B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition), страница 8

These living organisms appearextraordinarily diverse. What could be more different than a tiger and a piece ofseaweed, or a bacterium and a tree? Yet our ancestors, knowing nothing of cells orDNA, saw that all these things had something in common. They called that something “life,” marveled at it, struggled to define it, and despaired of explaining whatit was or how it worked in terms that relate to nonliving matter.The discoveries of the past century have not diminished the marvel—quite thecontrary.

But they have removed the central mystery regarding the nature of life.We can now see that all living things are made of cells: small, membrane-enclosedunits filled with a concentrated aqueous solution of chemicals and endowed withthe extraordinary ability to create copies of themselves by growing and then dividing in two.Because cells are the fundamental units of life, it is to cell biology—the studyof the structure, function, and behavior of cells—that we must look for answersto the questions of what life is and how it works. With a deeper understanding ofcells and their evolution, we can begin to tackle the grand historical problems oflife on Earth: its mysterious origins, its stunning diversity, and its invasion of everyconceivable habitat.

Indeed, as emphasized long ago by the pioneering cell biologist E. B. Wilson, “the key to every biological problem must finally be sought in thecell; for every living organism is, or at some time has been, a cell.”Despite their apparent diversity, living things are fundamentally similar inside.The whole of biology is thus a counterpoint between two themes: astonishingvariety in individual particulars; astonishing constancy in fundamental mechanisms. In this first chapter, we begin by outlining the universal features commonto all life on our planet. We then survey, briefly, the diversity of cells.

And we seehow, thanks to the common molecular code in which the specifications for allliving organisms are written, it is possible to read, measure, and decipher thesespecifications to help us achieve a coherent understanding of all the forms of life,from the smallest to the greatest.1IN THIS CHAPTERTHE UNIVERSAL FEATURES OFCELLS ON EARTHTHE DIVERSITY OF GENOMESAND THE TREE OF LIFEGENETIC INFORMATION INEUKARYOTES2Chapter 1: Cells and Genomes(A)(B)(C)100 µm(E)50 µm(D)THE 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 itself faithfully, yielding progeny that belong to the same species: the parent organism handsdown information specifying, in extraordinary detail, the characteristics that theoffspring shall have.

This phenomenon of heredity is central to the definition oflife: it distinguishes life from other processes, such as the growth of a crystal, or theburning of a candle, or the formation of waves on water, in which orderly structures are generated but without the same typeMBoC5of link1.01/1.01between the peculiarities ofparents and the peculiarities of offspring. Like the candle flame, the living organism must consume free energy to create and maintain its organization.

But lifeemploys the free energy to drive a hugely complex system of chemical processesthat are specified by hereditary information.Most living organisms are single cells. Others, such as ourselves, are vast multicellular cities in which groups of cells perform specialized functions linked byintricate systems of communication. But even for the aggregate of more than 1013cells that form a human body, the whole organism has been generated by celldivisions from a single cell.

The single cell, therefore, is the vehicle for all of thehereditary information that defines each species (Figure 1–1). This cell includesthe machinery to gather raw materials from the environment and to constructfrom them a new cell in its own image, complete with a new copy of its hereditaryinformation. Each and every cell is truly amazing.All Cells Store Their Hereditary Information in the Same LinearChemical Code: DNAComputers 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 bytes for the music on a CD, and so on. Computers have also made us well aware that the same information can be recordedin many different physical forms: the discs and tapes that we used 20 years ago forour electronic archives have become unreadable on present-day machines. Living50 µm(F)Figure 1–1 The hereditary informationin the fertilized egg cell determinesthe nature of the whole multicellularorganism.

Although their starting cellslook superficially similar, as indicated: asea urchin egg gives rise to a sea urchin(A and B). A mouse egg gives rise to amouse (C and D). An egg of the seaweedFucus gives rise to a Fucus seaweed(E and F). (A, courtesy of David McClay;B, courtesy of M. Gibbs, Oxford ScientificFilms; C, courtesy of Patricia Calarco, fromG. Martin, Science 209:768–776, 1980.With permission from AAAS; D, courtesy ofO. Newman, Oxford Scientific Films; E andF, courtesy of Colin Brownlee.)THE UNIVERSAL FEATURES OF CELLS ON EARTH(A)building block of DNA3(D)double-stranded DNAphosphatesugar+sugarphosphate(B)GGbasenucleotideATTGCCAGTGTAACGGTCADNA strandGTAACGGTsugar-phosphatebackboneAC(E)(C)Chydrogen-bondedbase pairsDNA double helixtemplated polymerization of new strandnucleotidemonomersCCATTGTAAGCGGTCACGGCATAGGCAGTAcells, like computers, store information, and it is estimated that they have beenevolving and diversifying for over 3.5 billion years.

It is scarcely to be expectedthat they would all store their information in the same form, or that the archivesof one type of cell should be readable by the information-handling machinery ofanother. And yet it is so. All living cells on Earth store their hereditary information in the form of double-stranded molecules of DNA—long, unbranched, pairedpolymer chains, formed always of the same four types of monomers. These monomers, chemical compounds known as nucleotides, have nicknames drawn froma four-letter alphabet—A, T, C, G—and they are strung together in a long linearsequence that encodes the genetic information, just as the sequence of 1s and 0sencodes the information in a computer file.

We can take a piece of DNA from ahuman cell and insert it into a bacterium, or a piece of bacterial DNA and insertit into a human cell, and the information will be successfully read, interpreted,and copied. Using chemical methods, scientists have learned how to read out thecomplete sequence of monomers in any DNA molecule—extending for many milm1.02/1.02lions of nucleotides—and thereby decipher all of theMBoC6hereditaryinformation thateach organism contains.All 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 phosphate groupattached 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 the phosphate group, creating a polymer chain composed of a repetitive sugar-phosphatebackbone with a series of bases protruding from it. The DNA polymer is extendedby adding monomers at one end. For a single isolated strand, these monomerscan, 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. In theliving cell, however, DNA is not synthesized as a free strand in isolation, but ona template formed by a preexisting DNA strand.

The bases protruding from theTCGACCAFigure 1–2 DNA and its building blocks.(A) DNA is made from simple subunits,called nucleotides, each consisting of asugar-phosphate molecule with a nitrogencontaining side group, or base, attached to it.The bases are of four types (adenine, guanine,cytosine, and thymine), corresponding tofour distinct nucleotides, labeled A, G, C,and T.

(B) A single strand of DNA consistsof nucleotides joined together by sugarphosphate linkages. Note that the individualsugar-phosphate units are asymmetric,giving the backbone of the strand a definitedirectionality, or polarity. This directionalityguides the molecular processes by which theinformation in DNA is interpreted and copiedin cells: the information is always “read” ina consistent order, just as written Englishtext is read from left to right.

(C) Throughtemplated polymerization, the sequence ofnucleotides in an existing DNA strand controlsthe sequence in which nucleotides are joinedtogether in a new DNA strand; T in onestrand pairs with A in the other, and G in onestrand with C in the other. The new strandhas a nucleotide sequence complementaryto that of the old strand, and a backbonewith opposite directionality: correspondingto the GTAA... of the original strand, it has...TTAC. (D) A normal DNA molecule consistsof two such complementary strands.

Thenucleotides within each strand are linkedby strong (covalent) chemical bonds; thecomplementary nucleotides on oppositestrands are held together more weakly, byhydrogen bonds. (E) The two strands twistaround each other to form a double helix—arobust structure that can accommodate anysequence of nucleotides without altering itsbasic structure (see Movie 4.1).4Chapter 1: Cells and Genomestemplate strandnew strandFigure 1–3 The copying of geneticinformation by DNA replication. In thisprocess, the two strands of a DNA doublehelix are pulled apart, and each servesas a template for synthesis of a newcomplementary strand.new strandparent DNA double helixtemplate strandexisting strand bind to bases of the strand being synthesized, according to a strictrule defined by the complementary structures of the bases: A binds to T, and Cbinds to G.