Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 2003 Chapter 3 Chapter 3 Cellular Form and Function 123 Dynein arms Protofilaments Tubulin (c) (b) (a) Figure 3.32 Microtubules. (a) A microtubule is composed of 13 protofilaments. Each protofilament is a spiral chain of globular proteins called tubulin. (b) One of the nine microtubule pairs that form the axonemes of cilia and flagella. (c) One of the nine microtubule triplets that form a centriole. Table 3.4 Summary of Organelles and Other Cellular Structures Structure Appearance to TEM Function Plasma membrane Two dark lines at cell surface, separated by narrow Prevents escape of cell contents; regulates exchange of (figs. 3.3 and 3.6) light space materials between cytoplasm and extracellular fluid; involved in intercellular communication Microvilli Short, densely spaced, hairlike processes or scattered Increase absorptive surface area; some sensory roles (figs. 3.10 and 3.11a–b) bumps on cell surface; interior featureless or with bundle (hearing, equilibrium, taste) of microfilaments Cilia Long hairlike projections of apical cell surface; axoneme Move substances along cell surface; some sensory roles (figs. 3.11c–e and 3.12) with 9 ϩ 2 array of microtubules (hearing, equilibrium, smell, vision) Flagellum Long, single, whiplike process with axoneme Sperm motility Nucleus Largest organelle in most cells, surrounded by double unit Genetic control center of cell; directs protein synthesis (figs. 3.3 and 3.25) membrane with nuclear pores Rough ER Extensive sheets of parallel unit membranes with Protein synthesis and manufacture of cellular membranes (fig. 3.26a) ribosomes on outer surface Smooth ER Branching network of tubules with smooth surface Lipid synthesis, detoxification, calcium storage (fig. 3.26b) (no ribosomes); usually broken into numerous small segments in TEM photos Ribosomes Small dark granules free in cytosol or on surface of Interpret the genetic code and synthesize polypeptides (fig. 3.26a) rough ER Golgi complex Several closely spaced, parallel cisternae with thick edges, Receives and modifies newly synthesized polypeptides, (fig. 3.27) usually near nucleus, often with many Golgi vesicles nearby synthesizes carbohydrates, adds carbohydrates to glycoproteins; packages cell products into Golgi vesicles Golgi vesicles Round to irregular sacs near Golgi complex, usually with Become secretory vesicles and carry cell products to (fig. 3.27) light, featureless contents apical surface for exocytosis, or become lysosomes (continued) Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 2003 Chapter 3 124 Part One Organization of the Body Table 3.4 Summary of Organelles and Other Cellular Structures, (continued) Structure Appearance to TEM Function Lysosomes Round to oval sacs with single unit membrane, often a dark Contain enzymes for intracellular digestion, autophagy, (fig. 3.28a) featureless interior but sometimes with protein layers programmed cell death, and glucose mobilization or crystals Peroxisomes Similar to lysosomes; often lighter in color Contain enzymes for detoxification of free radicals, (fig. 3.28b) alcohol, and other drugs; oxidize fatty acids Mitochondria Round, rod-shaped, bean-shaped, or threadlike structures ATP synthesis (fig. 3.29) with double unit membrane and shelflike infoldings called cristae Centrioles Short cylindrical bodies, each composed of a circle of nine Form mitotic spindle during cell division; unpaired (fig. 3.30) triplets of microtubules centrioles form basal bodies of cilia and flagella Centrosome Clear area near nucleus containing a pair of centrioles Organizing center for formation of microtubules of (fig. 3.5) cytoskeleton and mitotic spindle Basal body Unpaired centriole at the base of a cilium or flagellum Point of origin, growth, and anchorage of a cilium or (fig. 3.11e) flagellum; produces axoneme Microfilaments Thin protein filaments (6 nm diameter), often in parallel Support microvilli; involved in muscle contraction and (figs. 3.10 and 3.31) bundles or dense networks in cytoplasm other cell motility, endocytosis, and cell division Intermediate filaments Thicker protein filaments (8–10 nm diameter) extending Give shape and physical support to cell; anchor cells to (fig. 3.31) throughout cytoplasm or concentrated at cell-to-cell each other and to extracellular material; compartmentalize junctions cell contents Microtubules Hollow protein cylinders (25 nm diameter) Form axonemes of cilia and flagella, centrioles, basal (figs. 3.31 and 3.32) bodies, and mitotic spindles; enable motility of cell parts; direct organelles and macromolecules to their destinations within a cell Inclusions Highly variable—fat droplets, glycogen granules, protein Storage products or other products of cellular metabolism, (fig. 3.26b) crystals, dust, bacteria, viruses; never enclosed in unit or foreign matter retained in cytoplasm membranes Insight 3.4 Evolutionary Medicine Mitochondria—Evolution and Clinical Significance It is virtually certain that mitochondria evolved from bacteria that invaded another primitive cell, survived in its cytoplasm, and became permanent residents. Certain modern bacteria called ricketsii live in the cytoplasm of other cells, showing that this mode of life is feasible. The two unit membranes around the mitochondrion suggest that the original bacterium provided the inner membrane and the host cell’s phagosome provided the outer membrane when the bacterium was phagocytized. Several comparisons show the apparent relationship of mitochon- dria to bacteria. Their ribosomes are more like bacterial ribosomes than those of eukaryotic (nucleated) cells. Mitochondrial DNA (mtDNA) is a small, circular molecule that resembles the circular DNA of other bac- teria, not the linear DNA of the cell nucleus. It replicates independently of nuclear DNA. mtDNA codes for some of the enzymes employed in ATP synthesis. It consists of 16,569 base pairs (explained in chapter 4), comprising 37 genes, compared to over a billion base pairs and about 35,000 genes in nuclear DNA. When a sperm fertilizes an egg, any mitochondria introduced by the sperm are quickly destroyed and only those provided by the egg are passed on to the developing embryo. Therefore, all mitochondrial DNA is inherited exclusively through the mother. While nuclear DNA is reshuffled in every generation by sexual reproduction, mtDNA remains unchanged except by random mutation. Biologists and anthropologists have used mtDNA as a “molecular clock” to trace evolutionary lineages in humans and other species. mtDNA has also been used as evidence in criminal law and to identify the remains of soldiers killed in action. mtDNA was used recently to identify the remains of the famed bandit Jesse James, who was killed in 1882. Anthropologists have gained evi- dence, although still controversial, that of all the women who lived in Africa 200,000 years ago, only one has any descendents still living today. This “mitochondrial Eve” is ancestor to us all. mtDNA is very exposed to damage from free radicals normally gen- erated in mitochondria by aerobic respiration. Yet unlike nuclear DNA, mtDNA has no effective mechanism for repairing damage. Therefore, it mutates about ten times as rapidly as nuclear DNA. Some of these mutations are responsible for rare hereditary diseases. Tissues and organs with the highest energy demands are the most vulnerable to mitochondrial dysfunctions—nervous tissue, the heart, the kidneys, and skeletal muscles, for example. Mitochondrial myopathy is a degen- erative muscle disease in which the muscle displays “ragged red fibers,” Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 2003 Chapter 3 Cellular Form and Function 125 Concepts of Cellular Structure (p. 94) 1. Cytology is the study of cellular structure and function. 2. All human structure and function is the result of cellular activity. 3. Cell shapes are described as squamous, polygonal, stellate, cuboidal, columnar, spheroid, ovoid, discoid, fusiform, and fibrous. 4. Most human cells are 10 to 15 ␮m in diameter. Cell size is limited in part by the ratio of surface area to volume. 5. A cell is enclosed in a plasma membrane and contains usually one nucleus. 6. The cytoplasm is everything between the plasma membrane and nucleus. It consists of a clear fluid, the cytosol or intracellular fluid (ICF), and embedded organelles and other structures. Fluid external to the cell is extracellular fluid (ECF). The Cell Surface (p. 98) 1. The plasma membrane is made of lipid and protein. 2. The most abundant lipid molecules in the membrane are phospholipids, which form a bilayer with their hydrophobic heads facing the ICF and ECF. Other membrane lipids include cholesterol and glycolipids. 3. Membrane proteins are called integral proteins if they are embedded in the lipid bilayer and extend all the way through it, and peripheral proteins if they only cling to the intracellular face of the lipid bilayer. 4. Membrane proteins serve as receptors, second-messenger systems, enzymes, channels, carriers, molecular motors, cell-identity markers, and cell-adhesion molecules. 5. Channel proteins are called gates if they can open and close. Gates are called ligand-regulated, voltage- regulated, or mechanically regulated depending on whether they open and close in response to chemicals, voltage changes across the membrane, or mechanical stress. 6. Second-messenger systems are systems for generating an internal cellular signal in response to an external one. One of the best-known examples results in the formation of a second messenger, cyclic AMP (cAMP), within the cell when certain extracellular signaling molecules bind to a membrane receptor. 7. All cells are covered with a glycocalyx, a layer of carbohydrate molecules bound to membrane lipids and proteins. The glycocalyx functions in immunity and other forms of protection, cell adhesion, fertilization, and embryonic development, among other roles. 8. Microvilli are tiny surface extensions of the plasma membrane that increase a cell’s surface area. They are especially well developed on absorptive cells, as in the kidney and small intestine. 9. Cilia are longer, hairlike surface extensions with a central axoneme, composed of a 9 ϩ 2 arrangement of microtubules. Some cilia are stationary and sensory in function, and some are motile and propel substances across epithelial surfaces. 10. A flagellum is a long, solitary, whiplike extension of the cell surface. The only functional flagellum in humans is the sperm tail. Membrane Transport (p. 106) 1. The plasma membrane is selectively permeable—it allows some substances to pass through it but prevents others from entering or leaving a cell. There are several methods of passage through a plasma membrane. 2. Filtration is the movement of fluid through a membrane under a physical force such as blood pressure, while the membrane holds back relatively large particles. 3. Simple diffusion is the spontaneous net movement of particles from a place of high concentration to a place of low concentration, such as respiratory gases moving between the pulmonary air sacs and the blood. The speed of diffusion depends on temperature, molecular weight, concentration differences, and the surface area and permeability of the membrane. 4. Osmosis is the diffusion of water through a selectively permeable membrane from the more watery to the less watery side. Channel proteins called aquaporins allow passage of water through plasma membranes. 5. The speed of osmosis depends on the relative concentrations, on the two sides of a membrane, of solute molecules that cannot penetrate the membrane. Osmotic pressure, the physical force that would be required Chapter Review Review of Key Concepts cells with abnormal mitochondria that stain red with a particular his- tological stain. Mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) is a mitochondrial disease involving seizures, paralysis, dementia, muscle deterioration, and a toxic accu- mulation of lactic acid in the blood. Leber hereditary optic neuropathy (LHON) is a form of blindness that usually appears in young adulthood as a result of damage to the optic nerve. Kearns-Sayre syndrome (KSS) involves paralysis of the eye muscles, degeneration of the retina, heart disease, hearing loss, diabetes, and kidney failure. Damage to mtDNA has also been implicated as a possible factor in Alzheimer disease, Huntington disease, and other degenerative diseases of old age. Chapter 3 Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 2003 Chapter 3 126 Part One Organization of the Body to stop osmosis, is proportional to the concentration of nonpermeating solutes on the side to which water is moving. 6. An osmole is one mole of dissolved particles in a solution. Osmolarity is the number of osmoles of solute per liter of solution. The osmolarity of body fluids is usually expressed in milliosmoles per liter (mOsm/L). 7. Tonicity is the ability of a solution to affect the fluid volume and pressure in a cell. A solution is hypotonic, isotonic, or hypertonic to a cell if it contains, respectively, a lower, equal, or greater concentration of nonpermeating solutes than the cell cytoplasm does. Cells swell and burst in hypotonic solutions and shrivel in hypertonic solutions. 8. Carrier-mediated transport employs membrane proteins to move solutes through a membrane. A given carrier is usually specific for a particular solute. 9. Membrane carriers can become saturated with solute molecules and then unable to work any faster. The maximum rate of transport is the transport maximum (T m ). 10. A uniport is a carrier that transports only one solute at a time; a symport carries two or more solutes through the membrane in the same direction (a process called cotransport); and an antiport carries two or more solutes in opposite directions (a process called countertransport). 11. Facilitated diffusion is a form of carrier-mediated transport that moves solutes through a membrane down a concentration gradient, without an expenditure of ATP. 12. Active transport is a form of carrier- mediated transport that moves solutes through a membrane up (against) a concentration gradient, with the expenditure of ATP. 13. The Na ϩ -K ϩ pump is an antiport that moves Na ϩ out of a cell and K ϩ into it. It serves for control of cell volume, secondary active transport, heat production, and maintenance of an electrical membrane potential. 14. Vesicular transport is the movement of substances in bulk through a membrane in membrane-enclosed vesicles. 15. Endocytosis is any form of vesicular transport that brings material into a cell, including phagocytosis, pinocytosis, and receptor-mediated endocytosis. 16. Exocytosis is a form of vesicular transport that discharges material from a cell. It functions in the release of cell products and in replacement of plasma membrane removed by endocytosis. The Cytoplasm (p. 115) 1. The cytoplasm is composed of a clear gelatinous cytosol in which are embedded organelles, the cytoskeleton, and inclusions (table 3.4). 2. Organelles are internal structures in the cytoplasm that carry out specialized tasks for a cell. 3. Membranous organelles are enclosed in one or two layers of unit membrane similar to the plasma membrane. These include the nucleus, endoplasmic reticulum (which has rough and smooth portions), ribosomes, the Golgi complex, lysosomes, peroxisomes, and mitochondria. The centrioles and ribosomes are nonmembranous organelles. 4. The cytoskeleton is a supportive framework of protein filaments and tubules in a cell. It gives a cell its shape, organizes the cytoplasmic contents, and functions in movements of cell contents and the cell as a whole. It is composed of microfilaments of the protein actin; intermediate filaments of keratin or other proteins; and cylindrical microtubules of the protein tubulin. 5. Inclusions are either stored cellular products such as glycogen, pigments, and fat, or foreign bodies such as bacteria, viruses, and dust. Inclusions are not vital to cell survival. Selected Vocabulary cytoplasm 96 plasma membrane 97 organelle 97 cytoskeleton 97 cytosol 97 intracellular fluid 97 extracellular fluid 97 receptor 100 channel protein 100 ligand-regulated gate 100 voltage-regulated gate 100 carrier 101 microvillus 103 cilium 103 filtration 106 simple diffusion 106 osmosis 107 osmolarity 108 hypotonic 108 hypertonic 108 isotonic 108 uniport 110 symport 110 antiport 110 facilitated diffusion 110 active transport 110 sodium-potassium pump 110 endocytosis 112 exocytosis 112 phagocytosis 112 endoplasmic reticulum 116 ribosome 118 Golgi complex 118 lysosome 119 peroxisome 119 mitochondrion 120 centriole 121 microfilament 120 intermediate filament 120 microtubule 121 Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 2003 Chapter 3 Chapter 3 Cellular Form and Function 127 True or False Determine which five of the following statements are false, and briefly explain why. 1. If a cell were poisoned so it could not make ATP, osmosis through its membrane would cease. 2. Material can move either into a cell or out by means of active transport. 3. A cell’s second messengers serve mainly to transport solutes through the membrane. 4. The Golgi complex makes lysosomes but not peroxisomes. 5. Some membrane channels are peripheral proteins. 6. The plasma membrane consists primarily of protein molecules. 7. The brush border of a cell is composed of cilia. 8. Human cells swell or shrink in any solution other than an isotonic solution. 9. Osmosis is not limited by the transport maximum (T m ). 10. It is very unlikely for a cell to have more centrosomes than ribosomes. Answers in Appendix B Answers in Appendix B Testing Your Recall 1. The clear, structureless gel in a cell is its a. nucleoplasm. b. protoplasm. c. cytoplasm. d. neoplasm. e. cytosol. 2. The Na ϩ -K ϩ pump is a. a peripheral protein. b. an integral protein. c. a G protein. d. a glycolipid. e. a phospholipid. 3. Which of the following processes could occur only in the plasma membrane of a living cell? a. facilitated diffusion b. simple diffusion c. filtration d. active transport e. osmosis 4. Cells specialized for absorption of matter from the ECF are likely to show an abundance of a. lysosomes. b. microvilli. c. mitochondria. d. secretory vesicles. e. ribosomes. 5. Osmosis is a special case of a. pinocytosis. b. carrier-mediated transport. c. active transport. d. facilitated diffusion. e. simple diffusion. 6. Membrane carriers resemble enzymes except for the fact that carriers a. are not proteins. b. do not have binding sites. c. are not selective for particular ligands. d. change conformation when they bind a ligand. e. do not chemically change their ligands. 7. The cotransport of glucose derives energy from a. a Na ϩ concentration gradient. b. the glucose being transported. c. a Ca 2ϩ gradient. d. the membrane voltage. e. body heat. 8. The function of cAMP in a cell is a. to activate a G protein. b. to remove phosphate groups from ATP. c. to activate kinases. d. to bind to the first messenger. e. to add phosphate groups to enzymes. 9. Most cellular membranes are made by a. the nucleus. b. the cytoskeleton. c. enzymes in the peroxisomes. d. the endoplasmic reticulum. e. replication of existing membranes. 10. Matter can leave a cell by any of the following means except a. active transport. b. pinocytosis. c. an antiport. d. simple diffusion. e. exocytosis. 11. Most human cells are 10 to 15 ______ in diameter. 12. When a hormone cannot enter a cell, it activates the formation of a/an ______ inside the cell. 13. ______ gates in the plasma membrane open or close in response to changes in the electrical charge difference across the membrane. 14. The force exerted on a membrane by water is called ______ . 15. A concentrated solution that causes a cell to shrink is ______ to the cell. 16. Fusion of a secretory vesicle with the plasma membrane, and release of the vesicle’s contents, is called ______ . 17. Two organelles that are surrounded by a double unit membrane are the ______ and the ______ . 18. Liver cells can detoxify alcohol with two organelles, the ______ and ______. 19. An ion gate in the plasma membrane that opens or closes when a chemical binds to it is called a/an ______ . 20. The space enclosed by the unit membrane of the Golgi complex and endoplasmic reticulum is called the ______ . Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 2003 Chapter 3 128 Part One Organization of the Body Answers to Figure Legend Questions 3.9 Adenylate cyclase is integral. The G protein is peripheral. 3.19 The Na ϩ -K ϩ pump requires ATP, whereas osmosis does not. A dead cell ceases to produce ATP. 3.23 Transcytosis is simply a combination of endocytosis and exocytosis. 3.25 Proteins and mRNA must be able to move through the nuclear envelope. These large molecules require large pores for their passage. 3.30 A centriole has 27 microtubules— 9 groups of 3 each. www.mhhe.com/saladin3 The Online Learning Center provides a wealth of information fully organized and integrated by chapter. You will find practice quizzes, interac- tive activities, labeling exercises, flashcards, and much more that will complement your learning and understanding of anatomy and physiology. Testing Your Comprehension 1. If someone bought a saltwater fish in a pet shop and put it in a freshwater aquarium at home, what would happen to the fish’s cells? What would happen if someone put a freshwater fish in a saltwater aquarium? Explain. 2. A farmer’s hand and forearm are badly crushed in a hay bailer. Upon hospital examination, his blood potassium level is found to be abnormal. Would you expect it to be higher or lower than normal? Explain. 3. Many children worldwide suffer from a severe deficiency of dietary protein. As a result, they have very low levels of blood albumin. How do you think this affects the water content and volume of their blood? Explain. 4. It is often said that mitochondria make energy for a cell. Why is this statement false? 5. Kartagener syndrome is a hereditary disease in which dynein arms are lacking from the axonemes of cilia and flagella. Predict the effect of Kartagener syndrome on a man’s ability to father a child. Predict its effect on his respiratory health. Explain both answers. Answers at the Online Learning Center Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4. Genetics and Cellular Function Text © The McGraw−Hill Companies, 2003 The Nucleic Acids 130 • Organization of the Chromatin 130 • DNA Structure and Function 130 • RNA Structure and Function 133 Protein Synthesis and Secretion 134 • Preview 134 • The Genetic Code 134 • Transcription 136 • Translation 136 • Chaperones and Protein Structure 137 • Posttranslational Modification 138 • Packaging and Secretion 139 DNA Replication and the Cell Cycle 139 • DNA Replication 139 • Errors and Mutations 142 • The Cell Cycle 142 • Mitosis 143 • Timing of Cell Division 145 Chromosomes and Heredity 145 • The Karyotype 146 • Genes and Alleles 147 • Multiple Alleles, Codominance, and Incomplete Dominance 148 • Polygenic Inheritance and Pleiotropy 148 • Sex Linkage 149 • Penetrance and Environmental Effects 149 • Dominant and Recessive Alleles at the Population Level 149 Chapter Review 152 INSIGHTS 4.1 Medical History: Miescher and the Discovery of DNA 130 4.2 Medical History: Discovery of the Double Helix 132 4.3 Clinical Application: Can We Replace Brain Cells? 143 4.4 Clinical Application: Cancer 151 4 CHAPTER Genetics and Cellular Function A single DNA molecule spilling from a ruptured bacterial cell (TEM) CHAPTER OUTLINE Brushing Up To understand this chapter, it is important that you understand or brush up on the following concepts: • Levels of protein structure (p. 80) • Functions of proteins (p. 80) • Exocytosis (p. 114) • Ribosomes, rough endoplasmic reticulum, and Golgi complex (pp. 116–119) • Centrioles and microtubules (pp. 120, 121) 129 Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4. Genetics and Cellular Function Text © The McGraw−Hill Companies, 2003 Chapter 4 130 Part One Organization of the Body S ome of the basic ideas of heredity have been known since antiquity, but a scientific understanding of how traits are passed from parent to offspring began with the Austrian monk Gregor Mendel (1822–84) and his famous experiments on garden peas. In the early twentieth century, the importance of Mendel’s work was realized and chromosomes were first seen with the microscope. Cytogenetics now uses techniques of cytology and microscopy to study chromosomes and their relationship to hered- itary traits. Molecular genetics uses the techniques of biochem- istry to study the structure and function of DNA. In this chapter, we bring together some of the findings of molecular genetics, cytogenetics, and mendelian heredity to explore what the genes are, how they regulate cellular function, and how they are passed on when cells divide and people reproduce. A few basic concepts of heredity are introduced as a foundation for understanding con- cepts ranging from color blindness to blood types in the chapters that follow. The Nucleic Acids Objectives When you have completed this section, you should be able to • describe how DNA is organized in the nucleus; and • compare the structures and functions of DNA and RNA. With improvements in the microscope, nineteenth-century cytologists saw that the nucleus divides in preparation for cell division, and they came to regard the nucleus as the most likely center of heredity. This led to a search for the biochemical keys to heredity in the nucleus, and thus to the discovery of deoxyribonucleic acid (DNA) (insight 4.1). DNA directly or indirectly regulates all cellular form and function. Insight 4.1 Medical History Miescher and the Discovery of DNA Swiss biochemist Johann Friedrich Miescher (1844–95) was one of the first scientists intent on identifying the hereditary material in nuclei. In order to isolate nuclei with minimal contamination, Miescher chose to work with cells that have large nuclei and very lit- tle cytoplasm. At first he chose white blood cells extracted from the pus in used bandages from a hospital; later, he used the sperm of salmon—probably more agreeable to work with than used bandages! Miescher isolated an acidic substance rich in phosphorus, which he named nuclein. His student, Richard Altmann, later called it nucleic acid—a term we now use for both DNA and RNA. Miescher correctly guessed that “nuclein” (DNA) was the hereditary matter of the cell, but he was unable to provide strong evidence for this conjecture, and his work was harshly criticized. He died of tuberculosis at the age of 51. Organization of the Chromatin A human cell usually has 46 molecules of DNA with an average length of 44 mm (total slightly over 2 m). Each molecule is 2 nm in diameter. To put this in perspective, if a DNA molecule were the thickness of a telephone pole (20 cm, or 8 in.), it would reach about 4,400 km (2,700 mi) into space—far higher than the orbits of satellites and space shuttles. Imagine trying to make a pole 20 cm thick and 4,400 km long without breaking it! The problem for a cell is even greater. It has 46 DNA molecules packed together in a single nucleus, and it has to make an exact copy of every one of them and distribute these equally to its two daughter cells when the cell divides. Keeping the DNA organized and intact is a tremendous feat. Molecular biology and high-resolution electron microscopy have provided some insight into how this task is accomplished. Chromatin looks like a granular thread (fig. 4.1a). The granules, called nucleosomes, consist of a cluster of eight proteins called histones, with the DNA molecule wound around the cluster. Histones serve as spools that protect and organize the DNA. Other nuclear proteins called nonhistones seem to provide structural support for the chromatin and regulate gene activity. Winding DNA around the nucleosomes makes the chromatin shorter and more compact, but chromatin also has higher orders of structure. The “granular thread,” about 10 nm wide, further twists into a coil about 30 nm wide. When a cell prepares to undergo division, the chromatin further supercoils into a fiber about 200 nm wide (fig. 4.1b). Thus, the 2 m of DNA in each cell becomes shortened and compacted in an orderly way that prevents tangling and breakage without interfering with genetic function. DNA Structure and Function Nucleic acids are polymers of nucleotides (NEW-clee-oh- tides). A nucleotide consists of a sugar, a phosphate group, and a single- or double-ringed nitrogenous (ny-TRODJ-eh- nus) base. Three bases—cytosine (C), thymine (T), and uracil (U)—have a single carbon-nitrogen ring and are clas- sified as pyrimidines (py-RIM-ih-deens). The other two bases—adenine (A) and guanine (G)—have double rings and are classified as purines (fig. 4.2). The bases of DNA are C, T, A, and G, whereas the bases of RNA are C, U, A, and G. The structure of DNA resembles a ladder (fig. 4.3a). Each sidepiece is a backbone composed of phosphate groups alternating with the sugar deoxyribose. The step- like connections between the backbones are pairs of nitrogenous bases. Imagine this as a soft rubber ladder that you can twist, so that the two backbones become entwined to resemble a spiral staircase. This is analogous to the shape of the DNA molecule, described as a double helix. The nitrogenous bases face the inside of the helix and hold the two backbones together with hydrogen bonds. Across from a purine on one backbone, there is a pyrimidine Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4. Genetics and Cellular Function Text © The McGraw−Hill Companies, 2003 Chapter 4 131 Metaphase chromosome Chromatid (700 nm in diameter) Supercoiled structure (200 nm in diameter) Chromatin fiber (10 nm in diameter) Nucleosome Histones DNA (2 nm in diameter) (b) Figure 4.1 Chromatin Structure. (a) Nuclear contents of a germ cell from an 8-week-old human embryo (colorized SEM). The center mass is the nucleolus. It is surrounded by granular fibers of chromatin. Each granule is a nucleosome. (b) The coiling of chromatin and its relationship to the histones. Supercoiling beyond the 10-nm level occurs only during mitosis. 50 nm (a) HC N C N NH 2 N H C C CH N H CH 2 O C NH 2 N NH C CH C H N NC HO O OH P H HOH HH O Adenine Adenine (A) Purines C O N NH C CH CN HN C NH 2 Guanine (G) H C NH 2 C N H C HC N O Cytosine (C) Uracil (U) C C O C O CH HN CH N H N H C C HC CH 3 NH O O Thymine (T) Phosphate Deoxyribose Pyrimidines ( b ) (a) Figure 4.2 Nucleotides and Nitrogenous Bases. (a) The structure of a nucleotide, one of the monomers of DNA and RNA. In RNA, the sugar is ribose. (b) The five nitrogenous bases found in DNA and RNA nucleotides. [...]... a protein to be sent to the wrong address, Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function © The McGraw−Hill Companies, 20 03 Text Chapter 4 Genetics and Cellular Function 137 5 The preceding tRNA hands off the growing peptide to the new tRNA, and the ribosome links the new amino acid to the peptide Free amino acids 3 tRNA binds an amino acid;... to the rough ER and dock on its surface Assembly of the amino acid chain is then completed on the rough ER and the protein is sent to the Golgi complex for final modification Thus, we turn to the functions of these organelles in the modification, packaging, and secretion of a protein (fig 4.10) Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function. .. induced by the X rays that were her window on DNA architecture Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function Text © The McGraw−Hill Companies, 20 03 Chapter 4 Genetics and Cellular Function 133 (a) RNA Structure and Function (b) DNA directs the synthesis of proteins by means of its smaller cousins, the ribonucleic acids (RNAs) There are... determining the number of cells in the body An inability to stop cycling and enter G0 is characteristic of cancer cells (see insight 4.4 at the end of the chapter) Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function © The McGraw−Hill Companies, 20 03 Text Chapter 4 Genetics and Cellular Function 1 43 Think About It What is the maximum number of DNA... division of the cytoplasm Cytokinesis is achieved by the motor protein myosin pulling on microfilaments of actin in the membrane skeleton This creates a crease called the cleavage furrow around the equator of the cell, and the cell eventually pinches in two Interphase has now begun for these new cells Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function. .. case in DNA A sequence of 3 DNA nucleotides that stands for 1 amino acid is called a base triplet The “mirror image” Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function © The McGraw−Hill Companies, 20 03 Text Chapter 4 Genetics and Cellular Function 135 Interstitial cell of testis DNA 1 Transcription From pituitary mRNA 3 2 Second messenger Translation... wrote The fundamental steps of the replication process are as follows: 1 The double helix unwinds from the histones 2 Like a zipper, an enzyme called DNA helicase opens up a short segment of the helix, exposing its nitrogenous bases The point where one strand of DNA is “unzipped” and separates from its Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function. .. vesicles enter the rough ER and are modified here and in the Golgi complex Such alterations are called posttranslational modification Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function Text © The McGraw−Hill Companies, 20 03 154 Part One Organization of the Body 4 Two genetically identical strands of a metaphase chromosome, joined at the centromere,... exocytosis (p 114) 157 Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 5 Histology © The McGraw−Hill Companies, 20 03 Text 158 Part One Organization of the Body W Chapter 5 ith its 50 trillion cells and thousands of organs, the human body may seem to be a structure of forbidding complexity Fortunately for our health, longevity, and self-understanding, the biologists of past generations... easy to observe The drawings show a hypothetical cell with only two chromosome pairs; in humans, there are 23 pairs Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 4 Genetics and Cellular Function © The McGraw−Hill Companies, 20 03 Text Chapter 4 Genetics and Cellular Function 145 Timing of Cell Division Centromere Sister chromatids (a) Before You Go On Answer the following . Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 20 03 Chapter 3 Chapter 3 Cellular Form and. fibers,” Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill Companies, 20 03 Chapter 3 Cellular Form and Function. disease, and other degenerative diseases of old age. Chapter 3 Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition 3. Cellular Form and Function Text © The McGraw−Hill