The cell on the right, which is not dividing, contains identical by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter. ^~Bruce*>AlbertsBook => Molecular Biology of the Cell 6th Edition PDF. This is one of my favorite textbooks of all time. A really good textbook is designed to. Essential Cell Biology, Fourth Edition- Alberts, Bray, Hopkin. Alexandra Duarte. CHAPTER ONE 1 Cells: The Fundamental Units of Life What does it mean to be.
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nents of the cell may be lost or distorted during specimen preparation. The only the cell, such as a nucleus, retards light passing through it. The phase of the. Bruce Alberts received his Ph.D. from Harvard university and is professor of Council Laboratory for Molecular Cell Biology and the Biology Department at. PDF | On Jan 1, , Bruce Alberts and others published Essential Cell Biology: An Introduction to the Molecular Biology of the Cell.
We have only about six times as many genes as E. B Drawings to convey a sense of scale between living cells and atoms. On occasion, the pattern of descent may be complicated by sexual repro- duction, in which two cells of the same species fuse, pooling their DNA. A series of optical sections at different depths allows a three-dimensional image to be constructed. This small, rod-shaped cell nor- mally lives in the gut of humans and other vertebrates, but it also grows happily and reproduces rapidly in a simple nutrient broth in a culture bottle. What they saw was to them profoundly baffling—a collection of tiny and scarcely visible objects whose relation- ship to the properties of living matter seemed an impenetrable mystery. What do the cells of a rose have in common with those of a dolphin?
It is almost certain that the organelles in the plant cell that Figure 1—12 A sulfur bacterium gets its energy from H2S. Beggiatoa, a prokaryote that lives in sulfurous environments, oxidizes H2S to produce sulfur and can fix carbon even in the dark. Courtesy of Ralph W. The Eukaryotic Cell 15 perform photosynthesis—the chloroplasts—have evolved from photosyn- thetic bacteria that long ago found a home inside the cytoplasm of a plant cell ancestor. Bacteria and Archaea Traditionally, all prokaryotes have been classified together in one large group.
But molecular studies reveal that there is a gulf within the class of prokaryotes, dividing it into two distinct domains called the bacteria and the archaea. Remarkably, at a molecular level, the members of these two domains differ as much from one another as either does from the eukaryotes. Most of the prokaryotes familiar from everyday life—the spe- cies that live in the soil or make us ill—are bacteria. Archaea are found not only in these habitats, but also in environments that are too hostile for most other cells: Many of these extreme environments resemble the harsh conditions that must have existed on the primitive Earth, where living things first evolved before the atmosphere became rich in oxygen.
Some live independent lives as single-celled organisms, such as amoebae and yeasts Figure 1—13 ; others live in multicellular assemblies. All of the more complex multicellular organisms—including plants, animals, and fungi—are formed from eukaryotic cells. By definition, all eukaryotic cells have a nucleus. But possession of a nucleus goes hand-in-hand with possession of a variety of other organelles, most of which are membrane-enclosed and common to all eukaryotic organisms.
In this section, we take a look at the main organelles found in eukaryotic cells from the point of view of their func- tions, and we consider how they came to serve the roles they have in the life of the eukaryotic cell. It is enclosed within two concentric membranes that form the nuclear envelope, and it contains molecules of DNA—extremely long polymers that encode the genetic information of the organism. In the light microscope, these giant DNA molecules become visible as individual chromosomes when they become more compact before a cell divides into two daughter cells Figure 1— DNA also carries the genetic infor- mation in prokaryotic cells; these cells lack a distinct nucleus not because they lack DNA, but because they do not keep their DNA inside a nuclear envelope, segregated from the rest of the cell contents.
Figure 1—13 Yeasts are simple free-living eukaryotes. The cells shown in this micrograph belong to the species of yeast, Saccharomyces cerevisiae, used to make dough rise and turn malted barley juice into beer. As can be seen in this image, the cells reproduce by growing a bud and then dividing asymmetrically into a large mother cell and a small daughter cell; for this reason, they are called budding yeast. A This drawing of a typical animal cell shows its extensive system of membrane-enclosed organelles.
The nucleus is colored brown, the nuclear envelope is green, and the cytoplasm the interior of the cell outside the nucleus is white. B An electron micrograph of the nucleus in a mammalian cell. B, courtesy of Daniel S.
In a fluorescence microscope, they appear as worm-shaped struc- tures that often form branching networks Figure 1— When seen with an electron microscope, individual mitochondria are found to be enclosed in two separate membranes, with the inner membrane formed into folds that project into the interior of the organelle Figure 1— Microscopic examination by itself, however, gives little indication of what mitochondria do.
Their function was discovered by breaking open cells and then spinning the soup of cell fragments in a centrifuge; this nucleus nuclear envelope condensed chromosomes Figure 1—15 Chromosomes become visible when a cell is about to divide.
As a eukaryotic cell prepares to divide, its DNA molecules become progressively more compacted condensed , forming wormlike chromosomes that can be distinguished in the light microscope.
Courtesy of Conly L. This budding yeast cell, which contains a green fluorescent protein in its mitochondria, was viewed in a super-resolution confocal fluorescence microscope. In this three-dimensional image, the mitochondria are seen to form complex branched networks. From A. Egner et al.
Natl Acad. USA With permission from the National Academy of Sciences. Purified mitochondria were then tested to see what chemical processes they could perform. This revealed that mitochondria are generators of chemi- cal energy for the cell.
Without mitochondria, animals, fungi, and plants would be unable to use oxygen to extract the energy they need from the food molecules that nourish them. The process of cel- lular respiration is considered in detail in Chapter A An electron micrograph of a cross section of a mitochondrion reveals the extensive infolding of the inner membrane. B This three-dimensional representation of the arrangement of the mitochondrial membranes shows the smooth outer membrane gray and the highly convoluted inner membrane red.
C In this schematic cell, the interior space of the mitochondrion is colored orange. A, courtesy of Daniel S.
The Fundamental Units of Life Figure 1—18 Mitochondria most likely anaerobic early aerobic evolved from engulfed bacteria. It is pre-eukaryotic cell eukaryotic cell virtually certain that mitochondria originate internal from bacteria that were engulfed by an membranes nucleus ancestral pre-eukaryotic cell and survived inside it, living in symbiosis with their host.
Note that the double membrane of present- day mitochondria is thought to have been derived from the plasma membrane and outer membrane of the engulfed bacterium. Because they resemble bacteria in so many ways, they are thought to have been derived from bacteria that were engulfed by some ancestor of present-day eukaryotic cells Figure 1— This evidently created a symbiotic relationship in which the host eukaryote and the engulfed bac- terium helped one another to survive and reproduce.
Chloroplasts Capture Energy from Sunlight Chloroplasts are large, green organelles that are found only in the cells of plants and algae, not in the cells of animals or fungi.
These organelles have an even more complex structure than mitochondria: Chloroplasts carry out photosynthesis—trapping the energy of sun- light in their chlorophyll molecules and using this energy to drive the manufacture of energy-rich sugar molecules.
In the process, they release chloroplasts chlorophyll- containing membranes Figure 1—19 Chloroplasts in plant cells inner capture the energy of sunlight. B A drawing of one of the chloroplasts, showing the inner and outer membranes, as well as the highly folded system of internal membranes containing the green chlorophyll molecules that absorb A B light energy.
A, courtesy of Preeti Dahiya. The bacteria are thought to have been taken up by early eukaryotic cells that already contained mitochondria. Plant cells can then extract this stored chemical energy when they need it, by oxidizing these sugars in their mitochondria, just as animal cells do.
Chloroplasts thus enable plants to get their energy directly from sunlight. And they allow plants to produce the food molecules—and the oxygen—that mitochondria use to generate chemical energy in the form of ATP. How these organelles work together is discussed in Chapter Like mitochondria, chloroplasts contain their own DNA, reproduce by dividing in two, and are thought to have evolved from bacteria—in this case, from photosynthetic bacteria that were engulfed by an early eukaryotic cell Figure 1— Internal Membranes Create Intracellular Compartments with Different Functions Nuclei, mitochondria, and chloroplasts are not the only membrane- enclosed organelles inside eukaryotic cells.
The cytoplasm contains a profusion of other organelles that are surrounded by single membranes see Figure 1—7A. The endoplasmic reticulum ER is an irregular maze of interconnected spaces enclosed by a membrane Figure 1— It is the site where most cell-membrane components, as well as materials destined for export from the cell, are made.
This organelle is enormously enlarged in cells that are specialized for the secretion of proteins. Stacks of flattened, membrane-enclosed sacs constitute the Golgi apparatus Figure 1—22 , which modifies and packages molecules made in the ER that are destined to be either secreted from the cell or transported to another cell com- partment.
Lysosomes are small, irregularly shaped organelles in which intracellular digestion occurs, releasing nutrients from ingested food par- ticles and breaking down unwanted molecules for either recycling within the cell or excretion from the cell. Indeed, many of the large and small molecules within the cell are constantly being broken down and remade. Peroxisomes are small, membrane-enclosed vesicles that provide a safe environment for a variety of reactions in which hydrogen peroxide is used to inactivate toxic molecules.
Membranes also form many different types of small transport vesicles that ferry materials between one mem- brane-enclosed organelle and another.
All of these membrane-enclosed organelles are sketched in Figure 1—23A. The Fundamental Units of Life Figure 1—21 The endoplasmic reticulum nucleus nuclear envelope endoplasmic reticulum produces many of the components of a eukaryotic cell. A Schematic diagram of an animal cell shows the endoplasmic reticulum ER in green. B Electron micrograph of a thin section of a mammalian pancreatic cell shows a small part of the ER, of which there are vast amounts in this cell type, which is specialized for protein secretion.
Note that the ER is continuous with the membranes of the nuclear envelope. The black particles studding the particular region of the ER shown here are ribosomes, structures that translate RNAs into proteins.
B, courtesy of Lelio Orci. The exchange is mediated by transport vesicles that pinch off from the membrane of one organelle and fuse with another, like tiny soap bubbles budding from and rejoining larger bubbles. At the surface of the cell, for example, portions of the plasma membrane tuck inward and pinch off to form vesicles that carry material captured from the external medium into the cell—a process called endocytosis Figure 1— Animal cells can nuclear envelope A B membrane- Figure 1—22 The Golgi apparatus is enclosed vesicles composed of a stack of flattened discs.
A Schematic diagram of an animal cell with the Golgi apparatus colored red. B More Golgi apparatus realistic drawing of the Golgi apparatus. Some of the vesicles seen nearby have endoplasmic reticulum pinched off from the Golgi stack; others are destined to fuse with it. Only one stack is shown here, but several can be present in a cell. C Electron micrograph that shows the C Golgi apparatus from a typical animal cell. C, courtesy of Brij J.
A The peroxisome membrane-enclosed organelles, shown cytosol in different colors, are each specialized to perform a different function. B The cytoplasm that fills the space outside nuclear Golgi of these organelles is called the cytosol apparatus colored blue.
In the reverse process, called exocytosis, vesicles from inside the cell fuse with the plasma membrane and release their contents into the external medium see Figure 1—24 ; most of the hormones and signal molecules that allow cells to communicate with one another are secreted from cells by exocytosis. How membrane-enclosed organelles move proteins and other molecules from place to place inside the cell is discussed in detail in Chapter The Cytosol Is a Concentrated Aqueous Gel of Large and Small Molecules If we were to strip the plasma membrane from a eukaryotic cell and then remove all of its membrane-enclosed organelles, including the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, and so on, we would be left with the cytosol see Figure 1—23B.
In other words, the cytosol is the part of the cytoplasm that is not contained within intracellular membranes. In most cells, the cytosol is the largest single compartment. It contains a host of large and small molecules, crowded together so closely that it behaves more like a water-based gel than a liquid solution Figure 1— The Cytoskeleton Is Responsible for Directed Cell Movements plasma membrane The cytoplasm is not just a structureless soup of chemicals and organelles.
Using an electron microscope, one can see that in eukaryotic cells the cytosol is criss-crossed by long, fine filaments. Frequently, the filaments are seen to be anchored at one end to the plasma membrane or to radi- ate out from a central site adjacent to the nucleus. This system of protein filaments, called the cytoskeleton, is composed of three major filament types Figure 1— The thickest fila- Figure 1—24 Eukaryotic cells engage in continual endocytosis and exocytosis.
In dividing cells, they become reorganized into a endocytosis and secrete intracellular spectacular array that helps pull the duplicated chromosomes in opposite materials by exocytosis. The Fundamental Units of Life Figure 1—25 The cytoplasm is stuffed with directions and distribute them equally to the two daughter cells Figure organelles and a host of large and small 1— Intermediate in thickness between actin filaments and microtu- molecules.
This schematic drawing, which extends across two pages and is based bules are the intermediate filaments, which serve to strengthen the cell. Proteins are blue, mechanical strength, controls its shape, and drives and guides its move- membrane lipids are yellow, and ribosomes ments Movie 1. The panorama begins on the far left at the plasma membrane, moves Because the cytoskeleton governs the internal organization of the cell through the endoplasmic reticulum, Golgi as well as its external features, it is as necessary to a plant cell—boxed apparatus, and a mitochondrion, and ends in by a tough wall of extracellular matrix—as it is to an animal cell that on the far right in the nucleus.
Courtesy of D. In a plant cell, for example, organelles such as mitochondria are driven in a constant stream around the cell interior along cytoskeletal tracks Movie 1.
And animal cells and plant cells alike depend on the cytoskeleton to separate their internal components into two daughter cells during cell division see Figure 1— Even bacteria contain proteins that are distantly related to those of eukaryotic actin filaments and microtubules, forming filaments that play a part in prokaryotic cell division. We examine the cytoskeleton in detail in Chapter 17, discuss its role in cell division in Chapter 18, and review how it responds to signals from outside the cell in Chapter The cytoskeleton is a dynamic evolve elaborate internal membrane jungle of protein ropes that are continually being strung together and systems that allow them to import taken apart; its filaments can assemble and then disappear in a matter substances from the outside, as of minutes.
Motor proteins use the energy stored in molecules of ATP to shown in Figure 1— Figure 1—26 The cytoskeleton is a network of protein filaments that criss- crosses the cytoplasm of eukaryotic cells. The three major types of filaments can be detected using different fluorescent stains.
Shown here are A actin filaments, B microtubules, and C intermediate filaments. A few of the key discoveries are listed in Table 1—1.
In addition, Panel 1—2 sum- marizes the differences between animal, plant, and bacterial cells. Eukaryotic Cells May Have Originated as Predators Eukaryotic cells are typically 10 times the length and times the vol- ume of prokaryotic cells, although there is huge size variation within each category.
They also possess a whole collection of features—a cytoskeleton, mitochondria, and other organelles—that set them apart from bacteria and archaea. When and how eukaryotes evolved these systems remains something of a mystery. Although eukaryotes, bacteria, and archaea must have diverged from one another very early in the history of life on Earth discussed in Chapter 14 , the eukaryotes did not acquire all of their distinctive features at the same time Figure 1— According to one theory, the ancestral eukaryotic cell was a predator that fed by capturing other cells.
The nuclear compartment may have evolved to keep the DNA segregated from this physical and chemical Discuss the relative advantages hurly-burly, so as to allow more delicate and complex control of the way and disadvantages of light and the cell reads out its genetic information.
How could Such a primitive cell, witha nucleus and cytoskeleton, was most likely you best visualize a a living skin cell, b a yeast mitochondrion, c a the sort of cell that engulfed the free-living, oxygen-consuming bacte- bacterium, and d a microtubule?
This partnership is thought to have been established 1. A subset of duplicated chromosomes Figure 1—27 Microtubules help distribute the chromosomes in a dividing cell.
When a cell divides, its nuclear envelope breaks down and its DNA condenses into visible chromosomes, each of which has duplicated to form a pair of conjoined chromosomes that will ultimately be pulled apart into separate cells by microtubules.
In the transmission electron micrograph left , microtubules the microtubules are seen to radiate from foci at opposite ends of the dividing cell. Photomicrograph courtesy of Conly L. Nine years later, he sees bacteria for the first time.
In one of the first applications of these techniques, Huxley shows that muscle contains arrays of protein filaments—the first evidence of a cytoskeleton. Perutz proposes a lower-resolution structure for hemoglobin. The likely history of these endosymbiotic events is illustrated in Figure 1— That single-celled eukaryotes can prey upon and swallow other cells is borne out by the behavior of many of the free-living, actively motile nonphotosynthetic photosynthetic fungi plants animals archaea bacteria bacteria chloroplasts Figure 1—28 Where did eukaryotes mitochondria come from?
The eukaryotic, bacterial, and archaean lineages diverged from one another very early in the evolution of life TIME on Earth. Some time later, eukaryotes are bacteria anaerobic ancestral eukaryote archaea thought to have acquired mitochondria; later still, a subset of eukaryotes acquired chloroplasts. Mitochondria are essentially the same in plants, animals, and fungi, and therefore were presumably acquired before these lines diverged.
The same colors are used, however, to distinguish the organelles chromatin DNA flagellum of the cell. The animal cell drawing is based on a nuclear pore fibroblast, a cell that inhabits connective tissue cell wall and deposits extracellular matrix. A micrograph of a living fibroblast is shown in microtubule Figure 1—6A. The plant cell drawing is typical of a young leaf cell. The bacterium shown vacuole ribosomes in is rod-shaped and has a single fluid-filled cytosol flagellum for motility; note its much smaller size compare scale bars.
B Didinium is seen ingesting another ciliated protozoan, a Paramecium. It has a globular body encircled by two fringes of cilia, and its front end is flattened except for a single pro- trusion rather like a snout Figure 1—29A.
Didinium swims at high speed by means of its beating cilia.
When it encounters a suitable prey, usually another type of protozoan, it releases numerous small, paralyzing darts from its snout region. Didinium then attaches to and devours the other cell, inverting like a hollow ball to engulf its victim, which can be almost as large as itself Figure 1—29B. Not all protozoans are predators. They can be photosynthetic or carnivo- rous, motile or sedentary. Their anatomy is often elaborate and includes such structures as sensory bristles, photoreceptors, beating cilia, stalk- like appendages, mouthparts, stinging darts, and musclelike contractile bundles Figure 1— Although they are single cells, protozoans can be as intricate and versatile as many multicellular organisms.
Much remains to be learned about fundamental cell biology from studies of these fasci- nating life-forms. Thus knowledge gained from the study of one organism contributes to our understanding of others, including ourselves. But certain organisms are easier than others to study in the laboratory. Some reproduce rapidly and are convenient for genetic manipulations; others are multicellular but transparent, so that one can directly watch the development of all their internal tissues and organs.
For reasons such as these, large communi- ties of biologists have become dedicated to studying different aspects of the biology of a few chosen species, pooling their knowledge to gain a deeper understanding than could be achieved if their efforts were spread over many different species.
Although the roster of these representa- tive organisms is continually expanding, a few stand out in terms of the breadth and depth of information that has been accumulated about them over the years—knowledge that contributes to our understanding of how all cells work.
In this section, we examine some of these model organ- isms and review the benefits that each offers to the study of cell biology and, in many cases, to the promotion of human health. To see the latter in action, watch Movie 1.
From M. Sleigh, The Biology of Protozoa. Edward Arnold, With permission from Edward Arnold. Molecular Biologists Have Focused on E. This small, rod-shaped cell nor- mally lives in the gut of humans and other vertebrates, but it also grows happily and reproduces rapidly in a simple nutrient broth in a culture bottle.
Most of our knowledge of the fundamental mechanisms of life—including how cells replicate their DNA and how they decode these genetic instruc- tions to make proteins—has come from studies of E.
Subsequent research has confirmed that these basic processes occur in essentially the same way in our own cells as they do in E.
But human cells are complicated and reproduce relatively slowly. To get a handle on the fundamental biology of eukaryotic cells, it is often advantageous to study a simpler cell that reproduces more rapidly. A popular choice has been the budding yeast Saccharomyces cerevisiae Figure 1—31 —the same microorganism that is used for brew- ing beer and baking bread. Like other fungi, it has a rigid cell wall, Your next-door neighbor has is relatively immobile, and possesses mitochondria but not chloroplasts.
Yet it carries out all the basic tasks that every eukaryotic cell that her money is being spent on must perform. How could to understanding many basic mechanisms in eukaryotic cells, including you put her mind at ease? In this scanning electron micrograph, a few yeast cells are seen in the process of dividing, which they do by budding. Another micrograph of the same species is shown in Figure 1— Courtesy of Ira Herskowitz and Eric Schabatach. Darwin himself would no doubt have been stunned by this dramatic example of evolutionary conservation.
Whereas bacteria, archaea, and eukaryotes separated from each other more than 3 billion years ago, plants, animals, and fungi diverged only about 1. The close evolutionary relationship among all flowering plants means that we can gain insight into their cell and molecular biology by focusing on just a few convenient species for detailed analysis. Out of the several hundred thousand species of flowering plants on Earth today, molecular biologists have focused their efforts on a small weed, the common wall cress Arabidopsis thaliana Figure 1—32 , which can be grown indoors in large numbers: Because genes found in Arabidopsis have counterparts in agricultural species, studying this simple weed provides insights into the development and physiology of the crop plants upon which our lives depend, as well as into the evolution of all the other plant species that dominate nearly every ecosystem on Earth.
Model Animals Include Flies, Fish, Worms, and Mice Multicellular animals account for the majority of all named species of living organisms, and the majority of animal species are insects. It is fit- ting, therefore, that an insect, the small fruit fly Drosophila melanogaster Figure 1—33 , should occupy a central place in biological research.
In fact, the foundations of classical genetics were built to a large extent on studies of this insect. More than 80 years ago, genetic analysis of the fruit fly provided definitive proof that genes—the units of heredity—are car- ried on chromosomes. In more recent times, Drosophila, more than any other organism, has shown us how the genetic instructions encoded in DNA molecules direct the development of a fertilized egg cell or zygote into an adult multicellular organism containing vast numbers of different cell types organized in a precise and predictable way.
Drosophila mutants with body parts strangely misplaced or oddly patterned have provided the key to identifying and characterizing the genes that are needed to make a properly structured adult body, with gut, wings, legs, eyes, and all the other bits and pieces in their correct places.
These genes—which are copied and passed on to every cell in the body—define how each cell will behave in its social interactions with its sisters and cousins, thus controlling the structures that the cells can create.
Moreover, the genes Figure 1—32 Arabidopsis thaliana, the common wall cress, is a model plant. This small weed has become the favorite organism of plant molecular and developmental biologists. Model Organisms 29 Figure 1—33 Drosophila melanogaster is a favorite among developmental biologists and geneticists. Molecular genetic studies on this small fly have provided a key to the understanding of how all animals develop.
Courtesy of E. Thus the fly serves as a valuable model for studying human development and disease. Another widely studied organism is the nematode worm Caenorhabditis elegans Figure 1—34 , a harmless relative of the eelworms that attack the roots of crops.
Smaller and simpler than Drosophila, this creature devel- ops with clockwork precision from a fertilized egg cell into an adult that has exactly body cells plus a variable number of egg and sperm cells —an unusual degree of regularity for an animal. We now have a minutely detailed description of the sequence of events by which this occurs—as the cells divide, move, and become specialized according to strict and predictable rules. Studies of nematode development, for example, have led to a detailed molecular understand- ing of apoptosis, a form of programmed cell death by which surplus cells are disposed of in all animals—a topic of great importance for cancer research discussed in Chapters 18 and Another organism that is providing molecular insights into developmen- tal processes, particularly in vertebrates, is the zebrafish.
Because this 0. Most individuals are hermaphrodites, producing both sperm and eggs the latter of which can be seen along the underside of the animal. Courtesy of Maria Gallegos. The researchers found that one of the behavior. But how deep do these similarities between Cdc genes they had identified, called Cdc2, was required cells—and the organisms they comprise—really run?
When that gene was inactivated by a mutation, the yeast Are parts from one organism interchangeable with parts cells would not divide. And when the cells were pro- from another? Would an enzyme that breaks down glu- vided with a normal copy of the gene, their ability to cose in a bacterium be able to digest the same sugar if it reproduce was restored. Are they functionally with a functioning Cdc2 gene from the same yeast equivalent from one organism to another?
Insights have should repair the damage and enable the cell to divide come from many sources, but the most stunning and normally. But what about using a similar cell-division dramatic answer came from experiments performed on gene from a different organism? These studies, which shocked the Nurse team tackled next. Next of kin Saccharomyces cerevisiae is another kind of yeast and Division and discovery is one of a handful of model organisms biologists have chosen to study to expand their understanding of how All cells come from other cells, and the only way to cells work.
Also used to brew beer, S. To reproduce, a parent cell must execute an orderly rates from the mother cell see Figures 1—13 and 1— This critical process of division, both rely on a complex network of interacting duplication and division—known as the cell-division proteins to get the job done.
But could the proteins from cycle, or cell cycle for short—is complex and carefully one type of yeast substitute for those of the other? Defects in any of the proteins involved can be devastating to the cell.
If a gene that kept the cells from dividing when the tem- protein is essential for a given process, a mutation that perature was elevated.
And they found that some of the results in an abnormal protein—or in no protein at all— mutant S. By when warm. If spread onto a culture plate containing isolating organisms that are defective in their cell-divi- a growth medium, the rescued cells could divide again sion cycle, scientists have worked backward to discover and again to form visible colonies, each containing mil- the proteins that control progress through the cycle.
They are contained the S. Yeast cycle by Lee Hartwell and colleagues. After all, how different can one yeast be from that control the cell-division cycle—the so-called Cdc another?
A more demanding test would be to use DNA genes—and have provided a detailed understanding of from a more distant relative.
And the work. But the results clearly showed that the introduce human and yeast proteins are functionally equivalent. Together with Tim Hunt, who dis- covered a different cell-cycle protein called cyclin, Nurse and Hartwell shared a Nobel Prize for their studies of key regulators of the cell cycle.
The Nurse experiments showed that proteins from very different eukaryotes can be functionally interchange- cells that received able and suggested that the cell cycle is controlled in a functional S. Apparently, the proteins that orchestrate the cycle at the warm temperature in eukaryotes are so fundamentally important that they have been conserved almost unchanged over more than Figure 1—35 S.
DNA is collected from S. The mutant yeast cells were rescued, not by S. We discuss how DNA can be manipulated and transferred into different cell types direct injection of the human protein, but by introduc- in Chapter These yeast cells are then spread onto a plate tion of a piece of human DNA.
Thus the yeast cells could containing a suitable growth medium and are incubated at a read and use this information correctly, indicating that, warm temperature, at which the mutant Cdc2 protein is inactive. A yeast cell has all the equipment it needs to interpret the instructions encoded in a human gene and to use that information to direct the production of a fully functional human protein.
Although it may sound paradoxi- Gene reading cal, the shortest, most efficient path to improving human This result was much more surprising—even to Nurse. So it was hard to believe that these yeast. Identities between the amino acid sequences of a region of the human Cdc2 protein and a similar region of the equivalent proteins in S. Each amino acid is represented by a single letter. The Fundamental Units of Life Figure 1—37 Zebrafish are popular models for studies of vertebrate development.
A These small, hardy, tropical fish are a staple in many home aquaria. But they are also ideal for developmental studies, as their transparent embryos B make it easy to observe cells moving and changing their characters in the living organism as it develops. A, courtesy of Steve Baskauf; B, from M. Rhinn et al.
With permission from BioMed Central Ltd. Mammals are among the most complex of animals, and the mouse has long been used as the model organism in which to study mammalian A genetics, development, immunology, and cell biology. Thanks to modern 1 cm molecular biological techniques, it is now possible to breed mice with deliberately engineered mutations in any specific gene, or with artificially constructed genes introduced into them. In this way, one can test what a given gene is required for and how it functions.
Almost every human gene has a counterpart in the mouse, with a similar DNA sequence and function. Thus, this animal has proven an excellent model for studying genes that are important in both human health and disease. Like bacteria or yeast, our individual cells can be harvested and grown in culture, where we can study their biology and more closely examine the genes that govern their functions. Given the appropriate surroundings, most human cells—indeed, most cells from animals or plants—will survive, proliferate, and even express specialized properties in a culture dish.
Although not true for all types of cells, many types of cells grown in culture display the differentiated properties appropriate to their origin: Because cultured cells are maintained in a controlled environment, they are accessible to study in ways that are often not possible in vivo.
For example, cultured cells can be exposed to hormones or growth factors, and the effects that these signal molecules have on the shape or behavior of the cells can be easily explored.
In addition to studying human cells in culture, humans are also exam- ined directly in clinics. Much of the research on human biology has been driven by medical interests, and the medical database on the human spe- cies is enormous.
Although naturally occurring mutations in any given human gene are rare, the consequences of many mutations are well doc- umented. This is because humans are unique among animals in that they report and record their own genetic defects: Nevertheless, the extent of our ignorance is still daunting.
A Phase-contrast micrograph of fibroblasts a fish, or how the DNA in a human egg cell directs the development of in culture. B Micrograph of cultured a human rather than a mouse. Yet the revelations of molecular biology myoblasts, some of which have fused have made the task seem eminently approachable. As much as anything, to form multinucleate muscle cells that this new optimism has come from the realization that the genes of one spontaneously contract in culture.
Movie 1. We all have a com- muscle cell beating in culture. A, courtesy mon evolutionary origin, and under the surface it seems that we share of Daniel Zicha; B, courtesy of Rosalind the same molecular mechanisms. Flies, worms, fish, mice, and humans Zalin; C, from K. Chua et al. Natl thus provide a key to understanding how animals in general are made Acad.
We can see in present-day organisms many features that have been preserved through more than 3 billion years of life on Earth—about one-fifth of the age of the universe. This evolutionary conservatism provides the founda- tion on which the study of molecular biology is built. To set the scene for the chapters that follow, therefore, we end this chapter by considering a little more closely the family relationships and basic similarities among all living things.
This topic has been dramatically clarified in the past few years by technological advances that have allowed us to determine the complete genome sequences of thousands of organisms, including our own species as discussed in more detail in Chapter 9. Prokaryotes carry very little superfluous genetic baggage and, nucleotide- Figure 1—39 Different species share similar genes. The human baby and the mouse shown here have similar white patches on their foreheads because they both have defects in the same gene called Kit , which is required for the development and maintenance of some pigment cells.
Peter Walter Narrated by: Julie Theriot Producer: Michael Morales Interface Design: He is the editor-in-chief of Science magazine. For 12 years he served as President of the U. National Academy of Sciences Dennis Bray received his Ph.
In he was awarded the Microsoft European Science Award. Karen Hopkin received her Ph. Alexander Johnson received his Ph. Julian Lewis received his D. Martin Raff received his M. Keith Roberts received his Ph.
Peter Walter received his Ph. Ceaselessly re-engineered and diversified by evolution, extraordinarily versatile and adaptable, the cell still retains a core of complex self-replicating chemical machinery that is shared and endlessly repeated by every living organism on the face of the Earth, in every animal, every leaf, every bacterium in a piece of cheese, every yeast in a vat of wine.
We are made of cells, we feed on cells, and our world is made habitable by cells. We need to understand cell biology to understand our- selves; to look after our health; to take care of our food supplies; and to protect our endangered ecosystems.
The challenge for scientists is to deepen knowledge and find new ways to apply it. But all of us, as citizens, need to know something of the subject to grapple with the modern world, from our own health affairs to the great public issues of environmental change, biomedical technologies, agriculture, and epidemic disease. Cell biology is a big subject, and it has links with almost every other branch of science. The study of cell biology therefore provides a great scientific education.
However, it is easy to become lost in the detail and distracted by an overload of information and technical terminology. In this book we therefore focus on providing a digestible, straightforward, and engaging account of only the essential principles.
We seek to explain, in a way that can be understood even by a reader approaching modern biology for the first time, how the living cell works: