B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 16
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The functional characterization of each new family of gene products, unlike the description of the gene sequences, presents a fresh challenge to the biologist’s ingenuity.Moreover, we will never fully understand the function of a gene until we learn itsrole in the life of the organism as a whole. To make ultimate sense of gene functions, therefore, we have to study whole organisms, not just molecules or cells.Molecular Biology Began with a Spotlight on E. coliBecause living organisms are so complex, the more we learn about any particularspecies, the more attractive it becomes as an object for further study.
Each discovery raises new questions and provides new tools with which to tackle generalquestions in the context of the chosen organism. For this reason, large communities of biologists have become dedicated to studying different aspects of the samemodel organism.In the early days of molecular biology, the spotlight focused intensely on justone species: the Escherichia coli, or E.
coli, bacterium (see Figures 1–13 and 1–14).This small, rod-shaped bacterial cell normally lives in the gut of humans and othervertebrates, but it can be grown easily in a simple nutrient broth in a culture bottle. It adapts to variable chemical conditions and reproduces rapidly, and it canevolve by mutation and selection at a remarkable speed. As with other bacteria,different strains of E.
coli, though classified as members of a single species, differ genetically to a much greater degree than do different varieties of a sexuallyreproducing organism such as a plant or animal. One E. coli strain may possessmany hundreds of genes that are absent from another, and the two strains couldhave as little as 50% of their genes in common. The standard laboratory strainE. coli K-12 has a genome of approximately 4.6 million nucleotide pairs, containedin a single circular molecule of DNA that codes for about 4300 different kinds ofproteins (Figure 1–24).In molecular terms, we know more about E.
coli than about any other livingorganism. Most of our understanding of the fundamental mechanisms of life—for example, how cells replicate their DNA, or how they decode the instructionsrepresented in the DNA to direct the synthesis of specific proteins—initially camefrom studies of E. coli. The basic genetic mechanisms have turned out to be highlyconserved throughout evolution: these mechanisms are essentially the same inour own cells as in E.
coli.SummaryProkaryotes (cells without a distinct nucleus) are biochemically the most diverseorganisms and include species that can obtain all their energy and nutrients frominorganic chemical sources, such as the reactive mixtures of minerals released athydrothermal vents on the ocean floor—the sort of diet that may have nourished thefirst living cells 3.5 billion years ago. DNA sequence comparisons reveal the familyrelationships of living organisms and show that the prokaryotes fall into two groupsthat diverged early in the course of evolution: the bacteria (or eubacteria) and thearchaea.
Together with the eukaryotes (cells with a membrane-enclosed nucleus),these constitute the three primary branches of the tree of life.Most bacteria and archaea are small unicellular organisms with compactgenomes comprising 1000–6000 genes. Many of the genes within a single organismshow strong family resemblances in their DNA sequences, implying that they originated from the same ancestral gene through gene duplication and divergence. Family resemblances (homologies) are also clear when gene sequences are comparedbetween different species, and more than 200 gene families have been so highlyGENETIC INFORMATION IN EUKARYOTES23origin ofreplication(A)Escherichia coli K-124,639,221 nucleotide pairsterminus ofreplication(B)Figure 1–24 The genome of E.
coli. (A) A cluster of E. coli cells. (B) A diagram of the genome ofE. coli strain K-12. The diagram is circular because the DNA of E. coli, like that of other prokaryotes,forms a single, closed loop. Protein-coding genes are shown as yellow or orange bars, dependingon the DNA strand from which they are transcribed; genes encoding only RNA molecules areindicated by green arrows. Some genes are transcribed from one strand of the DNA double helix (ina clockwise direction in this diagram), others from the other strand (counterclockwise). (A, courtesyof Dr. Tony Brain and David Parker/Photo Researchers; B, adapted from F.R. Blattner et al., Science277:1453–1462, 1997.)conserved that they can be recognized as common to most species from all threedomains of the living world. Thus, given the DNA sequence of a newly discoveredMBoC6m1.29/1.24gene, it is often possible to deduce thegene’sfunction from the known function of ahomologous gene in an intensively studied model organism, such as the bacteriumE.
coli.GENETIC INFORMATION IN EUKARYOTESEukaryotic cells, in general, are bigger and more elaborate than prokaryotic cells,and their genomes are bigger and more elaborate, too. The greater size is accompanied by radical differences in cell structure and function.
Moreover, manyclasses of eukaryotic cells form multicellular organisms that attain levels of complexity unmatched by any prokaryote.Because they are so complex, eukaryotes confront molecular biologists with aspecial set of challenges that will concern us in the rest of this book. Increasingly,biologists attempt to meet these challenges through the analysis and manipulation of the genetic information within cells and organisms.
It is therefore important at the outset to know something of the special features of the eukaryoticgenome. We begin by briefly discussing how eukaryotic cells are organized, how24Chapter 1: Cells and Genomesthis reflects their way of life, and how their genomes differ from those of prokaryotes. This leads us to an outline of the strategy by which cell biologists, by exploiting genetic and biochemical information, are attempting to discover how eukaryotic organisms work.Eukaryotic Cells May Have Originated as PredatorsBy definition, eukaryotic cells keep their DNA in an internal compartment calledthe nucleus. The nuclear envelope, a double layer of membrane, surrounds thenucleus and separates the DNA from the cytoplasm. Eukaryotes also have otherfeatures that set them apart from prokaryotes (Figure 1–25).
Their cells are, typically, 10 times bigger in linear dimension and 1000 times larger in volume. Theyhave an elaborate cytoskeleton—a system of protein filaments crisscrossing thecytoplasm and forming, together with the many proteins that attach to them, asystem of girders, ropes, and motors that gives the cell mechanical strength, controls its shape, and drives and guides its movements (Movie 1.1).
And the nuclearenvelope is only one part of a set of internal membranes, each structurally similarto the plasma membrane and enclosing different types of spaces inside the cell,many of them involved in digestion and secretion. Lacking the tough cell wall ofmost bacteria, animal cells and the free-living eukaryotic cells called protozoa canchange their shape rapidly and engulf other cells and small objects by phagocytosis (Figure 1–26).How all of the unique properties of eukaryotic cells evolved, and in whatsequence, is still a mystery.
One plausible view, however, is that they are all reflections of the way of life of a primordial cell that was a predator, living by capturingother cells and eating them (Figure 1–27). Such a way of life requires a large cellwith a flexible plasma membrane, as well as an elaborate cytoskeleton to supportmicrotubulecentrosome withpair of centrioles5 µmextracellular matrixchromatin (DNA)nuclear porenuclear envelopevesicleslysosomeactinfilamentsnucleolusperoxisomeribosomesin cytosolGolgi apparatusintermediatefilamentsplasma membranenucleusendoplasmicreticulummitochondrionFigure 1–25 The major features of eukaryotic cells. The drawing depicts a typical animal cell, but almost all the same components are found inplants and fungi as well as in single-celled eukaryotes such as yeasts and protozoa.
Plant cells contain chloroplasts in addition to the componentsshown here, and their plasma membrane is surrounded by a tough external wall formed of cellulose.GENETIC INFORMATION IN EUKARYOTES25Figure 1–26 Phagocytosis. This series ofstills from a movie shows a human whiteblood cell (a neutrophil) engulfing a redblood cell (artificially colored red) that hasbeen treated with an antibody that marks itfor destruction (see Movie 13.5). (Courtesyof Stephen E. Malawista and Anne deBoisfleury Chevance.)10 µmand move this membrane.
It may also require that the cell’s long, fragile DNA molecules be sequestered in a separate nuclear compartment, to protect the genomefrom damage by the movements of the cytoskeleton.Modern Eukaryotic Cells Evolved from a SymbiosisA predatory way of life helps to explain another feature of eukaryotic cells. Allsuch cells contain (or at one time did contain) mitochondria (Figure 1–28). Thesesmall bodies in the cytoplasm, enclosed by a double layer of membrane, take upoxygen and harness energy from the oxidation of food molecules—such as sugars—to produce most of the ATP that powers the cell’s activities. Mitochondria aresimilar in size to small bacteria, and, like bacteria, they have their own genome inMBoC6m1.31/1.26the form of a circular DNA molecule,theirown ribosomes that differ from thoseelsewhere in the eukaryotic cell, and their own transfer RNAs. It is now generally accepted that mitochondria originated from free-living oxygen-metabolizing(aerobic) bacteria that were engulfed by an ancestral cell that could otherwisemake no such use of oxygen (that is, was anaerobic).
Escaping digestion, thesebacteria evolved in symbiosis with the engulfing cell and its progeny, receiving(A)100 µm(B)Figure 1–27 A single-celled eukaryote that eats other cells. (A) Didinium is a carnivorousprotozoan, belonging to the group known as ciliates. It has a globular body, about 150 μm indiameter, encircled by two fringes of cilia—sinuous, whiplike appendages that beat continually; itsfront end is flattened except for a single protrusion, rather like a snout.
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