Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 13
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For example, the endsof chromosomes, the telomeres, are extremely dilute in mostcells. Human cells typically contain 92 telomeres (46 chromosomes 2 ends per chromosome). In contrast, some protozoa with unusual “fragmented” chromosomes containmillions of telomeres per cell. Recent discoveries abouttelomere structure have benefited greatly from using this natural variation for experimental advantage.individual proteins, hundreds of macromolecular machines,and most of our organelles, all as a result of our shared evolutionary history. New insights into molecular cell biologyarising from genomics are leading to a fuller appreciation ofthe elegant molecular machines that arose during billions ofyears of genetic tinkering and evolutionary selection for themost efficient, precise designs.
Despite all that we currentlyknow about cells, many new proteins, new macromolecularassemblies, and new activities of known ones remain to bediscovered. Once a more complete description of cells is inhand, we will be ready to fully investigate the rippling, flowing dynamics of living systems.1.5 A Genome Perspectiveon EvolutionAs humans, we probably have a biased and somewhat exaggerated view of our status in the animal kingdom. Pride inour swollen forebrain and its associated mental capabilitiesmay blind us to the remarkably sophisticated abilities ofother species: navigation by birds, the sonar system of bats,homing by salmon, or the flight of a fly.Comprehensive studies of genes and proteins from many organisms are giving us an extraordinary documentation of thehistory of life.
We share with other eukaryotes thousands ofMetabolic Proteins, the Genetic Code,and Organelle Structures Are Nearly UniversalEven organisms that look incredibly different share many biochemical properties. For instance, the enzymes that catalyzedegradation of sugars and many other simple chemical reactions in cells have similar structures and mechanisms in mostliving things.
The genetic code whereby the nucleotide sequences of mRNA specifies the amino acid sequences of proteins can be read equally well by a bacterial cell and a humancell. Because of the universal nature of the genetic code, bacterial “factories” can be designed to manufacture growth factors, insulin, clotting factors, and other human proteins withtherapeutic uses. The biochemical similarities among organisms also extend to the organelles found in eukaryotic cells.The basic structures and functions of these subcellular components are largely conserved in all eukaryotes.Computer analysis of DNA sequence data, now availablefor numerous bacterial species and several eukaryotes, canlocate protein-coding genes within genomes.
With the aid ofthe genetic code, the amino acid sequences of proteins can bededuced from the corresponding gene sequences. Althoughsimple conceptually, “finding” genes and deducing the aminoacid sequences of their encoded proteins is complicated inpractice because of the many noncoding regions in eukaryotic DNA (Chapter 9). Despite the difficulties and occasionalambiguities in analyzing DNA sequences, comparisons of thegenomes from a wide range of organisms provide stunning,compelling evidence for the conservation of the molecularmechanisms that build and change organisms and for thecommon evolutionary history of all species.Many Genes Controlling DevelopmentAre Remarkably Similar in Humansand Other Animals1.5 • A Genome Perspective on Evolution(a)GenesFlyMammal(b)(c)(d)(e)27 FIGURE 1-26 Similar genes, conserved during evolution,regulate many developmental processes in diverse animals.Insects and mammals are estimated to have had a commonancestor about half a billion years ago.
They share genes thatcontrol similar processes, such as growth of heart and eyes andorganization of the body plan, indicating conservation of functionfrom ancient times. (a) Hox genes are found in clusters on thechromosomes of most or all animals. Hox genes encode relatedproteins that control the activities of other genes. Hox genesdirect the development of different segments along the head-totail axis of many animals as indicated by corresponding colors.Each gene is activated (transcriptually) in a specific region alongthe head-to-toe axis and controls the growth of tissues there.
Forexample, in mice the Hox genes are responsible for thedistinctive shapes of vertebrae. Mutations affecting Hox genes inflies cause body parts to form in the wrong locations, such aslegs in lieu of antennae on the head. These genes provide ahead-to-tail address and serve to direct formation of the rightstructures in the right places. (b) Development of the largecompound eyes in fruit flies requires a gene called eyeless(named for the mutant phenotype). (c) Flies with inactivatedeyeless genes lack eyes. (d) Normal human eyes require thehuman gene, called Pax6, that corresponds to eyeless.
(e) Peoplelacking adequate Pax6 function have the genetic disease aniridia,a lack of irises in the eyes. Pax6 and eyeless encode highlyrelated proteins that regulate the activities of other genes, andare descended from the same ancestral gene. [Parts (a) and (b)Andreas Hefti, Interdepartmental Electron Microscopy (IEM) Biocenter,University of Basel. Part (d) © Simon Fraser/Photo Researchers, Inc.]This is not to say that all genes or proteins are evolutionarily conserved. Many striking examples exist of proteinsthat, as far as we can tell, are utterly absent from certain lineages of animals. Plants, not surprisingly, exhibit many suchdifferences from animals after a billion-year separation intheir evolution.
Yet certain DNA-binding proteins differ between peas and cows at only two amino acids out of 102!Despite all the evidence for evolutionary unity at the cellular and physiological levels, everyone expected that genesregulating animal development would differ greatly fromone phylum to the next. After all, insects and sea urchinsand mammals look so different. We must have many uniqueproteins to create a brain like ours . . . or must we? Thefruits of research in developmental genetics during the pasttwo decades reveal that insects and mammals, which havea common ancestor about half a billion years ago, possessmany similar development-regulating genes (Figure 1-26).Indeed, a large number of these genes appear to be conserved in many and perhaps all animals. Remarkably, thedevelopmental functions of the proteins encoded by thesegenes are also often preserved.
For instance, certain proteinsinvolved in eye development in insects are related to protein regulators of eye development in mammals. Same fordevelopment of the heart, gut, lungs, and capillaries and forplacement of body parts along the head-to-tail and backto-front body axes (Chapter 15).Darwin’s Ideas About the Evolution of WholeAnimals Are Relevant to GenesDarwin did not know that genes exist or how they change,but we do: the DNA replication machine makes an error, ora mutagen causes replacement of one nucleotide with another or breakage of a chromosome.
Some changes in thegenome are innocuous, some mildly harmful, some deadly;a very few are beneficial. Mutations can change the sequenceof a gene in a way that modifies the activity of the encodedprotein or alters when, where, and in what amounts the protein is produced in the body.Gene-sequence changes that are harmful will be lost froma population of organisms because the affected individualscannot survive as well as their relatives.
This selectionprocess is exactly what Darwin described without knowingthe underlying mechanisms that cause organisms to vary.Thus the selection of whole organisms for survival is reallya selection of genes, or more accurately sets of genes. A population of organisms often contains many variants that are28CHAPTER 1 • Life Begins with Cellsall roughly equally well-suited to the prevailing conditions.When conditions change—a fire, a flood, loss of preferredfood supply, climate shift—variants that are better able toadapt will survive, and those less suited to the new conditions will begin to die out. In this way, the genetic composition of a population of organisms can change over time.Human Medicine Is Informed by Researchon Other OrganismsMutations that occur in certain genes during the course ofour lives contribute to formation of various human cancers.The normal, wild-type forms of such “cancer-causing” genesgenerally encode proteins that help regulate cell proliferationor death (Chapter 23).
We also can inherit from our parentsmutant copies of genes that cause all manner of genetic diseases, such as cystic fibrosis, muscular dystrophy, sickle cellanemia, and Huntington’s disease. Happily we can also inherit genes that make us robustly resist disease. A remarkablenumber of genes associated with cancer and other humandiseases are present in evolutionarily distant animals. For example, a recent study shows that more than three-quarters ofthe known human disease genes are related to genes found inthe fruit fly Drosophila.With the identification of human disease genes in otherorganisms, experimental studies in experimentally tractableorganisms should lead to rapid progress in understandingthe normal functions of the disease-related genes and whatoccurs when things go awry.
Conversely, the disease statesthemselves constitute a genetic analysis with well-studiedphenotypes. All the genes that can be altered to cause a certain disease may encode a group of functionally relatedproteins. Thus clues about the normal cellular functions ofproteins come from human diseases and can be used toguide initial research into mechanism. For instance, genesinitially identified because of their link to cancer in humanscan be studied in the context of normal development in various model organisms, providing further insight about thefunctions of their protein products.2CHEMICALFOUNDATIONSPolysaccharide chains on the surface of cellulose visualized byatomic force microscopy. [Courtesy of M.
Miles from A. A. Baker etal., 2000, Biophys J. 79:1139–1145.]The life of a cell depends on thousands of chemical interactions and reactions exquisitely coordinated withone another in time and space and under the influenceof the cell’s genetic instructions and its environment. Howdoes a cell extract critical nutrients and information from itsenvironment? How does a cell convert the energy storedin nutrients into work (movement, synthesis of critical components)? How does a cell transform nutrients into the fundamental structures required for its survival (cell wall,nucleus, nucleic acids, proteins, cytoskeleton)? How does acell link itself to other cells to form a tissue? How do cellscommunicate with one another so that the organism asa whole can function? One of the goals of molecular cell biology is to answer such questions about the structure andfunction of cells and organisms in terms of the properties ofindividual molecules and ions.Life first arose in a watery environment, and the properties of this ubiquitous substance have a profound influenceon the chemistry of life.
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