Часть 1 (1120999), страница 69
Текст из файла (страница 69)
One type of nucleotide sequenceacts as a DNA replication origin, the location at which duplication of the DNA begins. Eucaryoticchromosomes contain many origins of replication to ensure that the entire chromosome can be replicated rapidly, as discussedin detail in Chapter 5.After replication, the two daughter chromosomes remain attached to oneanother and, as the cell cycle proceeds, are condensed further to producemitotic chromosomes. The presence of a second specialized DNA sequence,called a centromere, allows one copy of each duplicated and condensed chromosome to be pulled into each daughter cell when a cell divides.
A protein21OChapter4: DNA,Chromosomes,and GenomesINTERPHASEtelomerer e p li c a t i o noflgrnAMtTOStSINTERPHASEi!,EHHcentromereHHportion ofm i t o t i cs p i n d l etlLld u pl icatedchromosomesi n s e p a r a t ec e l l scomplex called a kinetochore forms at the centromere and attaches the duplicated chromosomes to the mitotic spindle, allowing them to be pulled apart(discussedin Chapter I7).The third specializedDNA sequenceforms telomeres, the ends of a chromosome.
Telomerescontain repeated nucleotide sequencesthat enable the ends ofchromosomes to be efficiently replicated. Telomeresalso perform another function: the repeated telomere DNA sequences,together with the regions adjoiningthem, form structures that protect the end of the chromosome from being mistaken by the cell for a broken DNA molecule in need of repair. We discuss boththis type of repair and the structure and function of telomeres in Chapter 5.In yeast cells, the three types of sequencesrequired to propagate a chromosome are relatively short (typically less than 1000base pairs each) and thereforeuse only a tiny fraction of the information-carrying capacity of a chromosome.Although telomere sequencesare fairly simple and short in all eucaryotes, theDNA sequencesthat form centromeres and replication origins in more complexorganisms are much longer than their yeast counterparts.
For example, experiments suggestthat human centromeres contain up to 100,000nucleotide pairsand may not require a stretch of DNA with a defined nucleotide sequence.Instead, as we shall discuss later in this chapter, they seem to consist of a large,regularly repeating protein-nucleic acid structure that can be inherited when achromosome replicates.DNAMoleculesAreHighlyCondensedin ChromosomesAll eucaryotic organisms have special ways of packaging DNA into chromosomes. For example, if the 48 million nucleotide pairs of DNA in human chromosome 22 could be laid out as one long perfect double helix, the moleculewould extend for about 1.5 cm if stretched out end to end.
But chromos ome 22measures only about 2 pm in length in mitosis (seeFigures 4-10 and 4-ll), representing an end-to-end compaction ratio of nearly 10,000-fold.This remarkable feat of compression is performed by proteins that successivelycoil and foldthe DNA into higher and higher levels of organization. Although much less condensed than mitotic chromosomes, the DNA of human interphase chromosomes is still tightly packed, with an overall compaction ratio of approximately500-fold (the length of a chromosome's DNA helix divided by the end-to-endlength of that chromosome).In reading these sections it is important to keep in mind that chromosomestructure is dynamic. We have seen that each chromosome condenses to anunusual degree in the M phase of the cell cycle. Much less visible, but of enormous interest and importance, specific regions of interphase chromosomesFigure4-21 The three DNAsequencesrequiredto producea eucaryoticchromosomethat can be replicatedandthen segregatedat mitosis.Eachchromosomehasmultipleoriginsofreplication,one centromere,and twotelomeres.Shownhere is the seouenceofeventsthat a typicalchromosomefollowsduringthe cellcycle.The DNAreplicatesin interphase,beginningat the originsofreplicationand proceedingbidirectionallyfrom the originsacrossthechromosome.In M phase,the centromereattachesthe duplicatedchromosomestothe mitoticspindleso that one copyisdistributedto eachdaughtercellduringmitosis.Thecentromerealsohelpstohold the duplicatedchromosomestogetheruntilthey arereadyto bemovedapart.The telomeresform specialcaosat eachchromosomeend.CHROMOSOMALDNA AND ITSPACKAGINGINTHECHROMATINFIBER211decondense as the cells gain access to specific DNA sequences for geneexpression, DNA repair, and replication-and then recondense when theseprocessesare completed.
The packaging of chromosomes is therefore accomplished in a way that allows rapid localized, on-demand accessto the DNA. Inthe next sections we discuss the specialized proteins that make this type ofpackagingpossible.NucleosomesArea BasicUnitof EucaryoticChromosomeStructureThe proteins that bind to the DNA to form eucaryotic chromosomes are traditionally divided into two general classes:the histones and the nonhistone chromosomal proteins.The complex of both classesof protein with the nuclear DNAof eucaryotic cells is knor,rmas chromatin.
Histones are present in such enormous quantities in the cell (about 60 million molecules of each t)?e per humancell) that their total mass in chromatin is about equal to that of the DNA.Histones are responsible for the first and most basic level of chromosomepacking, the nucleosome, a protein-DNA complex discovered in 1974.\Mheninterphase nuclei are broken open very gently and their contents examinedunder the electron microscope, most of the chromatin is in the form of a fiberwith a diameter of about 30 nm (Figure 4-22A).If this chromatin is subjected totreatments that cause it to unfold partially, it can be seen under the electronmicroscope as a series of "beads on a string" (Figure 4-228).
The string is DNA,and each bead is a "nucleosome core particle" that consists of DNA woundaround a protein core formed from histones. <ACTC>The structural organization of nucleosomes was determined after first isolating them from unfolded chromatin by digestion with particular enzymes(called nucleases) that break dor,rmDNA by cutting between the nucleosomes.After digestion for a short period, the exposed DNA between the nucleosomecore particles, the linker Dl/A, is degraded. Each individual nucleosome coreparticle consists of a complex of eight histone proteins-two molecules each ofhistones H2A, HzB, H3, and H4-and double-stranded DNA that is 147nucleotide pairs long. The histone octamer forms a protein core around whichthe double-stranded DNA is wound (Figure 4-23).Each nucleosome core particle is separated from the next by a region oflinker DNA, which can vary in length from a few nucleotide pairs up to about 80.(The term nucleosometechnically refers to a nucleosome core particle plus oneof its adjacent DNA linkers, but it is often used synonymously with nucleosomecore particle.) On average,therefore, nucleosomes repeat at intervals of about200 nucleotide pairs.
For example, a diploid human cell with 6.4 x 10enucleotidepairs contains approximately 30 million nucleosomes.The formation of nucleosomes converts a DNA molecule into a chromatin thread about one-third of itsinitial length.(A)Figure 4-22 Nucleosomesas seenin theelectron microscope,(A)Chromatinisolateddirectlyfrom an interphasenucleusappearsin the electronmicroscooeasa thread30 nm thick.(B)Thiselectronmicrographshowsalengthof chromatinthat hasbeenorunpacked,experimentallyafterisolationto showthedecondensed,(A,courtesyof Barbaranucleosomes.Hamkalo;B,courtesyof VictoriaFoe.)212Chapter4: DNA,Chromosomes,and GenomesFigure4-23 Structuralorganizationof the nucleosome.A nucleosomecontainsa proteincoremadeof eight histonemolecules.In biochemicalexperiments,the nucleosomecoreparticlecan be releasedfrom isolatedchromatinby digestionof the linkerDNAwith a nuclease,an enzymethatbreaksdown DNA,(Thenucleasecandegradethe exposedlinkerDNAbutcannotattackthe DNAwound tightlyaroundthe nucleosomecore.)Afterdissociationof the isolatednucleosomeinto its proteincoreand DNA,thelengthof the DNAthat waswound aroundthe corecan be determined.Thislengthof 147nucleotidepairsis sufficientto wrap 1.7 timesaroundthe histonecore.c o r eh i s t o n e sof nucleosomenucleosomeincludes" beads-on-a-string"-200 nucleotideformof chromatinp a i r so f D N AINUCLEASEIDTGESTSIL I N K EDRN AIV,.rd'..Q'...odTheStructureof the NucleosomeCoreParticleRevealsHow DNAls PackagedThe high-resolution structure of a nucleosome core particle, solved in 1997,revealed a disc-shaped histone core around which the DNAwas tightlywrapped1.7 turns in a left-handed coil (Figure 4-24).
All four of the histones that makeup the core of the nucleosome are relatively small proteins (102-135 aminoacids), and they share a structural motif, known asthe histonefold,formed fromthree cr helices connected by two loops (Figure 4-25).In assembling a nucleosome, the histone folds first bind to each other to form H3-H4 and H2A-H2Bdimers, and the H3-H4 dimers combine to form tetramers. An H3-H4 tetramerthen further combines with two HZA-H2B dimers to form the compact octamercore, around which the DNA is wound (Figure 4-26).The interface between DNA and histone is extensive: 142 hydrogen bondsare formed between DNA and the histone core in each nucleosome.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
