Часть 1 (B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (5th edition)), страница 5

A completetable of contentsand overviewof all electronicresourcesiscontained in the MBoCSMedia Viewing Guide,a PDF file located on the rootlevel of the DVD-ROMand in the Appendix of the media player.The DVD-ROMalso containsChapters21-25which covermulticellularsystems.The chaptersarein PDFformat and can be easilyprinted or searchedusingAdobe@Acrobat@Readeror other PDF software.TeachingSupplementsUpon request,teaching supplements for MolecularBiologt of the Cell are available to qualified instructors.MBoC1TransparencySetProvides200 frrll-color overheadacetatetransparenciesof the most importantfiguresfrom the book.MBoCSTestQuestionsA selection of test questions will be available.Written by Kirsten Benjamin(AmyrisBiotechnologies,Emeryville,California)and Linda Huang (UniversityofBoston),thesethoughtquestionswill teststudents'understandMassachusetts,ing of the chapter material.MBoCSLecture OutlinesLectureoutlines createdfrom the conceptheadsfor the text are provided.Garlnnd ScienceClasswirerMAll of the teachingsupplementson the DVD-ROM(theseinclude figuresin PowerPointand JPEGformat;Chapters2l-25 in PDFformat; 125videos,animations,and movies)and the test questionsand Iectureoutlines areavailableto qualifiedinstructorsonline at the GarlandScienceClasswire'"Web site.GarlandScienceClasswire'"offersaccessto other instructional resourcesfrom all of the GarlandSciencetextbooks,and providesfreeonline coursemanagementtools.

For addiortional information, pleasevisit http://www.classwire.com/garlandscienceInc.)(ClasswireofChalKree,is a trademarke-mail science@garland.com.Adobe and Acrobat are either registeredtrademarks or trademarks of Adobe SystemsIncorporatedin the United Statesandlor other countriesPowerPoint is either a registeredtrademark or trademark of Microsoft Corporation in the UnitedStatesand/or other countriesINTRODUCTIONTOTHECELLChapter 1Cells and GenomesThe surface of our planet is populated by living things—curious, intricatelyorganized chemical factories that take in matter from their surroundings anduse these raw materials to generate copies of themselves. The living organismsappear extraordinarily diverse.

What could be more different than a tiger and apiece of seaweed, or a bacterium and a tree? Yet our ancestors, knowing nothingof cells or DNA, saw that all these things had something in common. They calledthat something “life,” marveled at it, struggled to define it, and despaired ofexplaining what it was or how it worked in terms that relate to nonliving matter.The discoveries of the past century have not diminished the marvel—quitethe contrary.

But they have lifted away the mystery as to the nature of life. We cannow see that all living things are made of cells, and that these units of living matter all share the same machinery for their most basic functions. Living things,though infinitely varied when viewed from the outside, are fundamentally similar inside. The whole of biology is a counterpoint between the two themes:astonishing variety in individual particulars; astonishing constancy in fundamental mechanisms. In this first chapter we begin by outlining the universal features common to all life on our planet. We then survey, briefly, the diversity ofcells. And we see how, thanks to the common code in which the specificationsfor all living organisms are written, it is possible to read, measure, and decipherthese specifications to achieve a coherent understanding of all the forms of life,from the smallest to the greatest.1In This ChapterTHE UNIVERSAL FEATURESOF CELLS ON EARTH1THE DIVERSITY OFGENOMES AND THE TREEOF LIFE11GENETIC INFORMATIONIN EUCARYOTES26THE UNIVERSAL FEATURES OF CELLS ON EARTHIt is estimated that there are more than 10 million—perhaps 100 million—livingspecies on Earth today.

Each species is different, and each reproduces itselffaithfully, yielding progeny that belong to the same species: the parent organismhands down information specifying, in extraordinary detail, the characteristicsthat the offspring shall have. This phenomenon of heredity is central to the definition of life: it distinguishes life from other processes, such as the growth of acrystal, or the burning of a candle, or the formation of waves on water, in whichorderly structures are generated but without the same type of link between thepeculiarities of parents and the peculiarities of offspring.

Like the candleflame, the living organism consumes free energy to create and maintain itsorganization; but the free energy drives a hugely complex system of chemicalprocesses that is specified by the hereditary information.Most living organisms are single cells; others, such as ourselves, are vastmulticellular cities in which groups of cells perform specialized functions andare linked by intricate systems of communication. But in all cases, whether wediscuss the solitary bacterium or the aggregate of more than 1013 cells that forma human body, the whole organism has been generated by cell divisions from asingle cell.

The single cell, therefore, is the vehicle for the hereditary informationthat defines the species (Figure 1–1). And specified by this information, the cellincludes the machinery to gather raw materials from the environment, and toconstruct out of them a new cell in its own image, complete with a new copy ofthe hereditary information. Nothing less than a cell has this capability.12Chapter 1: Cells and Genomes(A)(B)(E)(C)50 mm50 mm100 mm(D)(F)Figure 1–1 The hereditary information in the fertilized egg cell determines the nature of the whole multicellularorganism.

(A and B) A sea urchin egg gives rise to a sea urchin. (C and D) A mouse egg gives rise to a mouse. (E and F) Anegg of the seaweed Fucus gives rise to a Fucus seaweed. (A, courtesy of David McClay; B, courtesy of M. Gibbs, OxfordScientific Films; C, courtesy of Patricia Calarco, from G. Martin, Science 209:768–776, 1980. With permission from AAAS;D, courtesy of O. Newman, Oxford Scientific Films; E and F, courtesy of Colin Brownlee.)All Cells Store Their Hereditary Information in the Same LinearChemical Code (DNA)Computers have made us familiar with the concept of information as a measurable quantity—a million bytes (to record a few hundred pages of text or an imagefrom a digital camera), 600 million for the music on a CD, and so on.

They havealso made us well aware that the same information can be recorded in many different physical forms. As the computer world has evolved, the discs and tapesthat we used 10 years ago for our electronic archives have become unreadable onpresent-day machines.

Living cells, like computers, deal in information, and it isestimated that they have been evolving and diversifying for over 3.5 billion years.It is scarcely to be expected that they should all store their information in thesame form, or that the archives of one type of cell should be readable by the information-handling machinery of another. And yet it is so. All living cells on Earth,without any known exception, store their hereditary information in the form ofdouble-stranded molecules of DNA—long unbranched paired polymer chains,formed always of the same four types of monomers. These monomers have nicknames drawn from a four-letter alphabet—A, T, C, G—and they are strungtogether in a long linear sequence that encodes the genetic information, just asthe sequence of 1s and 0s encodes the information in a computer file.

We can takea piece of DNA from a human cell and insert it into a bacterium, or a piece of bacterial DNA and insert it into a human cell, and the information will be successfully read, interpreted, and copied. Using chemical methods, scientists can readout the complete sequence of monomers in any DNA molecule—extending formillions of nucleotides—and thereby decipher the hereditary information thateach organism contains.THE UNIVERSAL FEATURES OF CELLS ON EARTH3All Cells Replicate Their Hereditary Information by TemplatedPolymerizationThe mechanisms that make life possible depend on the structure of the doublestranded DNA molecule.

Each monomer in a single DNA strand—that is, eachnucleotide—consists of two parts: a sugar (deoxyribose) with a phosphategroup attached to it, and a base, which may be either adenine (A), guanine (G),cytosine (C) or thymine (T) (Figure 1–2). Each sugar is linked to the next via thephosphate group, creating a polymer chain composed of a repetitive sugarphosphate backbone with a series of bases protruding from it. The DNA polymeris extended by adding monomers at one end.

For a single isolated strand, thesecan, in principle, be added in any order, because each one links to the next in thesame way, through the part of the molecule that is the same for all of them. Inthe living cell, however, DNA is not synthesized as a free strand in isolation, buton a template formed by a preexisting DNA strand. The bases protruding fromthe existing strand bind to bases of the strand being synthesized, according to astrict rule defined by the complementary structures of the bases: A binds to T,and C binds to G.