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1、10MOLECULARSTRUCTUREOFGENESANDCHROMOSOMESThesebrightlycoloredRxFISH-paintedchromosomesarebothbeautifulandusefulinrevealingchromosomeanomaliesandincomparingkaryotypesofdifferentspecies.[©DepartmentofClinicalCytogenetics,AddenbrookesHospital/PhotoResearchers,Inc.]ythebeginningofthetwe
2、nty-firstcentury,molecularrareinbacteria.Sequencingofthesameprotein-codinggenebiologistshadcompletedsequencingtheentiregenomesinavarietyofeukaryoticspecieshasshownthatevolution-Bofhundredsofviruses,scoresofbacteria,andthebud-arypressureselectsformaintenanceofrelativelysimilarse-ding
3、yeastS.cerevisiae,aunicellulareukaryote.Inaddition,quencesinthecodingregions,orexons.Incontrast,widethevastmajorityofthegenomesequenceisnowknownsequencevariation,evenincludingtotalloss,occursamongforseveralmulticellulareukaryotesincludingtheroundwormintrons,suggestingthatmostintrons
4、equenceshavelittleC.elegans,thefruitflyD.melanogaster,andhumans(seefunctionalsignificance.Figure9-34).DetailedanalysisofthesesequencingdatahasThesheerlengthofcellularDNAisasignificantprob-revealedthatalargeportionofthegenomesofhighereukary-lemwithwhichcellsmustcontend.TheDNAinasingl
5、eotesdoesnotencodemRNAsoranyotherRNAsrequiredbyhumancell,whichmeasuresabout2metersintotallength,theorganism.Remarkably,suchnoncodingDNAconstitutesmustbecontainedwithincellswithdiametersoflessthan5morethan95percentofhumanchromosomalDNA.10m,acompactionratioofgreaterthan10.Specialized
6、ThenoncodingDNAinmulticellularorganismscontainseukaryoticproteinsassociatedwithnuclearDNAfoldandmanyregionsthataresimilarbutnotidentical.VariationsorganizeitintothestructuresofDNAandproteinvisualizedwithinsomestretchesofthisrepetitiousDNAaresogreatasindividualchromosomesduringmitosi
7、s.MitochondriaandthateachsinglepersoncanbedistinguishedbyaDNA“fin-gerprint”basedonthesesequencevariations.Moreover,somerepetitiousDNAsequencesarenotfoundinconstantpositionsintheDNAofindividualsofthesamespecies.SuchOUTLINE“mobile”DNAelements,whicharepresentinbothprokary-10.1Molecular
8、DefinitionofaGeneot
