli. A nucleolus is the site of ribosome manufacture. Specific parts of the DNA become organized within the nucleus to produce ribosomes. A nucleolus is composed of this DNA, specific granules, and partially completed ribosomes.

The final component of the nucleus is its liquid matrix, called the nucleoplasm. It is a mixture composed of water, nucleic acids, the molecules used in the construction of ribosomes, and other nuclear material (figure 4.20).

FIGURE 4.20. The Nucleus

The nucleus is bounded by two layers of membrane which separate it from the cytoplasm. The nucleus contains DNA and associated proteins in the form of chromatin material or chromosomes, nucleoli, and the nucleoplasm. Chromosomes are tightly coiled chromatin.

4.6. CONCEPT REVIEW

11. Define the following terms: chromosome, chromatin.

12. What is a nucleolus?

13. What other type of molecules are attached to DNA in chromosomes?