endoplasmic reticulum nuclear pore outer nuclear membrane inner nuclear membrane nuclear envelope nuclear lamina microtubule centrosome nucleolus DNA and associated proteins (chromatin), plus many RNA and protein molecules 2 µm (A) (B) nucleolus nuclear envelope heterochromatinheterochromatin
Figure 4–9 A cross-sectional view of a typical cell nucleus. (A) Electron micrograph of a thin section through the nucleus of a human fibroblast. (b) Schematic drawing, showing that the nuclear envelope consists of two membranes, the outer one being continuous with the endoplasmic reticulum (ER) membrane (see also Figure 12–7). The space inside the endoplasmic reticulum (the ER lumen) is colored yellow; it is continuous with the space between the two nuclear membranes. The lipid bilayers of the inner and outer nuclear membranes are connected at each nuclear pore. A sheetlike network of intermediate filaments (brown) inside the nucleus forms the nuclear lamina (brown), providing mechanical support for the nuclear envelope (for details, see Chapter 12). The dark-staining heterochromatin contains specially condensed regions of DNA that will be discussed later. (A, courtesy of E.g. Jordan and J. mcgovern.)
Eukaryotic DNA Is Packaged into a Set of Chromosomes
Each chromosome in a eukaryotic cell consists of a single, enormously long linear DNA molecule along with the proteins that fold and pack the fine DNA thread into a more compact structure. In addition to the proteins involved in packaging, chromosomes are also associated with many other proteins (as well as numerous RNA molecules). These are required for the processes of gene expression, DNA replication, and DNA repair. The complex of DNA and tightly bound protein is called chromatin (from the Greek chroma, “color,” because of its staining properties).
Bacteria lack a special nuclear compartment, and they generally carry their genes on a single DNA molecule, which is often circular (see Figure 1–24). This DNA is also associated with proteins that package and condense it, but they are different from the proteins that perform these functions in eukaryotes. Although the bacterial DNA with its attendant proteins is often called the bacterial “chromosome,” it does not have the same structure as eukaryotic chromosomes, and less is known about how the bacterial DNA is packaged. Therefore, our discussion of chromosome structure will focus almost entirely on eukaryotic chromosomes.
With the exception of the gametes (eggs and sperm) and a few highly specialized cell types that cannot multiply and either lack DNA altogether (for example, red blood cells) or have replicated their DNA without completing cell division (for example, megakaryocytes), each human cell nucleus contains two copies of each chromosome, one inherited from the mother and one from the father. The maternal and paternal chromosomes of a pair are called homologous chromosomes (homologs). The only nonhomologous chromosome pairs are the sex chromosomes in males, where a Y chromosome is inherited from the father and an X chromosome from the mother. Thus, each human cell contains a total of 46 chromosomes—22 pairs common to both males and females, plus two so-called sex chromosomes (X and Y in males, two Xs in females). These human chromosomes can be readily distinguished by “painting” each one a different color using a technique based on DNA hybridization (Figure 4–10). In this method (described in detail in Chapter 8), a short strand of nucleic acid tagged with a fluorescent dye serves as a “probe” that picks out its complementary DNA sequence, lighting up the target chromosome at any site where it binds. Chromosome painting is most frequently done at the stage in the cell cycle called mitosis, when chromosomes are especially compacted and easy to visualize (see below).