THE NUCLEUS

 

The nucleus of the cell appears as a rounded or elongated structure situated near the center of the cell (see Fig. 4-1). The nucleus, which is enclosed by the nuclear envelope (membrane), contains genetic material known as chromatin and a distinct region called the nucleolus. All eukaryotic cells

 

 

Golgi apparatus

Nucleolus ■

Nuclear envelope surrounding nucleus ■

Rough ------

endoplasmic reticulum

Cell membrane

Chromatin

Smooth endoplasmic reticulum

FIGURE 4-1 Composite cell de­signed to show in one cell all of the various components of the nu­cleus and cytoplasm.

THE FUNCTIONAL ORGANIZATION OF THE CELL

>- Cells are the smallest functional unit of the body. They con­tain structures that are strikingly similar to those needed to maintain total body function.

>- The nucleus is the control center for the cell. It also con­tains most of the hereditary material.

>- The organelles, which are analogous to the organs of the body, are contained in the cytoplasm. They include the mitochondria, which supply the energy needs of the cell; the ribosomes, which synthesize proteins and other materials needed for cell function; and the lysosomes and proteosomes, which function as the cell's digestive system.

>- The cell membrane encloses the cell and provides for intra-cellular and intercellular communication, transport of materials into and out of the cell, and maintenance of the electrical activities that power cell function.

have at least one nucleus (prokaryotic cells, such as bacte­ria, lack a nucleus and nuclear membrane). Some cells con­tain more than one nucleus; osteoclasts (a type of bone cell) typically contain 12 or more. The platelet-producing cell, the megakaryocyte, has only one nucleus but usually con­tains 16 times the normal chromatin amount.

The nucleus is the control center for the cell. It contains deoxyribonucleic acid (DNA) that is essential to the cell because its genes contain the information necessary for the synthesis of proteins that the cell must produce to stay alive. These proteins include structural proteins and enzymes used to synthesize other substances, including carbohydrates and lipids. Genes also represent the individual units of inheritance that transmit infor­mation from one generation to another. The nucleus is also the site of ribonucleic acid (RNA) synthesis. Three types of RNA exist in the cell: messenger RNA (mRNA), which copies and carries the DNA instructions for protein synthesis to the cytoplasm; ribosomal RNA (rRNA), which moves to the cytoplasm, where it becomes the site of protein synthesis; and transfer RNA (tRNA), which also moves into the cytoplasm, where it transports amino acids to the elongating protein being synthesized (see Chapter 6).

Chromatin is the term used to describe the complex structure of DNA and DNA-associated proteins dispersed in the nuclear matrix. Two extremely long, double-stranded helical chains containing variable sequences of four nitro­genous bases comprise each DNA molecule. These bases form the genetic code. Each double-stranded DNA mole­cule periodically coils around basic proteins called his­tones, forming regularly spaced spherical structures called nucleosomes that resemble beads on a string (Fig. 4-2). This string of beads further winds into filaments that make up the structure of chromatin. Additional coiling produces structures known as chromosomes, which are visible during cell division.

RE 4-2 Increasing orders of DNA compaction in chromatin and mitotic chromosomes. (From Cormack D.H. [1993]. Essential histol­ogy. Philadelphia: J.B. Lippincott)

Although each DNA molecule contains many genetic instructions, it also contains additional material. Stretches of meaningless DNA can lie between one gene and the next and sometimes reside within the gene sequence itself. The amino acid sequences of the gene interpreted are the exon, whereas introns are the interspersed, meaningless portions. In cells that are about to divide, the DNA must be replicated before mitosis or cell division occurs. Repli­cation generates two complementary DNA strands, such that each daughter cell receives an identical set of genes.

Chromosome condensation occurs during mitosis, a process rendering the chromosomes visible under the mcroscope; at other times during the cell cycle, they are not observable. In the mature or nondividing cell, chromatin may exist in a less active, condensed form called hetero-chromatin or a transcriptionally more active form called euchromatin. Because heterochromatic regions of the nucleus stain more intensely than regions consisting of euchroma­tin, nuclear staining can be a guide to cell activity. A nu­cleus containing primarily euchromatin stains less intense.

The nucleus also contains the darkly stained round body called the nucleolus. Although nucleoli were first de­scribed in 1781, their function was unknown until the early 1960s: it was determined that transcription of RNA occurs exclusively in the nucleolus. Nucleoli are structures com­posed of regions from five different chromosomes, each with a part of the genetic code needed for the synthesis of rRNA. Euchromatic nuclei and prominent nucleoli are characteristic of cells that are actively synthesizing proteins.

Surrounding the nucleus is a doubled membrane called the nuclear envelope or nuclear membrane. The nu­clear membrane contains many structurally complex cir­cular pores where the two membranes fuse to form a gap filled with a thin protein diaphragm. Evidence suggests that many classes of molecules, including fluids, electro­lytes, RNA, some proteins, and perhaps some hormones, can move in both directions through the nuclear pores. Nuclear pores apparently regulate the exchange of mole­cules between the cytoplasm and the nucleus.