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CELLS, TISSUES, AND SKIN
Figure 5.1 The image here is a diagrammatic view of the nuclear envelope, a double-membrane barrier separated by the perinuclear space. The outer nuclear membrane is continuous with the rough ER of the cytoplasm. The inner membrane is lined with a network of protein filaments, the nuclear lamina, that maintain the shape of the nucleus. Note the presence of nuclear pores, a complex of proteins that regulate entry and exit of large particles. The nuclear envelope encloses a gel-like fluid called the nucleoplasm in which other elements are suspended. Chromatin is composed of approximately equal amounts of DNA (our genetic material) and histone proteins, which provide a physical means for packing the very long DNA molecules.
Each gene is a segment of DNA that codes for a specific protein, and it is estimated humans have close to 35,000 different genes. The histone protein molecules associated with chromatin are used to help package a great deal of DNA into a small space. To fully appreciate this task, keep in mind that a nucleus is only about 5 micrometers in diameter. However, the total length of DNA in each nucleus is around 5 centimeters. Thus, in order to pack DNA into a nucleus, there is nearly a 10,000-fold reduction in length! This is accomplished by wrapping DNA around clusters of histone proteins, forming structures that look like beads on a string. In addition, proteins