Часть 1 (1120999), страница 71
Текст из файла (страница 71)
Thus,when nuclei are very gently lysed onto an electron microscope grid, most of thechromatin is seen to be in the form of a fiber with a diameter of about 30 nm,which is considerably wider than chromatin in the "beads on a string" form (seeFigure 4-22).217CHROMOSOMALDNAAND IT5PACKAGINGFIBERIN THECHROMATINF i g u r e 4 - 3 1A z i g z a g m o d ef lo r t h e 3 0 - n m c h r o m a t i n f i b e r . (TAh) e c o n f o r m a t i o n o f t w o o f t h e f o u r(B)Schematicofnucleosomesin a tetranucleosome,from a structuredeterminedby x-raycrystallography.is not visible,beingstackedon the bottom nucleosomethe entiretetranucleosome;the fourth nucleosomeof a possiblezigzagstructurethat couldaccountillustrationand behindit in this diagram.(C)Diagrammatic2005'Withfor the 30-nm chromatinfiber.(Adaptedfrom C.L.Woodcock,Ndf.Sttuct.Mol.Biol.12:639-640,permissionLtd.)from MacmillanPublishersHow are nucleosomes packed in the 30-nm chromatin fiber? This questionhas not yet been answered definitively, but important information concerningthe structure has been obtained.
In particular, high-resolution structural analyses have been performed on homogeneous short strings of nucleosomes, prepared from purified histones and purified DNA molecules. The structure of atetranucleosome, obtained by X-ray crystallography,has been used to support azigzag model for the stacking of nucleosomes in the 30-nm fiber (Figure 4-3f ).But cryoelectron microscopy of longer strings of nucleosomes supports a verydifferent solenoidal structure with intercalated nucleosomes (Figure 4-32).\Arhatcauses the nucleosomes to stack so tightly on each other in a 30-nmfiber? The nucleosome to nucleosome linkages formed by histone tails, mostnotably the H4 tail (Figure 4-33) constitute one important factor.
Another(A)(c)1 0n mFigure4-32 An interdigitatedsolenoidmodelfor the 30-nmchromatinfiber.(A)Drawingsin whichstringsof color(B)Schematicdiagramof finalstructurein (A).codednucleosomesareusedto illustratehow the solenoidis generated.arraysimagesof nucleosome(C)Structuralmodel.The modelis derivedfrom high-resolutionmicroscopycryoelectronoctamersandBothnucleosomeof specificlengthand sequence.reconstitutedfrom purifiedhistonesand DNAmoleculesa linkerhistone(discussedbelow)wereusedto produceregularlyrepeatingarrayscontainingup to 72 nucleosomes'1, 2006.With(Adaptedfrom P.Robinson,L. Fairall,V.
Huynhand D. Rhodes,Proc.NatlAcad.Sci.U.S.A.103:6506-651permissionfrom NationalAcademyof Sciences.)218Chapter4: DNA, Chromosomes,and GenomesH 4t a i lH 2 At a i lH 2 Bt a i l. . H 3t a i lH4 tailH3 tailimportant factor is an additional histone that is often present in a l-to-1 ratiowith nucleosome cores, knor,r,nas histone Hl.
This so-called linker histone islarger than the individual core histones and it has been considerably less wellconserved during evolution. A single histone Hl molecule binds to each nucleosome, contacting both DNA and protein, and changing the path of the DNA asit exits from the nucleosome. Although it is not understood in detail how Hlpulls nucleosomes together into the 30-nm fiber, a change in the exit path inDNA seems crucial for compacting nucleosomal DNA so that it interlocks toform the 30-nm fiber (Figure 4-34).
Most eucaryotic organisms make severalhistone Hl proteins of related but quite distinct amino acid sequences.It is possible that the 30-nm structure found in chromosomes is a fluidmosaic of several different variations. For example, a linker histone in the Hlfamily was present in the nucleosomal arrays studied in Figure 4-32 but wasmissing from the tetranucleosome in Figure 4-31. Moreover, we saw earlier thatthe linker DNA that connects adjacent nucleosomes can vary in length; thesedifferences in linker length probably introduce local perturbations into thestructure.
And the presenceof many other DNA-binding proteins, as well as proteins that bind directly to histones, will certainly add important additional features to any array of nucleosomes.Figure4-33 A speculativemodel for therole playedby histonetailsin theformationof the 30-nmfiber.(A)Thisschematicdiagramshowstheapproximateexit pointsof the eighthistonetails,one from eachhistoneprotein.that extendfrom eachnucleosome.Theactualstructureis shownto its right.In the high-resolutionstructureof the nucleosome,the tailsarelargelyunstructured,suggestingthat they arehighlyflexible.(B)A speculativemodelshowinghow the histonetailsmay helptopacknucleosomestogetherinto the30-nmfiber.Thismodelis basedon (1)experimentalevidencethat histonetailsaid in the formationof the 30-nmfiber,and (2)the x-raycrystalstructureof thenucleosome,in whichthe tailsof onenucleosomecontactthe histonecoreof anadjacentnucleosomein the crystallattice.Su m m a r yA geneis a nucleotidesequencein a DNA moleculethat actsas a functional unit for theproduction of a protein, a structural RNA,or a catalytic or regulatory RNAmolecule.Ineucaryotes,protein-codinggenesare usually composedof a string of alternating intronsand exonsassociatedwith regulatory regionsof DNA.
A chromosomeisformeclfrom asingle,enormously long DNA moleculethat contains a linear array of many genes.Thehuman genomecontains3.2x ]d DNA nucleotidepairs,diuidedbetween22 dffirentautosomesand 2 sexchromosomes.only a small percentageof this DNA codesfor proteins or functional RNAmolecules.A chromosomal DNA moleculealso contains threeother filpes of functionally important nucleotide sequences:replication origins andtelomeresallow the DNA molecule to be fficiently replicated, while a centromereattaches the daughter DNA moleculesto the mitotic spindle, ensuring their accuratesegregationto daughter cellsduring the M phaseof the cell cycle.Figure4-34 How the linkerhistonebindsto the nucleosome.The positionand structureof the globularregionofhistoneH1 areshown.As indicated,thisregionconstrainsan additional20 nucleotidepairsof DNAwhereit exitsfrom the nucleosomecore.Thistype ofbindingby H1 isthoughtto be importantfor formingthe 30-nmchromatinfiber.The long C-terminaltail of histoneH1 isalsorequiredfor the high-affinitybindingof H1 to chromatin,but neitheritspositionor that of the N-terminaltail is(B)structure.known.(A)Schematic,(8,from D.
Brown,T. lzardand T. Misteli,Nat,Struct.Mol. Biol. 13:250-255,2006.With permissionfrom MacmillanPublishersLtd.)219THEREGULATIONOF CHROMATINSTRUCTUREThe DNA in eucaryotesis tightly bound to an equal massof histones,which formrepeatedarrays of DNA-protein particles called nucleosomes.The nucleosomeis composedof an octameric core of histone proteins around which the DNA double helix iswrapped.Nucleosomesare spacedat interuals of about 200 nucleotidepairs, and theyare usually packed together (with the aid of histone Hl molecules)into quasi-regulararrays to form a 30-nm chromatin fiber.
Despite the high degreeof compaction inchromatin, its structure must be highly dynamic to allow accessto the DNA. ThereissomespontaneousDNA unwrapping and rewrappingin the nucleosomeitself;how'euer,the general strategyfor reuersiblychanging local chromatin structure featuresATP-driuen chromatin remodeling complexes.Cells contain a large set of such complexes,which are targeted to speciflc regionsof chromatin at appropriate times. Theremodeling complexescollaborate with histone chaperonesto allow nucleosomecoresto be repositioned,reconstitutedwith dffirent histones,or completelyremouedtoexposethe underlying DNA.THEREGULATIONOFCHROM IN STRUCTUREHaving described how DNA is packagedinto nucleosomesto create a chromatinfiber, we now turn to the mechanisms that create different chromatin structuresin different regions of a cell's genome.
We now know that mechanisms of this typeare used to control many genesin eucaryotes.Most importantly, certain types ofchromatin structure can be inherited; that is, the structure can be directly passeddonm from a cell to its decendents.Becausethe cell memory that results is basedon an inherited protein structure rather than on a change in DNA sequence,thisis a form of epigenetic inheritance. The prefix epl is Greek for "on"; this is appropriate, becauseepigeneticsrepresentsa form of inheritance that is superimposedon the genetic inheritance based on DNA (Figure,t-35).In Chapter 7, we shall introduce the many different ways in which theexpression of genes is regulated. There we discuss epigenetic inheritance indetail and present severaldistinct mechanisms that can produce it.
Here, we areconcerned with only one, that based on chromatin structure. We begin this section with an introduction to inherited chromatin structures and then describethe basis for them-the covalent modification of histones in nucleosomes.Weshall see that these modifications serve as recognition sites for protein modulesthat bring specific protein complexes to the appropriate regions of chromatin,thereby producing specific effects on gene expressionor inducing other biological functions. Through such mechanisms, chromatin structure plays a centralrole in the development, growth, and maintenance of eucaryotic organisms'including ourselves.G E N E T IICNHERITANCEE P I G E N E TI NI CH E R I T A N C Eg e n eY o ng e n eX o nseeuerucrI oruncHANGEIV cHnovnrtruI CHANGECIITI!gene X oflo e n eY o f fCELLSMULTTPLTCATTONOF SOMATTC/\gene X off*E*ilgene Y offP R O D U C T I OONF G E R MC E L L S:ililiii:li:i.t,t:]it::]iltl::.lulgene X offIItiiilillii*:iiilisi:i:liitlFigure4-35 A comparisonof geneticinheritancewith an epigeneticinheritancebasedon chromatinis basedstructures.Geneticinheritanceof DNAon the directinheritanceduringDNAnucleotidesequencesDNAsequencechangesarereplication.not only transmittedfaithfullyfrom abutsomaticcellto all of its descendents,alsothroughgerm cellsfrom onegenerationto the next.Thefieldofgenetics,reviewedin Chapter8, is basedof thesechangeson the inheritanceThe type ofbetweengenerations.shownhereisepigeneticinheritancebasedon other moleculesboundto theDNA,and it is thereforelesspermanentinthan a changein DNAsequence;particular,epigeneticinformationisusually(but not always)erasedduringthe formationof eggsand sPerm.thatOnlyone epigeneticmechanism,of chromatinbasedon an inheritancein this chapter.is discussedstructures,areOtherepigeneticmechanismspresentedin Chapter7, whichfocuseson(seethe controlof geneexpressionFigure7-86).220Chapter4: DNA,Chromosomes,and GenomesSomeEarlyMysteriesConcerningChromatinStructureThirty years ago, histones were viewed as relatively uninteresting proteins.Nucleosomes were known to cover all of the DNA in chromosomes, and theywere thought to exist to allow the enormous amounts of DNA in many eucaryotic cells to be packaged into compact chromosomes.
Extrapolating from whatwas knor.m in bacteria, many scientists believed that gene regulation in eucaryotes would simply bypass nucleosomes, treating them as uninvolvedbystanders.But there were reasons to challenge this view. Thus, for example, biochemists had determined that mammalian chromatin consists of an approximately equal mass of histone and non-histone proteins. This would mean that,on auerLge,every 200 nucleotide pairs of DNA in our cells is associated withmore than 1000 amino acids of non-histone proteins (that is, a mass of proteinequivalent to the total mass of the histone octamer plus histone Hl).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
