H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 3

Yeasts and molds, which collectively constitute the fungi, have an important ecological role in breaking down plant and animal remains for reuse. They also(a)Mating between haploid1 cells of opposite matingtypeαa2Vegetative growthof diploid cellsDiploid cells (a/α)Bud5Vegetativegrowthof haploidcells4make numerous antibiotics and are used in the manufactureof bread, beer, wine, and cheese. Not so pleasant are fungaldiseases, which range from relatively innocuous skin infections, such as jock itch and athlete’s foot, to life-threateningPneumocystis carinii pneumonia, a common cause of deathamong AIDS patients.Even Single Cells Can Have SexThe common yeast used to make bread and beer, Saccharomyces cerevisiae, appears fairly frequently in this book because it has proven to be a great experimental organism.

Likemany other unicellular organisms, yeasts have two matingtypes that are conceptually like the male and female gametes(eggs and sperm) of higher organisms. Two yeast cells of opposite mating type can fuse, or mate, to produce a third celltype containing the genetic material from each cell (Figure1-5). Such sexual life cycles allow more rapid changes in genetic inheritance than would be possible without sex, resulting in valuable adaptations while quickly eliminatingdetrimental mutations. That, and not just Hollywood, isprobably why sex is so ubiquitous.Viruses Are the Ultimate ParasitesFour haploidascosporeswithin ascusAscus ruptures,spores germinateStarvation causes3 ascus formation,meiosis(b)Budding (S.

cerevisiae)▲ FIGURE 1-5 The yeast Saccharomyces cerevisiaereproduces sexually and asexually. (a) Two cells that differ inmating type, called a and , can mate to form an a/ cell 1 .The a and cells are haploid, meaning they contain a single copyof each yeast chromosome, half the usual number. Mating yieldsa diploid a/ cell containing two copies of each chromosome.During vegetative growth, diploid cells multiply by mitoticbudding, an asexual process 2 . Under starvation conditions,diploid cells undergo meiosis, a special type of cell division, toform haploid ascospores 3 .

Rupture of an ascus releases fourhaploid spores, which can germinate into haploid cells 4 . Thesealso can multiply asexually 5 . (b) Scanning electron micrographof budding yeast cells. After each bud breaks free, a scar is leftat the budding site so the number of previous buds can becounted. The orange cells are bacteria. [Part (b) M. Abbey/VisualsUnlimited, Inc.]Virus-caused diseases are numerous and all too familiar:chicken pox, influenza, some types of pneumonia, polio,measles, rabies, hepatitis, the common cold, and many others.

Smallpox, once a worldwide scourge, was eradicated bya decade-long global immunization effort beginning in themid-1960s. Viral infections in plants (e.g., dwarf mosaicvirus in corn) have a major economic impact on crop production. Planting of virus-resistant varieties, developed bytraditional breeding methods and more recently by geneticengineering techniques, can reduce crop losses significantly.Most viruses have a rather limited host range, infecting certain 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 mustinfect a host cell and take over its internal machinery to synthesize viral proteins and in some cases to replicate the viralgenetic material.

When newly made viruses are released, thecycle starts anew. Viruses are much smaller than cells, on theorder of 100 nanometer (nm) in diameter; in comparison,bacterial cells are usually 1000 nm (1 nm109 meters). Avirus is typically composed of a protein coat that encloses acore containing the genetic material, which carries the information for producing more viruses (Chapter 4). The coatprotects a virus from the environment and allows it to stickto, or enter, specific host cells. In some viruses, the proteincoat is surrounded by an outer membrane-like envelope.The ability of viruses to transport genetic material intocells and tissues represents a medical menace and a medicalopportunity.

Viral infections can be devastatingly destructive,causing cells to break open and tissues to fall apart. However,many methods for manipulating cells depend upon using71.1 • The Diversity and Commonality of Cells(a) T4 bacteriophage(b) Tobacco mosaic virus50 nm(c) Adenovirus100 nm50 nm▲ FIGURE 1-6 Viruses must infect a host cell to grow andreproduce. These electron micrographs illustrate some of thestructural variety exhibited by viruses. (a) T4 bacteriophage(bracket) attaches to a bacterial cell via a tail structure.

Virusesthat infect bacteria are called bacteriophages, or simply phages.(b) Tobacco mosaic virus causes a mottling of the leaves ofinfected tobacco plants and stunts their growth. (c) Adenoviruscauses eye and respiratory tract infections in humans. This virushas an outer membranous envelope from which longglycoprotein spikes protrude. [Part (a) from A. Levine, 1991, Viruses,viruses to convey genetic material into cells. To do this, theportion of the viral genetic material that is potentially harmful is replaced with other genetic material, including humangenes. The altered viruses, or vectors, still can enter cells toting the introduced genes with them (Chapter 9). One day, diseases caused by defective genes may be treated by using viralvectors to introduce a normal copy of a defective gene intopatients. Current research is dedicated to overcoming the considerable obstacles to this approach, such as getting the introduced genes to work at the right places and times.sues, organs, and appendages.

Our two hands have the samekinds of cells, yet their different arrangements—in a mirrorimage—are critical for function. In addition, many cells exhibit distinct functional and/or structural asymmetries, aproperty often called polarity. From such polarized cells ariseScientific American Library, p. 20. Part (b) courtesy of R. C. Valentine.Part (c) courtesy of Robley C.

Williams, University of California.] FIGURE 1-7 The first(a)(b)(c)[Claude Edelmann/PhotoResearchers, Inc.]MEDIA CONNECTIONSIn 1827, German physician Karl von Baer discovered thatmammals grow from eggs that come from the mother’sovary. Fertilization of an egg by a sperm cell yields a zygote,a visually unimpressive cell 200 m in diameter. Everyhuman being begins as a zygote, which houses all the necessary instructions for building the human body containingabout 100 trillion (1014) cells, an amazing feat. Developmentbegins with the fertilized egg cell dividing into two, four, theneight cells, forming the very early embryo (Figure 1-7).

Continued cell proliferation and then differentiation into distinctcell types gives rise to every tissue in the body. One initialcell, the fertilized egg (zygote), generates hundreds of different kinds of cells that differ in contents, shape, size, color,mobility, and surface composition. We will see how genesand signals control cell diversification in Chapters 15 and 22.Making different kinds of cells—muscle, skin, bone, neuron, blood cells—is not enough to produce the human body.The cells must be properly arranged and organized into tis-Video: Early Embryonic DevelopmentWe Develop from a Single Cellfew cell divisions of afertilized egg set thestage for all subsequentdevelopment. A developingmouse embryo is shown at(a) the two-cell, (b) four-cell,and (c) eight-cell stages.The embryo is surroundedby supporting membranes.The corresponding stepsin human developmentoccur during the first fewdays after fertilization.8CHAPTER 1 • Life Begins with Cellsasymmetric, polarized tissues such as the lining of the intestines and structures like hands and hearts.

The features thatmake some cells polarized, and how they arise, also are covered in later chapters.Stem Cells, Cloning, and Related TechniquesOffer Exciting Possibilities but RaiseSome ConcernsIdentical twins occur naturally when the mass of cells composing an early embryo divides into two parts, each of whichdevelops and grows into an individual animal. Each cell inan eight-cell-stage mouse embryo has the potential to giverise to any part of the entire animal. Cells with this capability are referred to as embryonic stem (ES) cells. As we learnin Chapter 22, ES cells can be grown in the laboratory (cultured) and will develop into various types of differentiatedcells under appropriate conditions.The ability to make and manipulate mammalian embryosin the laboratory has led to new medical opportunities aswell as various social and ethical concerns.

In vitro fertilization, for instance, has allowed many otherwise infertile couples to have children. A new technique involves extraction ofnuclei from defective sperm incapable of normally fertilizing an egg, injection of the nuclei into eggs, and implantationof the resulting fertilized eggs into the mother.In recent years, nuclei taken from cells of adult animalshave been used to produce new animals. In this procedure,the nucleus is removed from a body cell (e.g., skin or bloodcell) of a donor animal and introduced into an unfertilizedmammalian egg that has been deprived of its own nucleus.This manipulated egg, which is equivalent to a fertilized egg,is then implanted into a foster mother. The ability of such adonor nucleus to direct the development of an entire animalsuggests that all the information required for life is retainedin the nuclei of some adult cells.

Since all the cells in an animal produced in this way have the genes of the single original donor cell, the new animal is a clone of the donor (Figure1-8). Repeating the process can give rise to many clones. Sofar, however, the majority of embryos produced by this technique of nuclear-transfer cloning do not survive due to birthdefects. Even those animals that are born live have shownabnormalities, including accelerated aging.

The “rooting”of plants, in contrast, is a type of cloning that is readily accomplished by gardeners, farmers, and laboratory technicians.The technical difficulties and possible hazards of nucleartransfer cloning have not deterred some individuals from pursuing the goal of human cloning. However, cloning ofhumans per se has very limited scientific interest and is opposed by most scientists because of its high risk. Of greaterscientific and medical interest is the ability to generate specificcell types starting from embryonic or adult stem cells. The scientific interest comes from learning the signals that can unleash the potential of the genes to form a certain cell type.

Themedical interest comes from the possibility of treating the nu-▲ FIGURE 1-8 Five genetically identical cloned sheep. Anearly sheep embryo was divided into five groups of cells andeach was separately implanted into a surrogate mother, muchlike the natural process of twinning. At an early stage the cellsare able to adjust and form an entire animal; later in developmentthe cells become progressively restricted and can no longer doso. An alternative way to clone animals is to replace the nuclei ofmultiple single-celled embryos with donor nuclei from cells of anadult sheep.