1. Copyright 2007 by Saunders/Elsevier. All rights reserved. Chapter 3: Nucleus Color Textbook of Histology, 3rd ed. Gartner & Hiatt Copyright 2007 by Saunders/Elsevier. All rights reserved.

2. Nucleus The nucleus is the largest organelle of the cell. It contains most of the deoxyribonucleic acid (DNA) possessed by the cell as well as the mechanisms for ribonucleic acid (RNA) synthesis. It houses the nucleolus, the location for the assembly of ribosomal subunits. The nucleus, bounded by two lipid membranes, possesses three major components: ■ Chromatin, the genetic material of the cell ■ The nucleolus, the center for ribosomal RNA (rRNA) synthesis ■ Nucleoplasm, containing macromolecules and nuclear particles involved in the maintenance of the cell For more information see Chapter 3 of Gartner and Hiatt: Color Textbook of Histology, 3 rd ed. Philadelphia, W.B. Saunders, 2007. Figure 3–3 Nucleus. The outer nuclear membrane is studded with ribosomes on its cytoplasmic surface, and it is continuous with the rough endoplasmic reticulum. The space between the inner and outer nuclear membranes is the perinuclear cistern. Observe that the two membranes are united at the nuclear pores.

3. Copyright 2007 by Saunders/Elsevier. All rights reserved. Nuclear Pore Complex The nuclear pore complex is about 80 to 100 nm in diameter and spans the two nuclear membranes. It is composed of three ring-like arrays of proteins stacked on top of the other, each displaying eight-fold symmetry and interconnected by a series of spokes arranged in a vertical fashion. In addition, the nuclear pore complex has cytoplasmic fibers, a transporter, and a nuclear basket. The bidirectional traffic between the nucleus and the cytoplasm is mediated by a certain group of proteins known as (1) exportins, which transport macromolecules (e.g., RNA) from the nucleus to the cytoplasm, and (2) importins, which transport cargo (e.g., protein subunits of ribosomes) from the cytoplasm to the nucleus. Exportin and importin function is regulated by a family of GTP- binding proteins known as Ran. Transport across the nuclear pore complex is usually, but not necessarily always, an energy-requiring process. For more information see the Nuclear Pore section of Chapter 3: Gartner and Hiatt: Color Textbook of Histology, 3 rd ed. Philadelphia, W.B. Saunders, 2007. DNA Transcription Transcription of DNA into mRNA begins with attachment of RNA polymerase II to a core promoter, a specific DNA sequence located adjacent to a gene. In the presence of a series of cofactors, RNA polymerase II initiates transcription by unwinding the double helix of the DNA two turns, thus exposing the nucleotides and, therefore, codons on the DNA strand. The enzyme uses one of the exposed DNA strands as a template on which to assemble and polymerize complementary bases of the RNA molecule. The process repeats as a new region of the DNA double helix is unwound and more nucleotides are polymerized into the growing mRNA chain. As the enzyme moves along the DNA molecule, the polymerized mRNA chain is separated from the template DNA strand, permitting the two DNA strands to re-form into the double helix configuration. For more information see the Nuclear Pore section of Chapter 3: Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007.

4. Copyright 2007 by Saunders/Elsevier. All rights reserved. Cell Cycle The cell cycle is divided into two major events: mitosis, the short period of time during which the cell divides its nucleus and cytoplasm, giving rise to two daughter cells, and interphase, a longer period of time during which the cell increases its size and content and replicates its genetic material (Fig. 3–11). The cell cycle may be thought of as beginning at the conclusion of the telophase stage in mitosis (M), after which the cell enters interphase. Interphase is subdivided into three phases: ■ G 1 (gap) phase, when the synthesis of macromolecules essential for DNA duplication begins ■ S (synthetic) phase, when the DNA is duplicated ■ G 2 phase, when the cell undergoes preparations for mitosis ■ Cells that become highly differentiated after the last mitotic event may cease to undergo mitosis either permanently (neurons, muscle cells) or temporarily (peripheral lymphocytes) and return to the cell cycle at a later time. Cells that have left the cell cycle are said to be in a resting stage, the G 0 (outside) phase, or the stable phase. For more information see the Cell cycle section of Chapter 3: Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007. Figure 3–12 The cell cycle in actively dividing cells. Nondividing cells, such as neurons, leave the cycle to enter the G 0 phase (resting stage). Other cells, such as lymphocytes, may return to the cell cycle.

5. Copyright 2007 by Saunders/Elsevier. All rights reserved. Mitosis Mitosis (M) occurs at the conclusion of the G 2 phase and thus completes the cell cycle. Mitosis is the process whereby the cytoplasm and the nucleus of the cell are divided equally into two identical daughter cells. First, the nuclear material is divided in a process called karyokinesis, followed by division of the cytoplasm, called cytokinesis. The process of mitosis is divided into five distinct stages: prophase, prometaphase, metaphase, anaphase, and telophase. For more information see the Mitosis section of Chapter 3: Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007. Figure 3–16 Stages of mitosis in a cell containing a diploid (2n) number of 6 chromosomes.

6. Copyright 2007 by Saunders/Elsevier. All rights reserved. Meiosis Meiosis is a specialized type of cell division that produces the germ cells—the ova and the spermatozoa. This process has two crucial results: 1. Reduction in the number of chromosomes from the diploid (2n) to the haploid (1n) number, ensuring that each gamete carries the haploid amount of DNA and the haploid number of chromosomes. 2. Recombination of genes, ensuring genetic variability and diversity of the gene pool. Meiosis is divided into two separate events: Meiosis I, or reductional division (first event). Homologous pairs of chromosomes line up, members of each pair separate and go to opposite poles, and the cell divides; thus, each daughter cell receives half the number of chromosomes (haploid number). Meiosis II, or equatorial division (second event). The two chromatids of each chromosome are separated, as in mitosis, followed by migration of the chromatids to opposite poles and the formation of two daughter cells. These two events produce four cells (gametes), each with the haploid number of chromo For more information see the Meiosis section of Chapter 3: Gartner and Hiatt: Color Textbook of Histology, 3rd ed. Philadelphia, W.B. Saunders, 2007. Figure 3–18 Stages of meiosis in an idealized cell containing a diploid (2n) number of 4 chromosomes.