Часть 1 (B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (5th edition)), страница 8

If a cell is to grow andreproduce, it must be able to import raw materials and export waste across itsplasma membrane. All cells therefore have specialized proteins embedded intheir membrane that transport specific molecules from one side to the other(Figure 1–13). Some of these membrane transport proteins, like some of the proteins that catalyze the fundamental small-molecule reactions inside the cell,Figure 1–11 Gene regulation by protein binding to regulatory DNA.(A) A diagram of a small portion of the genome of the bacterium Escherichiacoli, containing genes (called LacI, LacZ, LacY, and LacA) coding for fourdifferent proteins.

The protein-coding DNA segments (red) have regulatoryand other noncoding DNA segments (yellow) between them. (B) An electronmicrograph of DNA from this region, with a protein molecule (encoded bythe LacI gene) bound to the regulatory segment; this protein controls therate of transcription of the LacZ, LacY, and LacA genes. (C) A drawing of thestructures shown in (B). (B, courtesy of Jack Griffith.)site of proteinbinding shownin micrograph (B)belowLacILacZnoncodingDNA segmentsLacY LacA2000 nucleotidepairs(A)(B)protein bound toregulatory segmentof DNA(C)segment of DNAcoding for protein10Chapter 1: Cells and Genomeshave been so well preserved over the course of evolution that we can recognizethe family resemblances between them in comparisons of even the most distantly related groups of living organisms.The transport proteins in the membrane largely determine which moleculesenter the cell, and the catalytic proteins inside the cell determine the reactionsthat those molecules undergo.

Thus, by specifying the proteins that the cell is tomanufacture, the genetic information recorded in the DNA sequence dictatesthe entire chemistry of the cell; and not only its chemistry, but also its form andits behavior, for these too are chiefly constructed and controlled by the cell’s proteins.phospholipidmonolayerOILphospholipidbilayerA Living Cell Can Exist with Fewer Than 500 GenesThe basic principles of biological information transfer are simple enough, buthow complex are real living cells? In particular, what are the minimum requirements? We can get a rough indication by considering a species that has one ofthe smallest known genomes—the bacterium Mycoplasma genitalium (Figure1–14).

This organism lives as a parasite in mammals, and its environment provides it with many of its small molecules ready-made. Nevertheless, it still has tomake all the large molecules—DNA, RNAs, and proteins—required for the basicprocesses of heredity. It has only about 480 genes in its genome of 580,070nucleotide pairs, representing 145,018 bytes of information—about as much asit takes to record the text of one chapter of this book. Cell biology may be complicated, but it is not impossibly so.The minimum number of genes for a viable cell in today’s environments isprobably not less than 200–300, although there are only about 60 genes in thecore set shared by all living species without any known exception.H+plasma membraneOUTSIDEINSIDEsugars(13)(A)aminoacids,peptides,amines(14)ions(16)other(3)(B)Figure 1–13 Membrane transport proteins.

(A) Structure of a molecule ofbacteriorhodopsin, from the archaeon (archaebacterium) Halobacteriumhalobium. This transport protein uses the energy of absorbed light topump protons (H+ ions) out of the cell. The polypeptide chain threads toand fro across the membrane; in several regions it is twisted into a helicalconformation, and the helical segments are arranged to form the walls of achannel through which ions are transported.

(B) Diagram of the set oftransport proteins found in the membrane of the bacterium Thermotogamaritima. The numbers in parentheses refer to the number of differentmembrane transport proteins of each type. Most of the proteins withineach class are evolutionarily related to one another and to their counterparts in other species.WATERFigure 1–12 Formation of a membraneby amphiphilic phospholipid molecules.These have a hydrophilic (water-loving,phosphate) head group and ahydrophobic (water-avoiding,hydrocarbon) tail.

At an interfacebetween oil and water, they arrangethemselves as a single sheet with theirhead groups facing the water and theirtail groups facing the oil. When immersedin water, they aggregate to form bilayersenclosing aqueous compartments.THE DIVERSITY OF GENOMES AND THE TREE OF LIFEFigure 1–14 Mycoplasma genitalium. (A) Scanning electron micrographshowing the irregular shape of this small bacterium, reflecting the lack ofany rigid wall. (B) Cross section (transmission electron micrograph) of aMycoplasma cell. Of the 477 genes of Mycoplasma genitalium, 37 code fortransfer, ribosomal, and other nonmessenger RNAs.

Functions are known,or can be guessed, for 297 of the genes coding for protein: of these,153 are involved in replication, transcription, translation, and relatedprocesses involving DNA, RNA, and protein; 29 in the membrane andsurface structures of the cell; 33 in the transport of nutrients and othermolecules across the membrane; 71 in energy conversion and thesynthesis and degradation of small molecules; and 11 in the regulation ofcell division and other processes. (A, from S.

Razin et al., Infect. Immun.30:538–546, 1980. With permission from the American Society forMicrobiology; B, courtesy of Roger Cole, in Medical Microbiology, 4th ed.[S. Baron ed.]. Galveston: University of Texas Medical Branch, 1996.)11(A)5 mmSummaryLiving organisms reproduce themselves by transmitting genetic information to theirprogeny. The individual cell is the minimal self-reproducing unit, and is the vehicle fortransmission of the genetic information in all living species. Every cell on our planetstores its genetic information in the same chemical form—as double-stranded DNA. Thecell replicates its information by separating the paired DNA strands and using each asa template for polymerization to make a new DNA strand with a complementarysequence of nucleotides.

The same strategy of templated polymerization is used totranscribe portions of the information from DNA into molecules of the closely relatedpolymer, RNA. These in turn guide the synthesis of protein molecules by the more complex machinery of translation, involving a large multimolecular machine, the ribosome, which is itself composed of RNA and protein. Proteins are the principal catalystsfor almost all the chemical reactions in the cell; their other functions include the selective import and export of small molecules across the plasma membrane that forms thecell’s boundary. The specific function of each protein depends on its amino acidsequence, which is specified by the nucleotide sequence of a corresponding segment ofthe DNA—the gene that codes for that protein.

In this way, the genome of the cell determines its chemistry; and the chemistry of every living cell is fundamentally similar,because it must provide for the synthesis of DNA, RNA, and protein. The simplestknown cells have just under 500 genes.THE DIVERSITY OF GENOMES AND THE TREEOF LIFEThe success of living organisms based on DNA, RNA, and protein, out of theinfinitude of other chemical forms that we might conceive of, has been spectacular. They have populated the oceans, covered the land, infiltrated the Earth’scrust, and molded the surface of our planet.

Our oxygen-rich atmosphere, thedeposits of coal and oil, the layers of iron ores, the cliffs of chalk and limestoneand marble—all these are products, directly or indirectly, of past biologicalactivity on Earth.Living things are not confined to the familiar temperate realm of land, water,and sunlight inhabited by plants and plant-eating animals. They can be found inthe darkest depths of the ocean, in hot volcanic mud, in pools beneath thefrozen surface of the Antarctic, and buried kilometers deep in the Earth’s crust.The creatures that live in these extreme environments are generally unfamiliar,not only because they are inaccessible, but also because they are mostly microscopic.

In more homely habitats, too, most organisms are too small for us to seewithout special equipment: they tend to go unnoticed, unless they cause a disease or rot the timbers of our houses. Yet microorganisms make up most of the(B)0.2 mm12Chapter 1: Cells and Genomestotal mass of living matter on our planet. Only recently, through new methods ofmolecular analysis and specifically through the analysis of DNA sequences, havewe begun to get a picture of life on Earth that is not grossly distorted by ourbiased perspective as large animals living on dry land.In this section we consider the diversity of organisms and the relationshipsamong them.