İlk chapter the-cell-a-molecular-approach-8th-edition-2.pdf

CHAPTER Introduction to Cells 1 and Cell Research U nderstanding the molecular biol...

CHAPTER Introduction to Cells 1 and Cell Research U nderstanding the molecular biology of cells is one of the most active and fundamental areas of research in the biological sciences. This is 1.1 The Origin and Evolution true not only from the standpoint of basic science, but also with respect of Cells 4 to the numerous applications of cell and molecular biology to medicine, biotechnology, and agriculture. Especially with the ability to obtain rapid 1.2 Experimental Models in sequences of complete genomes, progress in cell and molecular biology is Cell Biology 18 opening new horizons in the practice of medicine. Striking examples include 1.3 Tools of Cell Biology: genome editing; the identification of genes that contribute to susceptibility Microscopy and to a variety of common diseases, such as heart disease, rheumatoid arthritis, Subcellular Fractionation and diabetes; the development of new drugs specifically targeted to interfere 28 with the growth of cancer cells; and the potential use of stem cells to replace Key Experiment damaged tissues and treat patients suffering from conditions like diabetes, Parkinson’s disease, Alzheimer’s disease, and spinal cord injuries. HeLa Cells: The First Because cell and molecular biology is such a rapidly growing field of Immortal Cell Line 25 research, it is important to understand its experimental basis as well as the Molecular Medicine current state of our knowledge. This chapter will therefore focus on how cells Viruses and Cancer 26 are studied, as well as review some of their basic properties. Appreciating the similarities and differences between cells is particularly important to understanding cell biology. The first section of this chapter discusses both the unity and the diversity of present-day cells in terms of their evolution from a common ancestor. On the one hand, all cells share common fundamental properties that have been conserved throughout evolution. For example, all cells employ DNA as their genetic material, are surrounded by plasma membranes, and use the same basic mechanisms for energy metabolism. On the other hand, present-day cells have evolved a variety of different lifestyles. Many organisms, such as bacteria, amoebas, and yeasts, consist of single cells that are capable of independent self-replication. More complex organisms are composed of collections of cells that function in a coordinated manner, with different cells specialized to perform particular tasks. The human body, for example, is composed of more than 200 different kinds of cells, each specialized for such distinctive functions as memory, sight, movement, and digestion. The diversity exhibited by the many different kinds of cells is striking; for example, consider the differences between bacteria and the cells of the human brain. The fundamental similarities between different types of cells provide a unifying theme to cell biology, allowing the basic principles learned from experiments with one kind of cell to be extrapolated and generalized to other cell types. Several kinds of cells and organisms are widely used to study dif- ferent aspects of cell and molecular biology; the second section of this chapter discusses some of the properties of these cells that make them particularly 4 Chapter 1 valuable as experimental models. Finally, it is important to recognize that progress in cell biology depends heavily on the availability of experimental tools that allow scientists to make new observations or conduct novel kinds of experiments. This introductory chapter therefore concludes with a discus- sion of some of the experimental approaches used to study cells, as well as a review of some of the major historical developments that have led to our current understanding of cell structure and function. 1.1 The Origin and Evolution of Cells Learning Objectives You should be able to: Explain how the first cell originated. Describe the major steps in evolution of metabolism. Illustrate the structures of eukaryotic and prokaryotic cells. Outline the evolution of eukaryotic cells and multicellular organisms. Cells are divided into two main classes, initially defined by whether they contain a nucleus. Prokaryotic cells, such as bacteria, lack a nuclear envelope and are generally smaller and simpler than eukaryotic cells, which include the highly specialized cells of multicellular organisms. In spite of these dif- ferences, the same basic molecular mechanisms govern the lives of both pro- karyotes and eukaryotes, indicating that all present-day cells are descended from a single primordial ancestor. How did this first cell develop? And how did the complexity and diversity exhibited by present-day cells evolve? How did the first cell arise? It appears that life first emerged at least 3.8 billion years ago, approximately 750 million years after Earth was formed. How life originated and how the first cell came into being are matters of speculation, since these events cannot be reproduced in the laboratory. Nonetheless, several types of experiments provide important evidence bearing on some steps of the process. It was first suggested in the 1920s that simple organic molecules could Organic molecules formed spontaneously in primitive Earth’s form and spontaneously polymerize into macromolecules under the condi- atmosphere. tions thought to exist in primitive Earth’s atmosphere. At the time life arose, the atmosphere of Earth is thought to have contained little or no free oxygen, instead consisting principally of CO2 and N2 in addition to smaller amounts of gases such as H2, H2S, and CO. Such an atmosphere provides reducing condi- tions in which organic molecules, given a source of energy such as sunlight or electrical discharge, can form spontaneously. The spontaneous formation of organic molecules was first demonstrated experimentally in the 1950s when Stanley Miller (then a graduate student) showed that the discharge of electric sparks into a mixture of H2, CH4, and NH3, in the presence of water, leads to the formation of a variety of organic molecules, including several amino acids (Figure 1.1). Although Miller’s experiments did not precisely reproduce the conditions of primitive Earth, they clearly demonstrated the plausibility of the spontaneous synthesis of organic molecules, providing the basic materials from which the first living organisms arose.  Introduction to Cells and Cell Research 5 The next step in evolution was the formation of macromolecules. The monomeric building blocks Electrode of macromolecules have been demonstrated to CH4 polymerize spontaneously under plausible pre- NH3 biotic conditions. Heating dry mixtures of amino H2O Water vapor was refluxed H2O acids, for example, results in their polymerization H2 through an atmosphere to form polypeptides. But the critical characteristic CH4 consisting of H2, CH4, and H2 NH3, into which electric of the macromolecule from which life evolved NH3 Electric sparks were discharged. must have been the ability to replicate itself. discharge Only a macromolecule capable of directing the synthesis of new copies of itself would have been Cooling capable of reproduction and further evolution. Water Of the two major classes of informational macromolecules in present-day cells (nucleic acids and proteins), only the nucleic acids are capable of directing their own self-replication. Nucleic acids can serve as templates for their own synthesis as a result of specific base pairing Heat between complementary nucleotides (Figure 1.2). A critical step in understanding molecular evolution was thus reached in the early 1980s, Organic Analysis of the reaction when it was discovered in the laboratories of molecules products revealed the Alanine formation of a variety Sid Altman and Tom Cech that RNA is capable of organic molecules, Aspartic acid of catalyzing a number of chemical reactions, including the amino acids Glutamic acid alanine, aspartic acid, including the polymerization of nucleotides. Glycine glutamic acid, and glycine. Further studies have extended the known catalytic Urea activities of RNA, including the description of Lactic acid RNA molecules that direct the synthesis of a new Acetic acid RNA strand from an RNA template. RNA is thus Formic acid uniquely able to both serve as a template and to catalyze its own replication. Consequently, RNA is Figure 1.1 Spontaneous formation of organic molecules generally believed to have been the initial genetic system, and an early stage of chemical evolution is thought to have been based on self-replicating C C C G C G C G C G G G G G C G C G C G C C A A U A U A U A U U A G G C G C G C G C C A A A U A U A U U U U U U A U A U A U A U U U A U A U A U A A G G G C G C G C G C C A A U A U A U A U A U C C C G C G C C G G G Figure 1.2 Self-replication of RNA Complementary pairing between nucleo- tides (adenine [A] with uracil [U] and guanine [G] with cytosine [C]) allows one strand of RNA to serve as a template for the synthesis of a new strand with the complemen- tary sequence. Cooper_The Cell 8e Sinauer Dragonfly Media Group Figure# 01.01 06/01/18 6 Chapter 1 RNA molecules—a period of evolution known as the RNA world. Ordered RNA can catalyze its own replication. interactions between RNA and amino acids then evolved into the present-day genetic code, and DNA eventually replaced RNA as the genetic material. As discussed further in Chapter 4, all present-day cells use DNA as the All present-day cells use the same genetic mechanisms. genetic material and employ the same basic mechanisms for DNA replication and expression of the genetic information. Genes are the functional units of inheritance, corresponding to segments of DNA that encode proteins or RNA molecules. The nucleotide sequence of a gene is copied into RNA by a process called transcription. For RNAs that encode proteins, their nucleotide sequence is then used to specify the order of amino acids in a protein by a process called translation. The first cell is presumed to have arisen by the enclosure of self-replicating Phospholipids are the basic components of biological RNA in a membrane composed of phospholipids (Figure 1.3). As discussed membranes. in detail in the next chapter, phospholipids are the basic components of all present-day biological membranes, including the plasma membranes of both prokaryotic and eukaryotic cells. The key characteristic of the phospholipids that form membranes is that they are amphipathic molecules, meaning that one portion of the molecule is soluble in water and another portion is not. Phospholipids have long, water-insoluble (hydrophobic) hydrocarbon chains joined to water-soluble (hydrophilic) head groups that contain phosphate. When placed in water, phospholipids spontaneously aggregate into a bilayer with their phosphate-containing head groups on the outside in contact with water and their hydrocarbon tails in the interior in contact with each other. Such a phospholipid bilayer forms a stable barrier between two aqueous compartments—for example, separating the interior of the cell from its external environment. The enclosure of self-replicating RNA and associated molecules in a phospholipid membrane would thus have maintained them as a unit, capable of self-reproduction and further evolution. RNA-directed protein synthesis RNA Phospholipid membrane Water Phospholipid molecule: Hydrophilic head group Hydrophobic tail Water Figure 1.3 Enclosure of self-replicating RNA in a phospholipid membrane The first cell is thought to have arisen by the enclosure of self-replicating RNA and associated molecules in a membrane composed of phospholipids. Each phospholip- id molecule has two long hydrophobic tails attached to a hydrophilic head group. The hydrophobic tails are buried in the lipid bilayer; the hydrophilic heads are exposed to water on both sides of the membrane.  Introduction to Cells and Cell Research 7 may already have evolved by this time, in which case the first cell would have consisted of self-replicating RNA and its encoded proteins. The evolution of metabolism Because cells originated in a sea of organic molecules, they were able to ob- tain food and energy directly from their environment. But such a situation is self-limiting, so cells needed to evolve their own mechanisms for generating energy and synthesizing the molecules necessary for their replication. The generation and controlled utilization of metabolic energy is central to all cell activities, and the principal pathways of energy metabolism (discussed in detail in Chapter 3) are highly conserved in present-day cells. All cells use adenosine 5′-triphosphate (ATP) as their source of metabolic energy to drive the synthesis of cell constituents and carry out other energy-requiring activi- ties, such as movement (e.g., muscle contraction). The mechanisms used by cells for the generation of ATP are thought to have evolved in three stages, corresponding to the evolution of glycolysis, photosynthesis, and oxidative metabolism (Figure 1.4). The development of these metabolic pathways changed Earth’s atmosphere, thereby altering the course of further evolution. In the initially anaerobic atmosphere of Earth, the first energy-generating The first cells obtained energy by reactions presumably involved the breakdown of organic molecules in the glycolysis. absence of oxygen. These reactions are likely to have been a form of present- day glycolysis—the anaerobic breakdown of glucose to lactic acid, with the net energy gain of two molecules of ATP. In addition to using ATP as their source of intracellular chemical energy, all present-day cells carry out glycolysis, consistent with the notion that these reactions arose very early in evolution. Glycolysis provided a mechanism by which the energy in preformed organic Photosynthesis made cells molecules (e.g., glucose) could be converted to ATP, which could then be used independent of organic molecules as a source of energy to drive other metabolic reactions. The development of in the environment. photosynthesis is generally thought to have been the next major evolutionary step, which allowed the cell to harness energy from sunlight and provided independence from the utilization of preformed organic molecules. The first photosynthetic bacteria probably utilized H2S to convert CO2 to organic Glycolysis C6H12O6 2 C3H6O3 Generates 2 ATP FYI Glucose Lactic acid Existence of organisms in extreme conditions has led to the hypothesis Photosynthesis that life could exist in similar environments elsewhere in the solar 6 CO2 + 6 H2O C6H12O6 + 6 O2 system. The field of astrobiology (or Glucose exobiology) seeks to find signs of this extraterrestrial life. Oxidative metabolism C6H12O6 + 6 O2 6 CO2 + 6 H2O Generates 36–38 ATP Glucose Figure 1.4 Generation of metabolic energy Glycolysis is the anaerobic break- down of glucose to lactic acid. Photosynthesis utilizes energy from sunlight to drive the synthesis of glucose from CO2 and H2O, with the release of O2 as a by-product. The O2 released by photosynthesis is used in oxidative metabolism, in which glucose is broken down to CO2 and H2O, releasing much more energy than can be obtained from glycolysis. 8 Chapter 1 molecules—a pathway of photosynthesis still used by some bacteria. The use of H2O as a donor of electrons and hydrogen for the conversion of CO2 to organic compounds evolved later and had the important consequence of changing Earth’s atmosphere. The use of H2O in photosynthetic reactions produces the by-product free O2; this mechanism is thought to have been responsible for making O2 abundant in Earth’s atmosphere, which occurred about 2.4 billion years ago. The release of O2 as a consequence of photosynthesis changed the envi- The oxidation of glucose to carbon dioxide and water yields much ronment in which cells evolved and is commonly thought to have led to the more energy than glycolysis. development of oxidative metabolism. Alternatively, oxidative metabolism may have evolved before photosynthesis, with the increase in atmospheric O2 then providing a strong selective advantage for organisms capable of us- ing O2 in energy-producing reactions. In either case, O2 is a highly reactive molecule, and oxidative metabolism, utilizing this reactivity, has provided a mechanism for generating energy from organic molecules that is much more efficient than anaerobic glycolysis. For example, the complete oxida- tive breakdown of glucose to CO2 and H2O yields energy equivalent to that of 36 to 38 molecules of ATP, in contrast to the 2 ATP molecules formed by anaerobic glycolysis (see Figure 1.4). With few exceptions, present-day cells use oxidative reactions as their principal source of energy. Prokaryotes Prokaryotes include cells of two domains, the Archaea and the Bacteria, which diverged early in evolution. The Archaea include cells that live in extreme environments that are unusual today but may have been preva- lent in primitive Earth. For example, thermoacidophiles live in hot sulfur Plasma springs with temperatures as high as 80°C and pH values as low as 2. The membrane Bacteria include the common forms of present-day prokaryotes—a large group of organisms that live in a wide range of environments, including Cell wall soil, water, and other organisms (e.g., human pathogens). Prokaryotic cells are smaller and simpler than most eukaryotic cells, their Prokaryotes are smaller and simpler than eukaryotes. genomes are less complex, and they do not contain nuclei or cytoplasmic organelles (Table 1.1). Most prokaryotic cells are spherical, rod-shaped, or spiral, with diameters of 1 to 10 μm. Their DNA contents range from about 0.6 million to 5 million base pairs, an amount sufficient to encode about 5000 different proteins. The largest and most complex prokaryotes are the cyanobacteria—bacteria in which photosynthesis evolved. The structure of a typical bacterial cell is illustrated by Escherichia coli Nucleoid (E. coli), a common inhabitant of the human intestinal tract (Figure 1.5). The cell is rod-shaped, about 1 μm in diameter and about 2 μm long. Like most other prokaryotes, E. coli is surrounded by a rigid cell wall composed of polysaccharides and peptides. Beneath the cell wall is the plasma membrane, which is a bilayer of phospholipids and associated proteins. Whereas the cell wall is porous and readily penetrated by a variety of molecules, the plasma membrane provides the functional separation between the inside of the cell and its external environment. The DNA of E. coli is a single circular molecule in the nucleoid, which, in contrast to the nucleus of eukaryotes, Figure 1.5 Electron micrograph of E. coli The cell is surrounded by a cell wall, beneath which is the plasma membrane. DNA is located in the nucleoid. Artifi- 0.5 µm cial color has been added. (© Biophoto Associates/Science Source.)  Introduction to Cells and Cell Research 9 Table 1.1 Prokaryotic and Eukaryotic Cells Prokaryotes are smaller and simpler than eukaryotes. Characteristic Prokaryote Eukaryote Nucleus Absent Present Diameter of a typical cell ≈1 μm 10–100 μm Cytoplasmic organelles Absent Present 6 6 DNA content (base pairs) 1 × 10 to 5 × 10 1.5 × 107 to 5 × 109 Chromosomes Single circular DNA Multiple linear DNA molecule molecules is not surrounded by a membrane separating it from the cytoplasm. The cytoplasm contains approximately 30,000 ribosomes (the sites of protein synthesis), which account for its granular appearance. Eukaryotic cells Like prokaryotic cells, all eukaryotic cells are surrounded by a plasma mem- Eukaryotic cells contain nuclei and brane and contain ribosomes. However, eukaryotic cells are much more com- cytoplasmic organelles. plex and contain a nucleus and a variety of cytoplasmic organelles (Figure 1.6). The largest and most prominent organelle of eukaryotic cells is the nucleus, with a diameter of approximately 5 μm. The nucleus contains the genetic information of the cell, which in eukaryotes is organized as linear rather than circular DNA molecules. The nucleus is the site of DNA replica- tion and of RNA synthesis; the translation of RNA into proteins takes place on ribosomes in the cytoplasm. In addition to a nucleus, eukaryotic cells contain a variety of membrane- enclosed organelles within their cytoplasm. These organelles provide com- partments in which different metabolic activities are localized. Eukaryotic cells are generally much larger than prokaryotic cells, frequently having a cell volume at least a thousandfold greater. The compartmentalization provided by cytoplasmic organelles is what allows eukaryotic cells to function efficiently. Two of these organelles, mitochondria and chloroplasts, play critical roles in energy metabolism. Mitochondria, which are found in almost all eukaryotic cells, are the sites of oxidative metabolism and are thus responsible for gen- erating most of the ATP derived from the breakdown of organic molecules. Chloroplasts are the sites of photosynthesis and are found only in the cells of plants and green algae. Lysosomes and peroxisomes also provide special- ized metabolic compartments for the digestion of macromolecules and for various oxidative reactions, respectively. In addition, most plant cells contain large vacuoles that perform a variety of functions, including the digestion of macromolecules and the storage of both waste products and nutrients. Because of the size and complexity of eukaryotic cells, the transport of proteins to their correct destinations within the cell is a formidable task. Two cytoplasmic organelles, the endoplasmic reticulum (ER) and the Golgi appara- tus, are specifically devoted to the sorting and transport of proteins destined for secretion, incorporation into the plasma membrane, and incorporation into lysosomes and peroxisomes. The endoplasmic reticulum is an extensive network of intracellular membranes, extending from the nuclear envelope throughout the cytoplasm. It functions not only in the processing and transport of proteins (the rough endoplasmic reticulum, which is covered by ribosomes), but also in the synthesis of lipids (the smooth endoplasmic reticulum). From Animal cell Cytoskeleton Nucleolus Nucleus Rough endoplasmic reticulum Ribosomes Lysosome Mitochondrion Peroxisome Centrioles Golgi apparatus Plasma Smooth endoplasmic membrane reticulum Plant cell Ribosomes Nucleolus Nucleus Cell wall Vacuole Rough endoplasmic reticulum Peroxisome Mitochondrion Plasma membrane Chloroplast Golgi Cooper_The Cell 8e apparatus Smooth Sinauer endoplasmic Dragonfly Media Group reticulum Figure# 01.06 06/13/18 Plasmodesmata Cytoskeleton  Introduction to Cells and Cell Research 11 Figure 1.6 Structures of animal and plant cells Both animal and plant cells ▼ are surrounded by a plasma membrane and contain a nucleus, a cytoskeleton, and many cytoplasmic organelles in common. Plant cells are also surrounded by a cell wall and contain chloroplasts and large vacuoles. the endoplasmic reticulum, proteins are transported within small membrane vesicles to the Golgi apparatus, where they are further processed and sorted for transport to their final destinations. In addition to this role in protein transport, the Golgi apparatus serves as a site of lipid synthesis and (in plant cells) as the site of synthesis of some of the polysaccharides that compose the cell wall. The internal organization of eukaryotic cells is maintained by the cytoskel- eton, a network of protein filaments extending throughout the cytoplasm. The cytoskeleton provides the structural framework of the cell, determining cell shape and the general organization of the cytoplasm. In addition, the cytoskeleton is responsible for the movements of entire cells (e.g., the contraction of muscle cells) and for the intracellular transport and positioning of organelles and other structures, including the movements of chromosomes during cell division. The origin of eukaryotes Eukaryotic cells are the third domain of life, called the Eukarya, which arose as a branch from the Archaea (Figure 1.7). A critical step in the evolution of eu- karyotic cells was the acquisition of membrane-enclosed subcellular organelles, allowing the development of the complexity characteristic of these cells. It is likely that some organelles evolved from invaginations of the plasma membrane. Other Green Fungi Cyanobacteria bacteria Plants algae Animals (yeasts) Protists Archaebacteria Chloroplasts Eukarya Mitochondria Bacteria Archaea First cell Figure 1.7 Evolution of cells Present-day cells evolved from a common ances- tor that gave rise to the two prokaryotic domains of life, the Archaea and Bacteria. The evolution of eukaryotic cells (Eukarya) from the Archaea involved the formation of mitochondria by endosymbiosis. Plants and green algae subsequently evolved by the endosymbiotic formation of chloroplasts. 12 Chapter 1 For example, the nucleus is thought to have been formed by invaginations of the plasma membrane that surrounded the nucleoid of a prokaryotic ancestor. At least two organelles of eukaryotes, mitochondria and chloroplasts, Mitochondria and chloroplasts originated by endosymbiosis. arose by endosymbiosis—one cell living inside another (Figure 1.8). In particular, mitochondria are thought to have evolved from aerobic bacteria living inside the archaeal ancestor of eukaryotes and chloroplasts evolved from photosynthetic bacteria, such as cyanobacteria, living inside the ancestor FYI to plants and green algae. Both mitochondria and chloroplasts are similar to bacteria in size and, like bacteria, they reproduce by dividing in two. Most Certain present-day marine important, both mitochondria and chloroplasts contain their own DNA, which protists engulf algae to serve as encodes some of their components. The mitochondrial and chloroplast DNAs endosymbionts that carry out are replicated each time the organelle divides, and the genes they encode photosynthesis for their hosts. are transcribed within the organelle and translated on organelle ribosomes. Mitochondria and chloroplasts thus contain their own genetic systems, which are distinct from the nuclear genome of the cell and are more closely related to the genomes of bacteria than to the nuclear genomes of eukaryotes. The acquisition of aerobic bacteria would have provided an anaerobic cell with the ability to carry out oxidative metabolism. The acquisition of photosynthetic bacteria would have provided the ability to perform photosynthesis, thereby affording nutritional independence. Thus, these endosymbiotic associations were highly advantageous to their partners and were selected for in the course of evolution. Through time, most of the genes originally present in these bacteria apparently became incorporated into the nuclear genome of the cell, so only a few components of mitochondria and chloroplasts are still encoded by the organelle genomes. Archaea Bacteria Endosymbiosis Most genes of endocytosed bacterium are transferred to host genome Mitochondrion Figure 1.8 Endosymbiosis Mitochondria arose from aer- obic bacteria living with the archaeal ancestor to eukaryotes. Most bacterial genes were subsequently transferred to the nuclear genome.  Introduction to Cells and Cell Research 13 It is important to note that the genomes of eukaryotes are mosaics, with The genomes of eukaryotes are some eukaryotic genes more similar to bacterial genes and others more similar mosaics of archaeal and bacterial to archaeal genes. Curiously, most eukaryotic genes related to informational genes. processes (such as DNA replication, transcription, and protein synthesis) were derived from archaebacteria, whereas most eukaryotic genes related to basic cell operational processes (such as glycolysis and amino acid biosynthesis) were derived from bacteria. One hypothesis to explain the mosaic nature of eukaryotic genomes is that the genome of eukaryotes arose from a fusion of archaeal and bacterial genomes. According to this proposal, an endosymbiotic association between a bacterium and an archaeum was followed by fusion of the two prokaryotic genomes, giving rise to an ancestral eukaryotic genome with contributions from both bacteria and archaea (see Figure 1.8). The sim- plest version of this hypothesis is that an initial endosymbiotic relationship of a bacterium living inside an archaeum gave rise not only to mitochondria but also to the genome of eukaryotic cells, containing genes derived from both prokaryotic ancestors. The development of multicellular organisms Many eukaryotes are unicellular organisms that, like bacteria, consist of only single cells capable of self-replication. The simplest eukaryotes are the yeasts, which contain only slightly more genes than many bacteria (Table 1.2). Although yeasts are more complex than bacteria, they are much Table 1.2 Cell Genomes Haploid DNA content Organism (millions of base pairs) Protein-coding genes Archaebacteria Methanococcus jannaschii 1.7 1700 Bacteria Mycoplasma 0.6 470 E. coli 4.6 4200 Cyanobacterium 3.6 3200 Unicellular eukaryotes Saccharomyces cerevisiae (yeast) 12 6000 Dictyostelium discoideum 34 12,000 Paramecium 72 39,500 Chlamydomonas 118 14,500 Volvox 138 14,500 Plants Arabidopsis thaliana 125 26,000 Corn 2200 33,000 Apple 740 57,000 Animals Caenorhabditis elegans (nematode) 97 19,000 Drosophila melanogaster (fruit fly) 180 14,000 Zebrafish 1700 26,000 Mouse 3000 20,000 Human 3000 20,000 14 Chapter 1 Figure 1.9 Scanning electron micrograph of Saccharomyces cerevisiae Yeasts are the sim- plest eukaryotes. Artificial color has been added to the micrograph. (© Medical-on-Line/Alamy.) 5 µm smaller and simpler than most cells of animals or plants. For example, the commonly studied yeast Saccharomyces cerevisiae is about 6 μm in diam- eter and contains 12 million base pairs of DNA (Figure 1.9). Other unicel Video 1.1 lular eukaryotes, however, are far more complex cells, with substantially Paramecium Feeding larger and more complex genomes. They include organisms specialized to perform a variety of tasks, including photosynthesis, movement, and the capture and ingestion of other organisms as food. The ciliated protozoan Paramecium, for example, is a large, complex cell that can be up to 350 μm in length and is specialized for movement and feeding on bacteria and yeast (Figure 1.10). Surprisingly, the Paramecium genome contains almost twice as many genes as humans (see Table 1.2), illustrating the fact that neither genome size nor gene number is directly related to the complexity of an (A) Paramecium (B) Chlamydomonas Figure 1.10 Light micrograph of Paramecium and scanning electron mi- crograph of Chlamydomonas Paramecium and Chlamydomas are examples of unicellular eukaryotes that are more complex than yeast. (A, © M. I. Walker/Science Source; B, © Aaron J. Bell/Science Source.) Cooper_The Cell 8e Sinauer Dragonfly Media Group Figure# 01.09 03/30/18  Introduction to Cells and Cell Research 15 Figure 1.11 Multicellular green algae Volvox consists of approximately 16 germ cells and 2000 somatic cells embedded in a gelatinous matrix. (Courtesy of David Kirk.) organism—an unexpected result of genome sequencing projects that will be discussed further in Chapters 5 and 6. Other unicellular eukaryotes, such as the green alga Chlamydomonas (see Figure 1.10), contain chloroplasts and are able to carry out photosynthesis. The evolution of multicellular organisms from unicellular eukaryotes oc- Multicellular organisms evolved curred multiple times, independently for plants and animals. The algae, for from associations between example, contain both unicellular and multicellular species. The multicellular unicellular eukaryotes. green alga Volvox contains cells of two different types: approximately 16 large germ cells and 2000 somatic cells that resemble the unicellular Chlamydomo- nas (Figure 1.11). Both Chlamydomonas and Volvox have genomes of similar size and complexity, about 10 times larger than that of yeast and containing 14,000–15,000 genes (see Table 1.2). Another example of the transition to multicellularity is provided by the amoeba Dictyostelium discoideum, which is able to alternate between unicellular and multicellular forms depending on the availability of food (Figure 1.12). Increasing cell specialization and division of labor among the cells of Multicellular organisms evolved simple multicellular organisms then led to the complexity and diversity from associations between observed in the many types of cells that make up present-day plants and unicellular eukaryotes. animals, including human beings. Plants are composed of fewer cell types than are animals, but each different kind of plant cell is specialized to per- form specific tasks required by the organism as a whole (Figure 1.13). The cells of plants are organized into three main tissue systems: ground tissue, dermal tissue, and vascular tissue. The ground tissue contains parenchyma cells, which carry out most of the metabolic reactions of the plant, includ- ing photosynthesis. Ground tissue also contains two specialized cell types (collenchyma cells and sclerenchyma cells) that are characterized by thick (A) Unicellular amoebae (B) Fruiting body Figure 1.12 Dictyostelium discoideum If food is unavailable, the unicellular amoebae (A) aggregate to form a multicellular fruiting body, specialized for the disper- sal of spores (B). (A, courtesy of David Knecht, University of Connecticut; B, © David Scharf/Science Source.) 16 Chapter 1 (A) Collenchyma cells (B) Epidermal cells (C) Xylem vessel elements and tracheids Figure 1.13 Representative plant cells (A) Collenchyma cells (from spinach leaf vein) are specialized for support and have thickened cell walls. (B) Epidermal cells on the surface of a dayflower leaf. Tiny pores (stomata) are flanked by specialized cells called guard cells. (C) Vessel elements and tracheids of a squash stem are elongat- ed cells that are arranged end to end to form vessels of the xylem. (A, © Phil Gates/ Biological Photo Service; B, © Alfred Owczarzak/Biological Photo Service; C, © J. Robert Waaland/Biological Photo Service.) cell walls and provide structural support to the plant. Dermal tissue covers the surface of the plant and is composed of epidermal cells, which form a protective coat and allow the absorption of nutrients. Finally, several types of elongated cells form the vascular system (the xylem and phloem), which is responsible for the transport of water and nutrients throughout the plant. The cells found in animals are considerably more diverse than those of Animal cells evolved to perform specialized functions. plants. The human body, for example, is composed of more than 200 differ- ent kinds of cells, which are generally considered to be components of five main types of tissues: epithelial tissue, connective tissue, blood, nervous tissue, and muscle (Figure 1.14). Epithelial cells form sheets that cover the (A) Epithelial cells (B) Fibroblasts (C) Blood cells Cooper_The Cell 8e Sinauer Dragonfly Media Group Figure# 01.13 06/14/18 Erythrocyte Lymphocyte Figure 1.14 Representative animal cells (A) Epithelial cells of the mouth form a thick, multilayered sheet. (B) Fibroblasts are connective tissue cells characterized by their elongated spindle shape. (C) Erythrocytes and lymphocytes in human blood. (A, © G. W. Willis/Visuals Unlimited, Inc.; B, © Biophoto Associates/Science Source; C, © G. W. Willis/Visuals Unlimited, Inc.)  Introduction to Cells and Cell Research 17 surface of the body and line the internal organs. There are many different types of epithelial cells, each specialized for a specific function, including protection (the skin), absorption (e.g., cells lining the small intestine), and secretion (e.g., cells of the salivary gland). Connective tissues include bone, cartilage, and adipose tissue, each of which is formed by different types of cells (osteoblasts, chondrocytes, and adipocytes, respectively). The loose connective tissue that underlies epithelial layers and fills the spaces between organs and tissues in the body is formed by another cell type, the fibroblast. Blood contains several different types of cells: red blood cells (erythrocytes) function in oxygen transport, and white blood cells (granulocytes, monocytes, macrophages, and lymphocytes) function in inflammatory reactions and the immune response. Nervous tissue is composed of supporting cells and nerve cells, or neurons, which are highly specialized to transmit signals throughout the body. Various types of sensory cells, such as cells of the eye and ear, are further specialized to receive external signals from the environment. Finally, several different types of muscle cells are responsible for the production of force and movement. The evolution of animals clearly involved the development of consider- able diversity and specialization at the cellular level. Understanding the mechanisms that control the growth and differentiation of such a complex array of specialized cells, starting from a single fertilized egg, is one of the major challenges facing contemporary cell and molecular biology. 1.1 Review The first cell is thought to have arisen at least 3.8 billion years ago by the enclosure of self-replicating RNA in a phospholipid membrane. The earli- est reactions for generation of metabolic energy were a form of anaerobic glycolysis, followed by the evolution of photosynthesis and oxidative metabolism. Two domains of prokaryotic cells, Bacteria and Archaea, diverged early in evolution. Eukaryotic cells, which are larger and more complex than prokaryotic cells, contain a nucleus and cytoplasmic organ- elles. They evolved as a branch from the Archaea, with mitochondria and chloroplasts originating by endosymbiosis. Multicellular organisms then evolved from associations between unicellular eukaryotes, and division of labor led to the development of the many kinds of specialized cells that make up present-day plants and animals. Questions 1. What properties of RNA support the hypothesis that it was the first self-replicating molecule in early evolution? 2. How did the evolution of photosynthesis affect the development of oxidative metabolism? 3. Assume that the diameter of a eukaryotic cell is 50 μm. How much larger in volume is that cell compared to a bacterium with a diameter of 2 μm? What is the function of subcellular organelles in helping eukaryotic cells function despite their large size? 4. What is the evidence that mitochondria originated from bacteria that were engulfed by the precursor of eukaryotic cells? 18 Chapter 1 5. What present-day organisms do you expect the DNA sequence of chloroplasts to most closely resemble? 6. How would you compare the evolutionary origins of mitochondria and the endoplasmic reticulum? 7. How does genome complexity relate to the development of multicellular organisms? 1.2 Experimental Models in Cell Biology Learning Objectives You should be able to: Explain the advantages of E. coli for studying basic concepts of molecular biology. Contrast yeast with E. coli as a model system. Summarize the simple models for studying plant and animal development. Describe the advantages and disadvantages of studying vertebrates. Summarize the principles of animal cell culture. Explain how viruses can be used to study cell biology. The evolution of present-day cells from a common ancestor has important implications for cell and molecular biology as an experimental science. Be- cause the fundamental properties of all cells have been conserved during evolution, the basic principles learned from experiments performed with one type of cell are generally applicable to other cells. On the other hand, because of the diversity of present-day cells, many kinds of experiments can be more readily undertaken with one type of cell than with another. Several different kinds of cells and organisms are commonly used as experimental models to study various aspects of cell and molecular biology. The features of some of these cells that make them particularly advantageous as experi- mental models are discussed in the sections that follow. The availability of complete genome sequences further enhances the value of these organisms as model systems in understanding the molecular biology of cells. E. coli Because of their comparative simplicity, bacteria are ideal models for studying many fundamental aspects of biochemistry and molecular biology. The most thoroughly studied species of bacteria is Escherichia coli (E. coli), which has long been the favored organism for investigation of the basic mechanisms of molecular genetics. Most of our present concepts of molecular biology—in- cluding our understanding of DNA replication, the genetic code, gene expres- sion, and protein synthesis—derive from studies of this humble bacterium. E. coli has been especially useful to molecular biologists because of both its relative simplicity and the ease with which it can be propagated and studied in the laboratory. The genome of E. coli, for example, consists of approximately 4.6 million base pairs and contains about 4000 genes. The human genome is  Introduction to Cells and Cell Research 19 nearly a thousand times larger (approximately 3 billion base pairs) and is thought to contain about 20,000 protein-coding genes (see Table 1.2). The small size of the E. coli genome provides obvious advantages for genetic analysis. Molecular genetic experiments are further facilitated by the rapid growth of E. coli under well-defined laboratory conditions. Under optimal culture conditions, E. coli divide every 20 minutes. Moreover, a clonal population of E. coli, in which all cells are derived by division of a single cell of origin, can be readily isolated as a colony grown on semisolid agar-containing medium (Figure 1.15). Because bacterial colonies containing as many as 108 cells can develop overnight, selecting genetic variants of an E. coli strain—for example, mutants that are resistant to an antibiotic such as penicillin—is easy and rapid. The ease with which such mutants can be selected and analyzed was critical Figure 1.15 Bacterial colonies to the success of experiments that defined the basic principles of molecular Photograph of colonies of E. coli grow- genetics, discussed in Chapter 4. ing on the surface of an agar-containing The nutrient mixtures in which E. coli divide most rapidly include glucose, medium. (© A. M. Siegelman/Visuals salts, and various organic compounds, such as amino acids, vitamins, and Unlimited, Inc.) nucleic acid precursors. However, E. coli can also grow in much simpler media consisting only of salts, a source of nitrogen (such as ammonia), and a source of carbon and energy (such as glucose). In such a medium, the bacteria grow a little more slowly (with a division time of about 40 minutes) because they The ease of working with E. coli must synthesize all their own amino acids, nucleotides, and other organic made it the fundamental model for compounds. The ability of E. coli to carry out these biosynthetic reactions in molecular biology. simple defined media has made them extremely useful in elucidating the biochemical pathways involved. Thus, the rapid growth and simple nutritional requirements of E. coli have greatly facilitated fundamental experiments in both molecular biology and biochemistry. Yeasts Although bacteria have been an invaluable model for studies of many con- served properties of cells, they obviously cannot be used to study aspects of cell structure and function that are unique to eukaryotes. Yeasts, the simplest eukaryotes, have a number of experimental advantages similar to those of E. coli. Consequently, yeasts have provided a crucial model for studies of many fundamental aspects of eukaryotic cell biology. The genome of the most frequently studied yeast, Saccharomyces cerevisiae, consists of 12 million base pairs of DNA and contains about 6000 genes. Although the yeast genome is approximately three times larger than that of E. coli, it is far more manageable than the genomes of more complex eukaryotes, such as humans. Yet, even in its simplicity, the yeast cell exhibits the typical features of eukaryotic cells (Figure 1.16): It contains a distinct nucleus surrounded by a nuclear membrane, its genomic DNA is organized as 16 linear chromosomes, and its cytoplasm contains subcellular organelles. Yeasts can be readily grown in the laboratory and can be studied by many of the same molecular genetic approaches that have proved so successful with E. coli. Although yeasts do not replicate as rapidly as bacteria, they still divide as frequently as every 2 hours and they can easily be grown as colonies from a single cell. Consequently, yeasts can be used for a variety of genetic manipulations similar Figure 1.16 Transmission electron micro- to those that can be performed using bacteria. graph of Saccharomyces cerevisiae Yeasts These features have made yeast cells the most approachable are the simplest model for studying eukaryotic eukaryotic cells from the standpoint of molecular biology. Yeast cells. (© Biophoto Associates/Science Source.) 20 Chapter 1 Figure 1.17 Caenorhabditis elegans The nematode is widely used for studies of animal development. (From J. E. Sul- ston and H. R. Horvitz, 1977. Dev. Biol. 56: 110.) Ovary Intestine Pharynx Eggs Vulva Rectum Anus 1 mm mutants have been important in understanding many fundamental processes Yeasts are the simplest model for eukaryotic cells. in eukaryotes, including DNA replication, transcription, RNA processing, protein sorting, and the regulation of cell division, as will be discussed in subsequent chapters. The unity of molecular cell biology is made abundantly clear by the fact that the general principles of cell structure and function revealed by studies of yeasts apply to all eukaryotic cells. Caenorhabditis elegans and Drosophila melanogaster The unicellular yeasts are important models for studies of eukaryotic cells, but understanding the development of multicellular organisms requires the experimental analysis of plants and animals—organisms that are more com- plex. The nematode Caenorhabditis elegans (Figure 1.17) possesses several notable features that make it one of the most widely used models for studies of animal development and cell differentiation. Although the genome of C. elegans (close to 100 million base pairs) is C. elegans is a simple model for studies of animal development and larger than those of unicellular eukaryotes, it is smaller and more manageable differentiation. than the genomes of most animals. Despite its relatively small size, however, the genome of C. elegans contains approximately 19,000 genes—more than three times the number of genes in yeast, and nearly the same number of protein-coding genes in humans. Biologically, C. elegans is a relatively simple multicellular organism: Adult worms consist of only 959 somatic cells, plus 1000–2000 germ cells. In addition, C. elegans can be easily grown and subjected to genetic manipulations in the laboratory. The simplicity of C. elegans has enabled the course of its development to be studied in detail by microscopic observation. Such analyses have successfully traced the embryonic origin and lineage of all the cells in the adult worm. Genetic studies have also identified many of the mutations responsible for developmental abnormalities, leading to the isolation and characterization of critical genes that control nematode development and differentiation. Importantly, similar genes have also been found to function in complex animals (including humans), making C. elegans an important model for studies of animal development. Like C. elegans, the fruit fly Drosophila melanogaster (Figure 1.18) has been The genetics of Drosophila have made it a key model in aCooper_The Cell 8e crucial model organism in developmental biology. The genome of Drosophila Sinauer developmental biology. isDragonfly 180 million base pairs, larger than that of C. elegans, but the Drosophila Media Group genome only03/30/18 Figure# 01.17 contains about 14,000 genes. Furthermore, Drosophila can be easily maintained and bred in the laboratory, and its short reproductive cycle (about 2 weeks) makes it a very useful organism for genetic experiments.  Introduction to Cells and Cell Research 21 Many fundamental concepts of genetics—such as the relationship between genes and chromosomes—were derived from studies of Drosophila early in the twentieth century (see Chapter 4). Extensive genetic analysis of Drosophila has uncovered many genes that control development and differentiation, and current methods of molecular biology have allowed the functions of these genes to be analyzed in detail. Consequently, studies of Drosophila have led to striking advances in un- derstanding the molecular mechanisms that govern animal development, particularly with respect to formation of the body plan of complex multicel- lular organisms. As with C. elegans, similar genes and mechanisms exist in vertebrates, validating the use of Drosophila as a major experimental model in contemporary developmental biology. Arabidopsis thaliana The study of plant molecular biology and development is an active and expanding field of considerable economic importance as well as intel- Figure 1.18 Drosophila melano- lectual interest. Since the genomes of plants cover a range of complexity gaster The fruit fly is a key model for comparable to that of animal genomes (see Table 1.2), an optimal model genetics and developmental biology. for studies of plant development would be a relatively simple organism (Photo by David McIntyre.) with some of the advantageous properties of C. elegans and Drosophila. The small flowering plant Arabidopsis thaliana (mouse-ear cress) (Figure 1.19) meets these criteria and is therefore widely used as a model to study the molecular biology of plants. Arabidopsis is notable for its genome of only about 125 million base pairs. Although Arabidopsis contains a total of about 26,000 genes, many of these are repeated, so its number of unique genes is approximately 15,000—a com- plexity similar to that of C. elegans and Drosophila. In addition, Arabidopsis is Arabidopsis thaliana is the basic relatively easy to grow in the laboratory, and methods for molecular genetic plant model system. manipulations of this plant have been developed. These studies have led to the identification of genes involved in various aspects of plant development, such as the development of flowers. Analysis of these genes points to many similarities, but also to striking differences, between the mechanisms that control the development of plants and animals. Vertebrates The most complex animals are the vertebrates, including humans and other mammals. The human genome is approximately 3 billion base pairs—about 20 to 30 times larger than the genomes of C. elegans, Drosophila, or Arabidopsis—and contains about 20,000 protein-coding genes. Moreover, the human body is com- posed of more than 200 different kinds of specialized cell types. This complexity makes the vertebrates difficult to study from the standpoint of cell and molecular biology, but much of the interest in biological sciences nonetheless stems from the desire to understand the human organism. Moreover, an understanding of many questions of immediate practical importance (e.g., in medicine) must be based directly on studies of human (or closely related) cell types. The specialized properties of some highly differentiated cell types have made them important models for studies of particular aspects of cell biology. Figure 1.19 Arabidopsis thaliana Arabidopsis is a model for studying the mo- lecular biology of plants. (Photo by David McIntyre.) 22 Chapter 1 (A) (B) Figure 1.20 Zebrafish (A) A 24-hour-old embryo. (B) An adult fish. (A, courtesy of Charles Kimmel, University of Ore- gon; B, photo by David McIntyre.) Muscle cells, for example, are highly specialized to undergo contraction, producing force and movement. Because of this specialization, muscle cells are crucial models for studying cell movement at the molecular level. Another example is provided by nerve cells (neurons), which are specialized to conduct electrochemical signals over long distances. In humans, nerve cell axons may be more than a meter long, and some invertebrates, such as the squid, have giant neurons with axons as large as 1 mm in diameter. Because of their highly specialized structure and function, these giant neurons have provided important models for studies of ion transport across the plasma membrane, and of the role of the cytoskeleton in the transport of cytoplasmic organelles. The zebrafish (Figure 1.20) possesses a number of advantages for genetic The zebrafish is an important model for vertebrate development. studies of vertebrate development. These small fish are easy to maintain in the laboratory and they reproduce rapidly, with a generation time of 3–4 months. In addition, the embryos develop outside of the mother and are transparent, so that early stages of development can be easily observed. Powerful methods have been developed to facilitate the isolation of mutations affecting zebrafish development, and several thousand such mutations have now been identified. Because the zebrafish is an easily studied vertebrate, it promises to bridge the gap between humans and the simpler invertebrate systems, such as C. elegans and Drosophila. Among mammals, the mouse is the most suitable for genetic analysis. The mouse is the closest model for human biology. Although the technical difficulties in studying mouse genetics (compared, for example, with the genetics of yeasts or Drosophila) are formidable, many mutations affecting mouse development have been identified. Most important, recent advances in molecular biology have enabled the produc- tion of genetically engineered mice in which specific mutant genes have been introduced into the mouse germ line, allowing the functions of these genes to be studied in the context of the whole animal. The suitability of the mouse as a model for human development is indicated not only by the similarity of the mouse and human genomes but also by the fact that mutations in homologous genes result in similar developmental defects Cooper_The Cell 8e Sinauer in both species—piebaldism (a defect in pigmentation) offering a striking example (Figure 1.21). Dragonfly Media Group Figure# 01.20 03/30/18  Introduction to Cells and Cell Research 23 Figure 1.21 The mouse as a model for human development A child and a mouse show similar defects in pigmentation (piebaldism) as a result of mutations in a gene required for normal migration of melanocytes (the cells responsible for skin pig- mentation) during embryonic development. (From R. A. Fleischman et al., 1991. Proc. Natl. Acad. Sci. USA 88: 10885; courtesy of R. A. Fleischmann, Markey Cancer Center, University of Kentucky.) Animal cell culture One important approach to studying the cells of multicellular organisms is to grow isolated cells in culture, where they can be manipulated under con- trolled laboratory conditions. The use of cultured cells has allowed studies of many aspects of mammalian cell biology, including experiments that have elucidated the mechanisms of DNA replication, gene expression, protein synthesis and processing, and cell division. Moreover, the ability to grow animal cells in culture has allowed studies of the signaling mechanisms that control cell growth and differentiation within the intact organism. Although the process is technically far more difficult than the culture The growth of cells in culture of bacteria or yeasts, a wide variety of animal cells can be grown and allows them to be manipulated manipulated in culture. Cultures are initiated by the dispersion of a piece outside of intact organisms. of tissue into a suspension of its component cells, which is then added to a culture dish containing nutrient media (Figure 1.22). Most animal cell types, such as fibroblasts and epithelial cells, attach to and grow on the plastic surface of dishes used for cell culture. Because they contain rapidly growing cells, embryos or tumors are frequently used as starting material. Embryo fibroblasts grow particularly well in culture and consequently are one of the most widely studied types of animal cells. Under appropriate conditions, however, many specialized cell types can also be grown in culture, allowing their differentiated properties to be studied in a controlled experimental environment. Embryonic stem (ES) cells are a particularly notable example. Cooper_The Cell 8e These cells are established in culture from early embryos and maintain their ability to differentiate into all of the cell types present Sinauer Dragonfly in adultMedia Group organisms. Consequently, embryonic stem cells have played an Figure# 01.21 04/09/18 important role in studying development and differentiation, as well as 24 Chapter 1 Tissue Figure 1.22 Culture of animal cells Cells obtained from a tissue are grown on culture dishes in nutrient medium. offering the possibility of contributing to the treat- ment of human diseases by providing a source of A piece of tissue is tissue for transplantation therapies. dispersed into a suspension The growthcells. of individual of cells in culture The initial cell cultures established from a tis- allows them to be manipulated sue are called primary cultures (see Figure 1.22). outside of intact animals. The cells in a primary culture usually grow until they cover the culture dish surface. They can then The cells are plated in a culture dish be removed from the dish and replated at a lower Cell suspension in nutrient medium. density to form secondary cultures. This process can be repeated many times, but most normal cells cannot be grown in culture indefinitely. For example, normal human fibroblasts can usually be cultured for Liquid medium 50–100 population doublings, after which they stop Primary growing and die. In contrast, embryonic stem cells culture and cells derived from tumors frequently proliferate The cells in this primary culture indefinitely in culture and are referred to as immortal attach to the dish and grow until cell lines. In addition, a number of immortalized they cover the culture dish surface. rodent cell lines have been isolated from cultures of normal fibroblasts. Instead of dying as most of their counterparts do, a few cells in these cultures continue proliferating indefinitely, forming cell lines like those derived from tumors. Such permanent cell lines have been particularly useful for many types of experiments because they provide a continuous The cells can then be removed from the and uniform source of cells that can be manipulated, culture dish and replated at a lower cloned, and indefinitely propagated in the labora- density to form a secondary culture. tory. The first human cell line to be established were HeLa cells, which were isolated from a cervical cancer in 1951 and have been used in thousands of Secondary culture laboratories studying many aspects of human cell biology (see Key Experiment). Viruses Viruses are intracellular parasites that cannot rep- licate on their own. They reproduce by infecting host cells and usurping the cellular machinery to produce more virus particles. In their simplest forms, viruses consist only of genomic nucleic acid (either DNA or RNA) surrounded by a protein coat (Figure 1.23). Viruses are important in mo- lecular and cellular biology because they provide simple systems that Viruses are simple models for can be used to investigate the functions of cells. Because virus replication studying cells. depends on the metabolism of the infected cells, studies of viruses have revealed many fundamental aspects of cell biology. The rapid growth and small genome size of viruses have made them especially important for studies of mammalian cells. Most animal viruses replicate and can be readily studied in cultured cells, where they take over the machinery of the cell to produce new virus particles. The genomes of animal viruses are much smaller and simpler than those of cells, ranging Cooper_The Cell 8e from approximately 3000 to 300,000 base pairs and often containing less than Sinauer Dragonfly Media Group Figure# 01.22 04/09/18  Introduction to Cells and Cell Research 25 a dozen genes. Animal viruses are thus far more manageable than their host cells, making it comparatively easy to follow virus replication and undertake genetic analysis. Examples in which animal viruses have provided critically important models for investigations of mammalian cells include studies of DNA repli- cation, transcription, RNA processing, and protein transport and secretion. woman, Henrietta Lacks, in February Key Experiment 1951. Like many cancer specimens that Gey and his colleagues had previ- HeLa Cells: The First ously attempted to establish in culture, the tissue sample from Henrietta Lacks Human Cell Line was plated in medium containing chicken plasma, calf embryo extract, Tissue Culture Studies of and human umbilical blood. However, the Proliferative Capacity unlike many previous attempts, the of Cervical Carcinoma and cells from Henrietta Lacks’ cancer Normal Epithelium grew rapidly and continued growing in culture, making HeLa cells the first George O. Gey, Ward D. Coffman, established human cancer cell line. The and Mary T. Kubicek story behind this cell line and much Johns Hopkins Hospital and of the history of early cell culture is George O. Gey (Courtesy of University, Baltimore, MD described in The Immortal Life of Henri- The Alan Mason Chesney Cancer Research, Volume 12, 1952, etta Lacks by Rebecca Skloot, 2010. Medical Archives of The pages 264–265 Johns Hopkins Medical HeLa’s Impact Institutions.) Early Cell Culture HeLa cells have become the most The earliest cell cultures involved the widely used cell line for cancer re- signaling. As one indication of their growth of cells from fragments of tis- search and other studies of human cell importance, HeLa cells have been sue that were embedded in clots of biology. One of the first major uses of used in more than 90,000 published plasma—a culture system that was far HeLa cells was in development of the research papers. Moreover, many from suitable for experimental analysis. polio vaccine, because it was found other human cell lines have now been In the 1940s, a major advance was that the polio virus grew readily in established and these lines form the made by the establishment of cell lines these cells. They continue to be used basis for contemporary studies of that grew from isolated cells attached today as a model system for virtually human cell biology. to the surface of culture dishes. The all aspects of human molecular and first of these cell lines was a mouse cellular biology, including studies of Question: Why do you think cancer line called L cells that was established DNA replication, gene expression, cells were used to establish HeLa in 1942. However, it proved consider- cell division, virology, cancer, and cell and other early cell lines? ably more difficult to establish cell lines of human origin, which were highly desired as a model for cancer research. Despite many attempts, scientists were unable to grow human cells in culture for more than a few weeks. The breakthrough came in 1951, when George Gey and his colleagues established the first human cell line, HeLa cells, from tissue of a cervical cancer. The Origin of HeLa HeLa cells were cultured from a biopsy of a cervical can- Photomicrographs of HeLa cells at magnifications of 65, 160, and 250×. (From W. F. cer taken from a 30-year-old Scherer, J. T. Syverton, and G. O. Gey, 1953. J. Exp. Med. 97: 695.) 26 Chapter 1 eventually led to identification of a effective vaccines against hepatitis B Molecular Medicine specific cancer-causing gene (onco- virus and human papillomaviruses. gene) carried by the virus, and to the Other human cancers are caused by Viruses and Cancer discovery of related genes in normal mutations in normal cell genes, most cells of all vertebrate species, including of which occur during the lifetime of What Is Cancer? humans. Some cancers in humans are the individual rather than from in- Cancer is a family of diseases char- now known to be caused by viruses; heritance. Studies of cancer-causing acterized by uncontrolled cell prolif- others result from mutations in normal viruses have led to the identification eration. The growth of normal animal cell genes similar to the oncogene first of many of the genes responsible for cells is carefully regulated to meet the identified in RSV. non–virus-induced cancers, and to needs of the complete organism. In an understanding of the molecular contrast, cancer cells grow in an un- What Viruses Have Taught Us mechanisms responsible for cancer regulated manner, ultimately invading The human cancers that are caused development. Major efforts are now and interfering with the function of nor- by viruses include cervical and other under way to use these insights into mal tissues and organs. Cancer is the anogenital cancers (papillomavi- the molecular and cellular biology of second most common cause of death ruses), liver cancer (hepatitis B and C cancer to develop new approaches (next to heart disease) in the United viruses), and some types of lympho- to cancer treatment. Indeed, the first States. Approximately one out of every mas (Epstein-Barr virus and human designer drug effective in treating a three Americans will develop cancer at T-cell lymphotropic virus). Together, human cancer (the drug imatinib or some point in life and, despite major these virus-induced cancers account Gleevec, discussed in Chapter 20) was advances in treatment, nearly one out for 15–20% of worldwide cancer developed against a gene very similar of every four Americans ultimately die incidence. In principle, these cancers to the oncogene of RSV. of this disease. Understanding the could be prevented by vaccination causes of cancer and developing more against the responsible viruses, and Question: Why have viruses been effective methods of cancer treatment considerable progress in this area has useful for studying cancer, even therefore represent major goals of been made by the development of though they cause less than 20% of medical research. cancers in humans? The First Cancer-Causing Virus Cancer is now known to result from mutations in the genes that normally control cell proliferation. The major insights leading to identification of these genes came from studies of viruses that cause cancer in animals, the prototype of which was isolated by Peyton Rous in 1911. Rous found that sarcomas (cancers of connective tissues) in chickens could be transmit- ted by a virus, now known as Rous sarcoma virus, or RSV. Because RSV has a genome of only 10,000 base pairs, it can be subjected to molecu- lar analysis much more readily than the complex genomes of chickens or other animal cells. Such studies The transplantable tumor from which Rous sarcoma virus was isolated. (From P. Rous, 1911. J. Exp. Med. 13: 397.) It is also noteworthy that infection by some animal viruses can convert FYI normal cells into cancer cells (see Molecular Medicine). Studies of such cancer-causing viruses, first described by Peyton Rous in 1911, not only have Viruses cause 15–20% of human provided the basis for our current understanding of cancer at the level of cell cancers worldwide (see Chapter 20). and molecular biology, but also have led to the elucidation of many of the molecular mechanisms that control animal cell growth and differentiation.  Introduction to Cells and Cell Research 27 (A) (B) 50 nm DNA Capsid proteins Figure 1.23 Structure of an animal virus (A) Papillomavirus particles contain a small circular DNA molecule enclosed in a protein coat (the capsid). (B) Electron micrograph of human papillomavirus particles. Artificial color has been added. (B, © Linda Stannard/Science Photo Library/Science Source.) 1.2 Review Some organisms are widely used in cell and molecular biology because they can easily be studied in the laboratory. E. coli is the basic model for fundamental aspects of biochemistry and molecular biology, and yeasts are the simplest model for eukaryotic cells. C. elegans and Drosophila are widely used for studies of animal development, and Arabidopsis thaliana is the model plant. The closest model for hu