B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 17
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(B) A Didinium engulfing itsprey. Didinium normally swims around in the water at high speed by means of the synchronousbeating of its cilia. When it encounters a suitable prey (yellow), usually another type of protozoan, itreleases numerous small paralyzing darts from its snout region.
Then, the Didinium attaches to anddevours the other cell by phagocytosis, inverting like a hollow ball to engulf its victim, which can bealmost as large as itself. (Courtesy of D. Barlow.)MBoC6 m1.32/1.2726Chapter 1: Cells and Genomes(B)(C)(A)100 nmshelter and nourishment in return for the power generation they performed fortheir hosts. This partnership between a primitive anaerobic predator cell and anaerobic bacterial cell is thought to have been established about 1.5 billion yearsago, when the Earth’s atmosphere first became rich in oxygen.As indicated in Figure 1–29, recent genomic analyses suggest that the firsteukaryotic cells formed after an archaeal cell engulfedanm1.33/1.28aerobic bacterium.
ThisMBoC6would explain why all eukaryotic cells today, including those that live as strictanaerobes show clear evidence that they once contained mitochondria.Many eukaryotic cells—specifically, those of plants and algae—also containanother class of small membrane-enclosed organelles somewhat similar to mitochondria—the chloroplasts (Figure 1–30).
Chloroplasts perform photosynthesis,using the energy of sunlight to synthesize carbohydrates from atmospheric carbon dioxide and water, and deliver the products to the host cell as food. Like mitochondria, chloroplasts have their own genome. They almost certainly originatedas symbiotic photosynthetic bacteria, acquired by eukaryotic cells that alreadypossessed mitochondria (Figure 1–31).A eukaryotic cell equipped with chloroplasts has no need to chase after othercells as prey; it is nourished by the captive chloroplasts it has inherited from itsancestors. Correspondingly, plant cells, although they possess the cytoskeletal equipment for movement, have lost the ability to change shape rapidly andto engulf other cells by phagocytosis. Instead, they create around themselves atough, protective cell wall.
If the first eukaryotic cells were predators on otherorganisms, we can view plant cells as cells that have made the transition fromhunting to farming.Fungi represent yet another eukaryotic way of life. Fungal cells, like animalcells, possess mitochondria but not chloroplasts; but in contrast with animal cellsand protozoa, they have a tough outer wall that limits their ability to move rapidlyFigure 1–28 A mitochondrion. (A) A crosssection, as seen in the electron microscope.(B) A drawing of a mitochondrion withpart of it cut away to show the threedimensional structure (Movie 1.2). (C) Aschematic eukaryotic cell, with the interiorspace of a mitochondrion, containing themitochondrial DNA and ribosomes, colored.Note the smooth outer membrane and theconvoluted inner membrane, which housesthe proteins that generate ATP from theoxidation of food molecules.
(A, courtesyof Daniel S. Friend.)GENETIC INFORMATION IN EUKARYOTES27anaerobic cell derivedfrom an archaeonearly aerobiceukaryotic cellprimitivenucleusFigure 1–29 The origin of mitochondria.An ancestral anaerobic predator cell (anarchaeon) is thought to have engulfed thebacterial ancestor of mitochondria, initiatingnucleus a symbiotic relationship. Clear evidence ofa dual bacterial and archaeal inheritancecan be discerned today in the genomes ofall eukaryotes.internalmembranesbacterial outer membraneloss of membranederived fromarchaeal cellbacterial plasmamembranemitochondria withdouble membranesaerobic bacteriumor to swallow up other cells.
Fungi, it seems, have turned from hunters into scavengers: other cells secrete nutrient molecules or release them upon death, andfungi feed on these leavings—performing whatever digestion is necessary extracellularly, by secreting digestive enzymes to the exterior.Eukaryotes Have Hybrid GenomesThe genetic information of eukaryotic cells has a hybrid origin—from the ancestral anaerobic archaeal cell, and from the bacteria that it adopted as symbionts.MBoC6inm1.34/1.29Most of this information is storedthe nucleus, but a small amount remainsinside the mitochondria and, for plant and algal cells, in the chloroplasts.
Whenmitochondrial DNA and the chloroplast DNA are separated from the nuclear DNAand individually analyzed and sequenced, the mitochondrial and chloroplastgenomes are found to be degenerate, cut-down versions of the correspondingbacterial genomes. In a human cell, for example, the mitochondrial genome consists of only 16,569 nucleotide pairs, and codes for only 13 proteins, 2 ribosomalRNA components, and 22 transfer RNAs.chloroplastschlorophyllcontainingmembranesinnermembraneoutermembrane(A)10 µm(B)Figure 1–30 Chloroplasts. Theseorganelles capture the energy of sunlightin plant cells and some single-celledeukaryotes. (A) A single cell isolatedfrom a leaf of a flowering plant, seenin the light microscope, showing thegreen chloroplasts (Movie 1.3 and seeMovie 14.9). (B) A drawing of one of thechloroplasts, showing the highly foldedsystem of internal membranes containingthe chlorophyll molecules by which light isabsorbed.
(A, courtesy of Preeti Dahiya.)Chapter 1: Cells and Genomes28Figure 1–31 The origin of chloroplasts.An early eukaryotic cell, already possessingmitochondria, engulfed a photosyntheticbacterium (a cyanobacterium) and retainedit in symbiosis. Present-day chloroplastsare thought to trace their ancestry back toa single species of cyanobacterium thatwas adopted as an internal symbiont (anendosymbiont) over a billion years ago.eukaryotic cellcapable ofphotosynthesisearlyeukaryotic cellchloroplastsphotosyntheticbacteriumMany of the genes that are missing from the mitochondria and chloroplastshave not been lost; instead, they have moved from the symbiont genome into theDNA of the host cell nucleus.
The nuclear DNA of humans contains many genescoding for proteins that serveMBoC6essentialfunctions inside the mitochondria; inm1.36/1.31plants, the nuclear DNA also contains many genes specifying proteins required inchloroplasts. In both cases, the DNA sequences of these nuclear genes show clearevidence of their origin from the bacterial ancestor of the respective organelle.Eukaryotic Genomes Are BigNatural selection has evidently favored mitochondria with small genomes. By contrast, the nuclear genomes of most eukaryotes seem to have been free to enlarge.Perhaps the eukaryotic way of life has made large size an advantage: predatorstypically need to be bigger than their prey, and cell size generally increases in proportion to genome size. Whatever the reason, aided by a massive accumulation ofDNA segments derived from parasitic transposable elements (discussed in Chapter 5), the genomes of most eukaryotes have become orders of magnitude largerthan those of bacteria and archaea (Figure 1–32).The freedom to be extravagant with DNA has had profound implications.Eukaryotes not only have more genes than prokaryotes; they also have vastly moreDNA that does not code for protein.
The human genome contains 1000 times asmany nucleotide pairs as the genome of a typical bacterium, perhaps 10 times asMAMMALS, BIRDS, REPTILESFugu zebrafishAMPHIBIANS, FISHESDrosophilaCRUSTACEANS, INSECTSPLANTS, ALGAEMycoplasmanewtshrimpArabidopsiswheatlilyyeastmalarial parasitePROTOZOANSBACTERIAfrogCaenorhabditisNEMATODE WORMSFUNGIhumanamoebaE.
coliARCHAEA1051061071081091010nucleotide pairs per haploid genome10111012Figure 1–32 Genome sizes compared.Genome size is measured in nucleotidepairs of DNA per haploid genome, that is,per single copy of the genome. (The cellsof sexually reproducing organisms such asourselves are generally diploid: they containtwo copies of the genome, one inheritedfrom the mother, the other from the father.)Closely related organisms can vary widelyin the quantity of DNA in their genomes,even though they contain similar numbersof functionally distinct genes. (Data fromW.H. Li, Molecular Evolution, pp.
380–383.Sunderland, MA: Sinauer, 1997.)GENETIC INFORMATION IN EUKARYOTES29TABLE 1–2 Some Model Organisms and Their GenomesOrganismGenome size*(nucleotide pairs)Approximate numberof genesEscherichia coli (bacterium)4.6 × 1064300Saccharomyces cerevisiae (yeast)13 × 1066600Caenorhabditis elegans(roundworm)130 ×10621,000Arabidopsis thaliana (plant)220 × 10629,000Drosophila melanogaster (fruit fly)200 × 10615,000Danio rerio (zebrafish)1400 × 10632,000Mus musculus (mouse)2800 × 10630,000Homo sapiens (human)10630,0003200 ×*Genome size includes an estimate for the amount of highly repeated DNA sequence not ingenome databases.many genes, and a great deal more noncoding DNA (~98.5% of the genome for ahuman does not code for proteins, as opposed to 11% of the genome for the bacterium E.
coli). The estimated genome sizes and gene numbers for some eukaryotesare compiled for easy comparison with E. coli in Table 1–2; we shall discuss howeach of these eukaryotes serves as a model organism shortly.Eukaryotic Genomes Are Rich in Regulatory DNAMuch of our noncoding DNA is almost certainly dispensable junk, retained likea mass of old papers because, when there is little pressure to keep an archivesmall, it is easier to retain everything than to sort out the valuable informationand discard the rest.
Certain exceptional eukaryotic species, such as the pufferfish, bear witness to the profligacy of their relatives; they have somehow managedto rid themselves of large quantities of noncoding DNA. Yet they appear similar instructure, behavior, and fitness to related species that have vastly more such DNA(see Figure 4–71).Even in compact eukaryotic genomes such as that of puffer fish, there is morenoncoding DNA than coding DNA, and at least some of the noncoding DNA certainly has important functions.
In particular, it regulates the expression of adjacent genes. With this regulatory DNA, eukaryotes have evolved distinctive ways ofcontrolling when and where a gene is brought into play. This sophisticated generegulation is crucial for the formation of complex multicellular organisms.The Genome Defines the Program of Multicellular DevelopmentThe cells in an individual animal or plant are extraordinarily varied. Fat cells, skincells, bone cells, nerve cells—they seem as dissimilar as any cells could be (Figure1–33). Yet all these cell types are the descendants of a single fertilized egg cell, andall (with minor exceptions) contain identical copies of the genome of the species.The differences result from the way in which the cells make selective use oftheir genetic instructions according to the cues they get from their surroundingsin the developing embryo.
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