1 LIFE BEGINS WITH CELLS A single ~200 micrometer ((cid:1)m) cell,the human egg,with sperm,which are also single cells.From the union of an egg and sperm will arise the 10 trillion cells of a human body. [Photo Researchers, Inc.] L ike ourselves, the individual cells that form our bodies 1.1 The Diversity and Commonality can grow, reproduce, process information, respond to of Cells stimuli, and carry out an amazing array of chemical re- actions. These abilities define life. We and other multicellular Cells come in an amazing variety of sizes and shapes (Figure organisms contain billions or trillions of cells organized into 1-1). Some move rapidly and have fast-changing structures, as complex structures, but many organisms consist of a single we can see in movies of amoebae and rotifers. Others are cell. Even simple unicellular organisms exhibit all the hall- largely stationary and structurally stable. Oxygen kills some mark properties of life, indicating that the cell is the funda- cells but is an absolute requirement for others. Most cells in mental unit of life. As the twenty-first century opens, we face multicellular organisms are intimately involved with other an explosion of new data about the components of cells, cells. Although some unicellular organisms live in isolation, what structures they contain, how they touch and influence others form colonies or live in close association with other each other. Still, an immense amount remains to be learned, types of organisms, such as the bacteria that help plants to ex- particularly about how information flows through cells and tract nitrogen from the air or the bacteria that live in our in- how they decide on the most appropriate ways to respond. testines and help us digest food. Despite these and numerous Molecular cell biology is a rich, integrative science that brings together biochemistry, biophysics, molecular biology, microscopy, genetics, physiology, computer science, and de- OUTLINE velopmental biology. Each of these fields has its own em- phasis and style of experimentation. In the following 1.1 The Diversity and Commonality of Cells chapters, we will describe insights and experimental ap- 1.2 The Molecules of a Cell proaches drawn from all of these fields, gradually weaving the multifaceted story of the birth, life, and death of cells. We 1.3 The Work of Cells start in this prologue chapter by introducing the diversity of 1.4 Investigating Cells and Their Parts cells, their basic constituents and critical functions, and what we can learn from the various ways of studying cells. 1.5 A Genome Perspective on Evolution 1 2 CHAPTER 1 • Life Begins with Cells (a) (b) (c) (d) (e) (f) (g) (h) ▲ FIGURE 1-1 Cells come in an astounding assortment of Purkinje neuron of the cerebellum, which can form more than a shapes and sizes.Some of the morphological variety of cells is hundred thousand connections with other cells through the illustrated in these photographs. In addition to morphology, cells branched network of dendrites. The cell was made visible by differ in their ability to move, internal organization (prokaryotic introduction of a fluorescent protein; the cell body is the bulb at versus eukaryotic cells), and metabolic activities. (a) Eubacteria; the bottom. (g) Cells can form an epithelial sheet, as in the slice note dividing cells. These are Lactococcus lactis,which are used through intestine shown here. Each finger-like tower of cells, a to produce cheese such as Roquefort, Brie, and Camembert. villus, contains many cells in a continuous sheet. Nutrients are (b) A mass of archaebacteria (Methanosarcina) that produce their transferred from digested food through the epithelial sheet to the energy by converting carbon dioxide and hydrogen gas to blood for transport to other parts of the body. New cells form methane. Some species that live in the rumen of cattle give rise continuously near the bases of the villi, and old cells are shed to >150 liters of methane gas/day. (c) Blood cells, shown in false from the top. (h) Plant cells are fixed firmly in place in vascular color. The red blood cells are oxygen-bearing erythrocytes, the plants, supported by a rigid cellulose skeleton. Spaces between white blood cells (leukocytes) are part of the immune system the cells are joined into tubes for transport of water and food. and fight infection, and the green cells are platelets that provide [Part (a) Gary Gaugler/ Photo Researchers, Inc. Part (b) Ralph Robinson/ substances to make blood clot at a wound. (d) Large single cells: Visuals Inlimited, Inc. Part (c) NIH/Photo Researchers, Inc. Part (d) fossilized dinosaur eggs. (e) A colonial single-celled green alga, John D. Cunningham/Visuals Unlimited, Inc. Part (e) Carolina Volvox aureus. The large spheres are made up of many individual Biological/Visuals Unlimited, Inc. Part (f) Helen M. Blau, Stanford cells, visible as blue or green dots. The yellow masses inside are University. Part (g) Jeff Gordon, Washington University School of daughter colonies, each made up of many cells. (f) A single Medicine. Part (h) Richard Kessel and C. Shih/Visuals Unlimited, Inc.] other differences, all cells share certain structural features and gle closed compartment that is surrounded by the plasma carry out many complicated processes in basically the same membrane,lacks a defined nucleus,and has a relatively simple way. As the story of cells unfolds throughout this book, we internal organization (Figure 1-2a). All prokaryoteshave cells will focus on the molecular basis of both the differences and of this type. Bacteria, the most numerous prokaryotes, are similarities in the structure and function of various cells. single-celled organisms; the cyanobacteria, or blue-green algae, can be unicellular or filamentous chains of cells. Although bac- terial cells do not have membrane-bounded compartments, All Cells Are Prokaryotic or Eukaryotic many proteins are precisely localized in their aqueous interior, The biological universe consists of two types of cells— or cytosol,indicating the presence of internal organization. A prokaryotic and eukaryotic. Prokaryotic cells consist of a sin- single Escherichia coli bacterium has a dry weight of about 1.1 • The Diversity and Commonality of Cells 3 (a) Prokaryotic cell (b) Eukaryotic cell Nucleus Periplasmic space and cell wall Golgi vesicles Lysosome Mitochondrion Outer membrane Inner (plasma) Nucleoid 0.5 (cid:3)m membrane Endoplasmic reticulum 1 (cid:3)m Nucleoid Nuclear membrane Plasma membrane Golgi vesicles Nucleus Mitochondrion Peroxisome Lysosome Inner (plasma) membrane Cell wall Periplasmic space Outer membrane Rough endoplasmic Secretory vesicle reticulum ▲ FIGURE 1-2 Prokaryotic cells have a simpler internal characteristic of eukaryotic cells is segregation of the cellular DNA organization than eukaryotic cells.(a) Electron micrograph of a within a defined nucleus, which is bounded by a double thin section ofEscherichia coli,a common intestinal bacterium. membrane. The outer nuclear membrane is continuous with the The nucleoid, consisting of the bacterial DNA, is not enclosed rough endoplasmic reticulum, a factory for assembling proteins. within a membrane. E. coliand some other bacteria are Golgi vesicles process and modify proteins, mitochondria generate surrounded by two membranes separated by the periplasmic energy, lysosomes digest cell materials to recycle them, space. The thin cell wall is adjacent to the inner membrane. peroxisomes process molecules using oxygen, and secretory (b) Electron micrograph of a plasma cell, a type of white blood vesicles carry cell materials to the surface to release them. cell that secretes antibodies. Only a single membrane (the plasma [Part (a) courtesy of I. D. J. Burdett and R. G. E. Murray. Part (b) from membrane) surrounds the cell, but the interior contains many P. C. Cross and K. L. Mercer, 1993, Cell and Tissue Ultrastructure: membrane-limited compartments, or organelles. The defining A Functional Perspective,W. H. Freeman and Company.] 25 (cid:1)10(cid:2)14g. Bacteria account for an estimated 1–1.5 kg of branes that enclose other compartments, the organelles(Fig- the average human’s weight. The estimated number of bacte- ure 1-2b). The region of the cell lying between the plasma ria on earth is 5 (cid:1)1030, weighing a total of about 1012kg. membrane and the nucleus is the cytoplasm,comprising the Prokaryotic cells have been found 7 miles deep in the ocean cytosol (aqueous phase) and the organelles.Eukaryotescom- and 40 miles up in the atmosphere; they are quite adaptable! prise all members of the plant and animal kingdoms, includ- The carbon stored in bacteria is nearly as much as the carbon ing the fungi, which exist in both multicellular forms (molds) stored in plants. and unicellular forms (yeasts), and the protozoans (proto, Eukaryotic cells, unlike prokaryotic cells, contain a de- primitive; zoan,animal), which are exclusively unicellular. fined membrane-bound nucleus and extensive internal mem- Eukaryotic cells are commonly about 10–100 (cid:3)m across, 4 CHAPTER 1 • Life Begins with Cells Animals Plants so many similarities. In recent years, detailed analysis of the Fungi DNA sequences from a variety of prokaryotic organisms has Ciliates revealed two distinct types: the so-called “true” bacteria, or eu- Euglena Microsporidia EUKARYOTA bacteria,and archaea(also called archaebacteria or archaeans). Slime molds Working on the assumption that organisms with more similar Diplomonads genes evolved from a common progenitor more recently than (Giardia lamblia) those with more dissimilar genes, researchers have developed the evolutionary lineage tree shown in Figure 1-3. According to EUBACTERIA E. coli Sulfolobus this tree, the archaea and the eukaryotes diverged from the true ARCHAEA bacteria before they diverged from each other. Many archaeans grow in unusual, often extreme, envi- B. subtilus Thermococcus ronments that may resemble ancient conditions when life Thermotoga Methanobacterium first appeared on earth. For instance, halophiles (“salt lov- Halococcus ing”) require high concentrations of salt to survive, and Flavobacteria thermoacidophiles (“heat and acid loving”) grow in hot (80(cid:4) C) sulfur springs, where a pH of less than 2 is common. Still Green sulfur Halobacterium bacteria other archaeans live in oxygen-free milieus and generate Methanococcus methane (CH ) by combining water with carbon dioxide. Borrelia 4 jannaschii burgdorferi Unicellular Organisms Help and Hurt Us Bacteria and archaebacteria, the most abundant single-celled Presumed common progenitor of all extant organisms organisms, are commonly 1–2 (cid:3)m in size. Despite their small size and simple architecture, they are remarkable biochemi- Presumed common progenitor cal factories, converting simple chemicals into complex bio- of archaebacteria and eukaryotes logical molecules. Bacteria are critical to the earth’s ecology, ▲ FIGURE 1-3 All organisms from simple bacteria to but some cause major diseases: bubonic plague (Black Death) complex mammals probably evolved from a common,single- from Yersinia pestis,strep throat from Streptomyces,tuber- celled progenitor.This family tree depicts the evolutionary culosis from Mycobacterium tuberculosis, anthrax from relations among the three major lineages of organisms. The Bacillus anthracis, cholera from Vibrio cholerae, food poi- structure of the tree was initially ascertained from morphological soning from certain types of E. coliand Salmonella. criteria: Creatures that look alike were put close together. More Humans are walking repositories of bacteria, as are all recently the sequences of DNA and proteins have been plants and animals. We provide food and shelter for a stag- examined as a more information-rich criterion for assigning gering number of “bugs,” with the greatest concentration in relationships. The greater the similarities in these macromolecular our intestines. Bacteria help us digest our food and in turn sequences, the more closely related organisms are thought to are able to reproduce. A common gut bacterium, E. coliis be. The trees based on morphological comparisons and the fossil also a favorite experimental organism. In response to signals record generally agree well with those those based on molecular from bacteria such as E. coli,the intestinal cells form appro- data. Although all organisms in the eubacterial and archaean priate shapes to provide a niche where bacteria can live, thus lineages are prokaryotes, archaea are more similar to eukaryotes facilitating proper digestion by the combined efforts of the than to eubacteria (“true” bacteria) in some respects. For instance, archaean and eukaryotic genomes encode homologous bacterial and the intestinal cells. Conversely, exposure to in- histone proteins, which associate with DNA; in contrast, bacteria testinal cells changes the properties of the bacteria so that lack histones. Likewise, the RNA and protein components of they participate more effectively in digestion. Such commu- archaean ribosomes are more like those in eukaryotes than nication and response is a common feature of cells. those in bacteria. The normal, peaceful mutualism of humans and bacteria is sometimes violated by one or both parties. When bacteria begin to grow where they are dangerous to us (e.g., in the blood- generally much larger than bacteria. A typical human fi- stream or in a wound), the cells of our immune system fight broblast, a connective tissue cell, might be about 15 (cid:3)m back, neutralizing or devouring the intruders. Powerful antibi- across with a volume and dry weight some thousands of otic medicines, which selectively poison prokaryotic cells, times those of an E. colibacterial cell. An amoeba, a single- provide rapid assistance to our relatively slow-developing celled protozoan, can be more than 0.5 mm long. An ostrich immune response. Understanding the molecular biology of bac- egg begins as a single cell that is even larger and easily visi- terial cells leads to an understanding of how bacteria are nor- ble to the naked eye. mally poisoned by antibiotics, how they become resistant to All cells are thought to have evolved from a common pro- antibiotics, and what processes or structures present in bacter- genitor because the structures and molecules in all cells have ial but not human cells might be usefully targeted by new drugs. 1.1 • The Diversity and Commonality of Cells 5 (a) (b) V M Red blood cell 3 ide E Sporulation o D : P I la A Merozoites Merozoites 4 sm C 2 od O Liver Gametocytes iu N m N Human 5 S E p o C r o T 1 z o I Mosquito it O Sporozoites Sperm 6 Egg e En NS t e r in 7 g a n d 8 E Zygote x Oocyst it in g ▲ FIGURE 1-4 Plasmodium organisms,the parasites that event that brings on the fevers and shaking chills that are a L cause malaria,are single-celled protozoans with a the well-known symptoms of malaria. Some of the iv remarkable life cycle.Many Plasmodiumspecies are known, released merozoites infect additional RBCs, creating a e r and they can infect a variety of animals, cycling between insect cycle of production and infection. Eventually, some C e and vertebrate hosts. The four species that cause malaria in merozoites develop into male and female gametocytes ll humans undergo several dramatic transformations within their 5, another metamorphosis. These cells, which contain half human and mosquito hosts. (a) Diagram of the life cycle. the usual number of chromosomes, cannot survive for long Sporozoites enter a human host when an infected Anopheles unless they are transferred in blood to an Anopheles mosquito bites a person 1. They migrate to the liver where they mosquito. In the mosquito’s stomach, the gametocytes are develop into merozoites, which are released into the blood 2. transformed into sperm or eggs (gametes), yet another Merozoites differ substantially from sporozoites, so this metamorphosis marked by development of long hairlike transformation is a metamorphosis (Greek, “to transform” or flagella on the sperm 6. Fusion of sperm and eggs “many shapes”). Circulating merozoites invade red blood cells generates zygotes 7, which implant into the cells of the (RBCs) and reproduce within them 3. Proteins produced by stomach wall and grow into oocysts, essentially factories some Plasmodiumspecies move to the surface of infected for producing sporozoites. Rupture of an oocyst releases RBCs, causing the cells to adhere to the walls of blood vessels. thousands of sporozoites 8; these migrate to the salivary This prevents infected RBCs cells from circulating to the spleen glands, setting the stage for infection of another human where cells of the immune system would destroy the RBCs and host. (b) Scanning electron micrograph of mature oocysts the Plasmodiumorganisms they harbor. After growing and and emerging sporozoites. Oocysts abut the external reproducing in RBCs for a period of time characteristic of each surface of stomach wall cells and are encased within a Plasmodiumspecies, the merozoites suddenly burst forth in membrane that protects them from the host immune synchrony from large numbers of infected cells 4. It is this system. [Part (b) courtesy of R. E. Sinden.] Like bacteria, protozoa are usually beneficial members of tis; and Trypanosoma brucei, sleeping sickness. Each year the the food chain. They play key roles in the fertility of soil, con- worst of the protozoa, Plasmodium falciparumand related trolling bacterial populations and excreting nitrogenous and species, is the cause of more than 300 million new cases of phosphate compounds, and are key players in waste treat- malaria, a disease that kills 1.5 to 3 million people annually. ment systems—both natural and man-made. These unicellu- These protozoans inhabit mammals and mosquitoes alter- lar eukaryotes are also critical parts of marine ecosystems, nately, changing their morphology and behavior in response consuming large quantities of phytoplankton and harboring to signals in each of these environments. They also recog- photosynthetic algae, which use sunlight to produce biologi- nize receptors on the surfaces of the cells they infect. The cally useful energy forms and small fuel molecules. complex life cycle of Plasmodium dramatically illustrates However, some protozoa do give us grief: Entamoeba how a single cell can adapt to each new challenge it encoun- histolyticacauses dysentery; Trichomonas vaginalis,vagini- ters (Figure 1-4). All of the transformations in cell type that 6 CHAPTER 1 • Life Begins with Cells occur during the Plasmodiumlife cycle are governed by in- make numerous antibiotics and are used in the manufacture structions encoded in the genetic material of this parasite and of bread, beer, wine, and cheese. Not so pleasant are fungal triggered by environmental inputs. diseases, which range from relatively innocuous skin infec- The other group of single-celled eukaryotes, the yeasts, tions, such as jock itch and athlete’s foot, to life-threatening also have their good and bad points, as do their multicellular Pneumocystis carinii pneumonia, a common cause of death cousins, the molds. Yeasts and molds, which collectively con- among AIDS patients. stitute the fungi, have an important ecological role in break- ing down plant and animal remains for reuse. They also Even Single Cells Can Have Sex The common yeast used to make bread and beer, Saccha- romyces cerevisiae,appears fairly frequently in this book be- (a) cause it has proven to be a great experimental organism. Like Mating between haploid many other unicellular organisms, yeasts have two mating 1 cells of opposite mating types that are conceptually like the male and female gametes type Vegetative growth a α 2 of diploid cells (eggs and sperm) of higher organisms. Two yeast cells of op- Diploid cells (a/α) posite mating type can fuse, or mate, to produce a third cell type containing the genetic material from each cell (Figure Bud 1-5). Such sexual life cycles allow more rapid changes in ge- netic inheritance than would be possible without sex, result- ing in valuable adaptations while quickly eliminating detrimental mutations. That, and not just Hollywood, is probably why sex is so ubiquitous. 5 Vegetative growth Viruses Are the Ultimate Parasites of haploid cells Four haploid Virus-caused diseases are numerous and all too familiar: ascospores chicken pox, influenza, some types of pneumonia, polio, within ascus measles, rabies, hepatitis, the common cold, and many oth- Ascus ruptures, Starvation causes ers. Smallpox, once a worldwide scourge, was eradicated by 4 3 ascus formation, spores germinate a decade-long global immunization effort beginning in the meiosis mid-1960s. Viral infections in plants (e.g., dwarf mosaic virus in corn) have a major economic impact on crop pro- (b) duction. Planting of virus-resistant varieties, developed by traditional breeding methods and more recently by genetic engineering techniques, can reduce crop losses significantly. Most viruses have a rather limited host range, infecting cer- tain bacteria, plants, or animals (Figure 1-6). Because viruses cannot grow or reproduce on their own, they are not considered to be alive. To survive, a virus must Budding (S. cerevisiae) infect a host cell and take over its internal machinery to syn- thesize viral proteins and in some cases to replicate the viral ▲ FIGURE 1-5 The yeast Saccharomyces cerevisiae genetic material. When newly made viruses are released, the reproduces sexually and asexually.(a) Two cells that differ in cycle starts anew. Viruses are much smaller than cells, on the mating type, called aand (cid:7), can mate to form an a/(cid:7)cell 1. order of 100 nanometer (nm) in diameter; in comparison, The aand (cid:7)cells are haploid, meaning they contain a single copy bacterial cells are usually (cid:5)1000 nm (1 nm(cid:6)10(cid:2)9meters). A of each yeast chromosome, half the usual number. Mating yields virus is typically composed of a protein coat that encloses a a diploid a/(cid:7)cell containing two copies of each chromosome. core containing the genetic material, which carries the infor- During vegetative growth, diploid cells multiply by mitotic mation for producing more viruses (Chapter 4). The coat budding, an asexual process 2. Under starvation conditions, protects a virus from the environment and allows it to stick diploid cells undergo meiosis, a special type of cell division, to to, or enter, specific host cells. In some viruses, the protein form haploid ascospores 3. Rupture of an ascus releases four coat is surrounded by an outer membrane-like envelope. haploid spores, which can germinate into haploid cells 4. These The ability of viruses to transport genetic material into also can multiply asexually 5. (b) Scanning electron micrograph of budding yeast cells. After each bud breaks free, a scar is left cells and tissues represents a medical menace and a medical at the budding site so the number of previous buds can be opportunity. Viral infections can be devastatingly destructive, counted. The orange cells are bacteria. [Part (b) M. Abbey/Visuals causing cells to break open and tissues to fall apart. However, Unlimited, Inc.] many methods for manipulating cells depend upon using 1.1 • The Diversity and Commonality of Cells 7 (a) T4 bacteriophage (b) Tobacco mosaic virus 50 nm (c) Adenovirus 100 nm 50 nm ▲ FIGURE 1-6 Viruses must infect a host cell to grow and infected tobacco plants and stunts their growth. (c) Adenovirus reproduce. These electron micrographs illustrate some of the causes eye and respiratory tract infections in humans. This virus structural variety exhibited by viruses. (a) T4 bacteriophage has an outer membranous envelope from which long (bracket) attaches to a bacterial cell via a tail structure. Viruses glycoprotein spikes protrude. [Part (a) from A. Levine, 1991, Viruses, that infect bacteria are called bacteriophages, or simply phages. Scientific American Library, p. 20. Part (b) courtesy of R. C. Valentine. (b) Tobacco mosaic virus causes a mottling of the leaves of Part (c) courtesy of Robley C. Williams, University of California.] viruses to convey genetic material into cells. To do this, the sues, organs, and appendages. Our two hands have the same portion of the viral genetic material that is potentially harm- kinds of cells, yet their different arrangements—in a mirror ful is replaced with other genetic material, including human image—are critical for function. In addition, many cells ex- genes. The altered viruses, or vectors,still can enter cells tot- hibit distinct functional and/or structural asymmetries, a ing the introduced genes with them (Chapter 9). One day, dis- property often called polarity.From such polarized cells arise eases caused by defective genes may be treated by using viral vectors to introduce a normal copy of a defective gene into (cid:1)FIGURE 1-7 The first patients. Current research is dedicated to overcoming the con- few cell divisions of a siderable obstacles to this approach, such as getting the in- fertilized egg set the V M troduced genes to work at the right places and times. stage for all subsequent ide E development.A developing o D mouse embryo is shown at : E I We Develop from a Single Cell a A In 1827, German physician Karl von Baer discovered that (a) (aan) dt h(ce) tewigoh-ct-eclel,l l( bs)t afgoeusr-.c ell, rly E C m O mammals grow from eggs that come from the mother’s The embryo is surrounded b N ovary. Fertilization of an egg by a sperm cell yields a zygote, by supporting membranes. r a visually unimpressive cell 200 (cid:3)m in diameter. Every The corresponding steps yon NE hsaurmy ainn sbtreuincgti obnegsi nfos ra sb uai lzdyignogt et,h we hhiuchm haonu bsoesd ayl lc othnet anienciensg- ionc chuurm daunr indge vtehleo pfmirset nfte w ic De CT about 100 trillion (1014) cells, an amazing feat. Development days after fertilization. ve IO begins with the fertilized egg cell dividing into two, four, then [Claude Edelmann/Photo lop N eight cells, forming the very early embryo (Figure 1-7). Con- (b) Researchers, Inc.] m S e tinued cell proliferation and then differentiationinto distinct n t cell types gives rise to every tissue in the body. One initial cell, the fertilized egg (zygote), generates hundreds of differ- ent kinds of cells that differ in contents, shape, size, color, mobility, and surface composition. We will see how genes and signals control cell diversification in Chapters 15 and 22. Making different kinds of cells—muscle, skin, bone, neu- ron, blood cells—is not enough to produce the human body. (c) The cells must be properly arranged and organized into tis- 8 CHAPTER 1 • Life Begins with Cells asymmetric, polarized tissues such as the lining of the intes- tines and structures like hands and hearts. The features that make some cells polarized, and how they arise, also are cov- ered in later chapters. Stem Cells, Cloning, and Related Techniques Offer Exciting Possibilities but Raise Some Concerns Identical twins occur naturally when the mass of cells com- posing an early embryo divides into two parts, each of which develops and grows into an individual animal. Each cell in an eight-cell-stage mouse embryo has the potential to give rise to any part of the entire animal. Cells with this capabil- ity are referred to as embryonic stem (ES) cells.As we learn ▲ FIGURE 1-8 Five genetically identical cloned sheep. An in Chapter 22, ES cells can be grown in the laboratory (cul- early sheep embryo was divided into five groups of cells and tured) and will develop into various types of differentiated each was separately implanted into a surrogate mother, much cells under appropriate conditions. like the natural process of twinning. At an early stage the cells The ability to make and manipulate mammalian embryos are able to adjust and form an entire animal; later in development in the laboratory has led to new medical opportunities as the cells become progressively restricted and can no longer do well as various social and ethical concerns. In vitro fertiliza- so. An alternative way to clone animals is to replace the nuclei of tion, for instance, has allowed many otherwise infertile cou- multiple single-celled embryos with donor nuclei from cells of an ples to have children. A new technique involves extraction of adult sheep. Each embryo will be genetically identical to the nuclei from defective sperm incapable of normally fertiliz- adult from which the nucleus was obtained. Low percentages of ing an egg, injection of the nuclei into eggs, and implantation embryos survive these procedures to give healthy animals, and of the resulting fertilized eggs into the mother. the full impact of the techniques on the animals is not yet known. In recent years, nuclei taken from cells of adult animals [Geoff Tompkinson/Science Photo Library/Photo Researchers, Inc.] have been used to produce new animals. In this procedure, the nucleus is removed from a body cell (e.g., skin or blood cell) of a donor animal and introduced into an unfertilized merous diseases in which particular cell types are damaged mammalian egg that has been deprived of its own nucleus. or missing, and of repairing wounds more completely. This manipulated egg, which is equivalent to a fertilized egg, is then implanted into a foster mother. The ability of such a donor nucleus to direct the development of an entire animal 1.2 The Molecules of a Cell suggests that all the information required for life is retained in the nuclei of some adult cells. Since all the cells in an ani- Molecular cell biologists explore how all the remarkable mal produced in this way have the genes of the single origi- properties of the cell arise from underlying molecular events: nal donor cell, the new animal is a cloneof the donor (Figure the assembly of large molecules, binding of large molecules 1-8). Repeating the process can give rise to many clones. So to each other, catalytic effects that promote particular chem- far, however, the majority of embryos produced by this tech- ical reactions, and the deployment of information carried by nique of nuclear-transfer cloningdo not survive due to birth giant molecules. Here we review the most important kinds of defects. Even those animals that are born live have shown molecules that form the chemical foundations of cell struc- abnormalities, including accelerated aging. The “rooting” ture and function. of plants, in contrast, is a type of cloning that is readily ac- complished by gardeners, farmers, and laboratory technicians. Small Molecules Carry Energy, Transmit Signals, The technical difficulties and possible hazards of nuclear- and Are Linked into Macromolecules transfer cloning have not deterred some individuals from pur- suing the goal of human cloning. However, cloning of Much of the cell’s contents is a watery soup flavored with humans per se has very limited scientific interest and is op- small molecules (e.g., simple sugars, amino acids, vitamins) posed by most scientists because of its high risk. Of greater and ions (e.g., sodium, chloride, calcium ions). The locations scientific and medical interest is the ability to generate specific and concentrations of small molecules and ions within the cell types starting from embryonic or adult stem cells. The sci- cell are controlled by numerous proteins inserted in cellular entific interest comes from learning the signals that can un- membranes. These pumps, transporters, and ion channels leash the potential of the genes to form a certain cell type. The move nearly all small molecules and ions into or out of the medical interest comes from the possibility of treating the nu- cell and its organelles (Chapter 7). 1.2 • The Molecules of a Cell 9 One of the best-known small molecules is adenosine polysaccharides.These macromolecules are critical structural triphosphate(ATP),which stores readily available chemical components of plant cell walls and insect skeletons. A typical energy in two of its chemical bonds (see Figure 2-24). When polysaccharide is a linear or branched chain of repeating cells split apart these energy-rich bonds in ATP, the released identical sugar units. Such a chain carries information: the energy can be harnessed to power an energy-requiring number of units. However if the units are notidentical, then process like muscle contraction or protein biosynthesis. To the order and type of units carry additional information. As obtain energy for making ATP, cells break down food mole- we see in Chapter 6, some polysaccharides exhibit the greater cules. For instance, when sugar is degraded to carbon diox- informational complexity associated with a linear code made ide and water, the energy stored in the original chemical up of different units assembled in a particular order. This bonds is released and much of it can be “captured” in ATP property, however, is most typical of the two other types of (Chapter 8). Bacterial, plant, and animal cells can all make biological macromolecules—proteinsand nucleic acids. ATP by this process. In addition, plants and a few other or- ganisms can harvest energy from sunlight to form ATP in photosynthesis. Proteins Give Cells Structure and Perform Most Other small molecules act as signals both within and be- Cellular Tasks tween cells; such signals direct numerous cellular activities (Chapters 13–15). The powerful effect on our bodies of a The varied, intricate structures of proteins enable them to frightening event comes from the instantaneous flooding of carry out numerous functions. Cells string together 20 dif- the body with epinephrine, a small-molecule hormonethat ferent amino acids in a linear chain to form a protein (see mobilizes the “fight or flight” response. The movements Figure 2-13). Proteins commonly range in length from 100 to needed to fight or flee are triggered by nerve impulses that 1000 amino acids, but some are much shorter and others flow from the brain to our muscles with the aid of neuro- longer. We obtain amino acids either by synthesizing them transmitters,another type of small-molecule signal that we from other molecules or by breaking down proteins that we discuss in Chapter 7. eat. The “essential” amino acids, from a dietary standpoint, Certain small molecules (monomers)in the cellular soup are the eight that we cannot synthesize and must obtain from can be joined to form polymersthrough repetition of a single food. Beans and corn together have all eight, making their type of chemical-linkage reaction (see Figure 2-11). Cells combination particularly nutritious. Once a chain of amino produce three types of large polymers, commonly called acids is formed, it folds into a complex shape, conferring a macromolecules: polysaccharides, proteins, and nucleic distinctive three-dimensional structure and function on each acids. Sugars, for example, are the monomers used to form protein (Figure 1-9). Insulin DNA molecule Glutamine synthetase Hemoglobin Immunoglobulin Adenylate Lipid bilayer kinase ▲ FIGURE 1-9 Proteins vary greatly in size,shape,and (glutamine synthetase and adenylate kinase), an antibody function.These models of the water-accessible surface of some (immunoglobulin), a hormone (insulin), and the blood’s oxygen representative proteins are drawn to a common scale and reveal carrier (hemoglobin). Models of a segment of the nucleic acid the numerous projections and crevices on the surface. Each DNA and a small region of the lipid bilayer that forms cellular protein has a defined three-dimensional shape (conformation) membranes (see Section 1.3) demonstrate the relative width of that is stabilized by numerous chemical interactions discussed in these structures compared with typical proteins. [Courtesy of Chapters 2 and 3. The illustrated proteins include enzymes Gareth White.] 10 CHAPTER 1 • Life Begins with Cells Some proteins are similar to one another and therefore lecular weight of 52,700 (g/mol). Assuming this value is can be considered members of a protein family.A few hun- typical of eukaryotic proteins, we can calculate the total dred such families have been identified. Most proteins are de- number of protein molecules per liver cell as about 7.9 (cid:1) signed to work in particular places within a cell or to be 109from the total protein weight and Avogadro’s number, released into the extracellular (extra,“outside”) space. Elab- the number of molecules per mole of any chemical com- orate cellular pathways ensure that proteins are transported pound (6.02 (cid:1) 1023). To carry this calculation one step to their proper intracellular (intra,within) locations or se- further, consider that a liver cell contains about 10,000 creted (Chapters 16 and 17). different proteins; thus, a cell contains close to a million Proteins can serve as structural components of a cell, for molecules of each type of protein on average. In actuality example, by forming an internal skeleton (Chapters 5, 19, and the abundance of different proteins varies widely, from the 20). They can be sensors that change shape as temperature, ion quite rare insulin-binding receptor protein (20,000 mole- concentrations, or other properties of the cell change. They cules) to the abundant structural protein actin (5 (cid:1) 108 can import and export substances across the plasma mem- molecules). brane (Chapter 7). They can be enzymes,causing chemical re- actions to occur much more rapidly than they would without Nucleic Acids Carry Coded Information the aid of these protein catalysts(Chapter 3). They can bind to for Making Proteins at the Right Time and Place a specific gene, turning it on or off (Chapter 11). They can be extracellular signals, released from one cell to communicate The information about how, when, and where to produce each with other cells, or intracellular signals, carrying information kind of protein is carried in the genetic material, a polymer within the cell (Chapters 13–15). They can be motors that called deoxyribonucleic acid (DNA). The three-dimensional move other molecules around, burning chemical energy (ATP) structure of DNA consists of two long helical strands that are to do so (Chapters 19 and 20). coiled around a common axis, forming a double helix.DNA How can 20 amino acids form all the different proteins strands are composed of monomers called nucleotides;these needed to perform these varied tasks? Seems impossible at often are referred to as basesbecause their structures contain first glance. But if a “typical” protein is about 400 amino cyclic organic bases (Chapter 4). acids long, there are 20400 possible different protein se- Four different nucleotides, abbreviated A, T, C, and G, quences. Even assuming that many of these would be func- are joined end to end in a DNA strand, with the base parts tionally equivalent, unstable, or otherwise discountable, the projecting out from the helical backbone of the strand. Each number of possible proteins is well along toward infinity. DNA double helix has a simple construction: wherever there Next we might ask how many protein molecules a cell is an A in one strand there is a T in the other, and each C is needs to operate and maintain itself. To estimate this num- matched with a G (Figure 1-10). This complementarymatch- ber, let’s take a typical eukaryotic cell, such as a hepatocyte ing of the two strands is so strong that if complementary (liver cell). This cell, roughly a cube 15 (cid:3)m (0.0015 cm) on strands are separated, they will spontaneously zip back to- a side, has a volume of 3.4 (cid:1)10(cid:2)9cm3(or milliliters). As- gether in the right salt and temperature conditions. Such suming a cell density of 1.03 g/ml, the cell would weigh hybridizationis extremely useful for detecting one strand using 3.5 (cid:1)10(cid:2)9g. Since protein accounts for approximately 20 the other. For example, if one strand is purified and attached percent of a cell’s weight, the total weight of cellular pro- to a piece of paper, soaking the paper in a solution contain- tein is 7 (cid:1) 10(cid:2)10 g. The average yeast protein has a mo- ing the other complementary strand will lead to zippering, Parental Daughter strands strands A G T C ▲ FIGURE 1-10 DNA consists of two complementary as templates to produce complementary strands. The outcome is strands wound around each other to form a double helix. two copies of the original double helix, each containing one of (Left)The double helix is stabilized by weak hydrogen bonds the original strands and one new daughter (complementary) between the A and T bases and between the C and G bases. strand. (Right)During replication, the two strands are unwound and used