医学分子生物学-第二章 基因

医学分子生物学第二章基因结构与功能易光辉南华大学医学院分子生物学研究中心2009-11Genes:

StructureandFunctionbyProf.GuanghuiYighyi6108@163.constituteofCardiovascularDiseaseResearchCenterforMolecularBiologyMedicalCollege,UniversityofSouthChina2009-11基因的概念及其含义的演变参见“医学分子生物学---第一章绪论”相关学习网站：(NHGRI官方网站)EukaryoticCentralDogma翻译非翻译序列蛋白成熟RNARNA初始转录本外显子内含子RNA剪接转录启动子上游下游Gene:ADNAsegmentcontainingbiologicalinformationandhencecodingforanRNAand/orpolypeptidemolecule.基因是含有生物信息的DNA片段，根据这些生物信息可以编码具有生物功能的产物，包括RNA和多肽链。基因的分子本质

Genes:MolecularNature碱基配对A:TG:CTCGA双链DNA的结构右手方向双股螺旋反向平行共同轴心大沟和小沟Bases

InDNA,therearefourheterocyclicbases:adenine(A)（腺嘌啉）andquinine(G)（鸟嘌啉）arepurines;cytosine(C)（胞嘧啶）andthymine(T)（胸腺嘧啶）arepyrimidines.InRNA,thymineisreplacedbythestructurallyverysimilarpyrimidine,uracil(U)（尿嘧啶）.Nucleosides

InRNAthesugarisribose（核糖）andthecompoundsareribonucleosides（核糖核苷）,orjustnucleosides,whereasinDNAitis2’-deoxyribose,andthenucleosidesarenamed2’-deoxyribonucleosides（2‘-脱氧核糖核苷）,orjustdeoxynucleosides.Base+sugar=nucleoside.Nucleotides

Inribonucleotides(核苷酸）(CMP,CTP,ATP,GTP,UTP),the2’-positionishydroxylgroup.Ifthesugarisdeoxyribose,thencompoundsaredeoxynucleotides（脱氧核苷酸）(dATP,dGTP,dCTP,dTTP;dNTP).

Base+sugar+phosphate=nucleotide.

PhosphodiesterbondsBetweenthe5’-positionofonesugarandthe3’-positionofthenext,forminga3’,5’-phosphodiesterbond.Nucleicacidsarehighlychargedpolymerswithanegativecharge（负电荷）oneachphosphate.带负电荷的核酸在电场中向正极泳动，泳动速率与其分子量的对数近似负线性关系2000bp1500bp1000bp700bp500bp400bp300bp200bp100bp50bp900bp800bp700bp600bp500bp400bp300bpM12345Cathode(-)Anode(+)DirectionofmigrationSizeofDNAfragment1.5DNA/RNAsequenceThenucleicacidsequenceisthesequenceofbasesA,C,G,T/UintheDNAorRNAchain.Thesequenceisconventionallywrittenfromthefree5’-（游离5’-端）tothefree3’-end（游离3’-端)ofthemolecule.Examples:5’-ATAAGCTC-3’(DNA),5’-AUAGCUUGA-3’(RNA).DNAdoublehelixTwoseparateandantiparallelchains

（反向平行）ofDNAarewoundaroundeachotherinaright-handedhelical(coiled)path,withthesugar-phosphatebackbonesontheoutsideandthebases,pairedbyhydrogenbondingandstackedoneachother,ontheinside.Adeninepairswiththymine;guaninepairswithcytosine.Thetwochainsarecomplemently（互补）;onespecifiesthesequenceoftheother.Table1.1FormsofDNA

Form Pitch(nm) Residues Inclinationofbaseperturn pairfromhorizontal

A0.261120ºB0.34100ºC0.339.33D0.308.5Z0.45127º

RNA-DNAhybrid0.281120ºRNAsecondarystructureMostRNAmoleculesoccurasasinglestrand（单链）,whichmaybefoldedintoacomplexconformation,involvinglocalregionsofintramolecularbasepairing（分子内碱基配对）andotherhydrogenbondinginteractions.ModifiednucleicacidsCovalentmodificationsofnucleicacidshavespecificrolesinthecell.InDNA,thesearenormallyrestrictedtomethylationofadenineandcytosinebases,buttherangeofmodificationsofRNAismuchgreater.Example:methylation

（甲基化）oftheN-6positionofadenineandthe4-aminogroup,andthe5-positionofcytocine,phageDNAs;modificationsofRNA.ChemicalandPhysicalPropertiesofNucleicAcids核酸的物理化学性质核酸的物理化学性质Stabilityofnucleicacids（核酸稳定性）Effectofacid（酸效应）Effectofalkali（碱效应）Chemicaldenaturation（化学变性）Viscosity（粘滞性）Stabilityofnucleicacids(核酸的稳定性)AlthoughitmightseemobviousthatDNAdoublestrandsandRNAstructuresarestabilizedbyhydrogenbonding,thisisnotthecase.H-bondsdeterminethespecificityofthebasepairing,butthestabilityofanucleicacidhelixistheresultofhydrophobic（疏水）anddipole-dipoleinteractions（双极-双极相互作用）betweenthestacked（堆叠）basepairs.Effectofacid(酸效应)Highlyacidicconditionsmayhydrolyzenucleicacidstotheircomponents:bases,sugarandphosphate.Moderateacidcausesthehydrolysisofthepurinebaseglycosylicbonds

（糖苷键）toyieldapurinicacid（脱嘌啉核苷酸）.Morecomplexchemistryhasbeendevelopedtoremoveparticularbases,andisthebasisofchemicalDNAsequencing.Effectofalkali(碱效应)HighpHdenaturesDNAandRNAbyalteringthetautomeric（互变异构）

stateofthebases

anddisruptingspecifichydrogenbonding.RNAisalsosusceptibletohydrolysisathighpH,byparticipationofthe2’-OHinintramolecularcleavageofthephosphodiesterbackbone.Chemicaldenaturation(化学变性)Somechemicals,suchasurea（尿素）andformamide（甲酰胺）,candenatureDNAandRNAatneutralpHbydisruptingthehydrophobicforcesbetweenthestacked（堆叠）bases.Viscosity(黏滞性)DNAsolutionshavehighviscosity（粘滞性）.LongDNAmoleculesaresusceptibletocleavagebyshearing（剪切）(orbysonication（超声）insolution–thisprocesscanbeusedtogenerateDNAofaspecificaveragelength.SpectroscopicandThermalPropertiesofNucleicAcids

（核酸的热力学和光吸收特性）UVabsorption（紫外吸收）Hypochromacity/hyperchromacity（减色/增色效应）Quantitationofnucleicacids（核酸定量）PurityofDNA（DNA纯度）Thermaldenaturation（热变性）Renaturation（复性）Hybridization（杂交）UVabsorptionNucleicacidsabsorbUVlightduetotheconjugatedaromatic（共轭芳香基团）natureofthebases;thesugar-phosphatebackbonedoesnotcontributeappreciablytoabsorption.ThewavelengthofmaximumabsorptionoflightbybothDNAandRNAis260nm(λmax=260nm).Theabsorptionpropertiesofnucleicacidscanbeusedfordetection（检测）,quantitation（定量）andassessmentofpurity（纯度估计）.3.2HypochromicityandHyperchromicityQuantitationofnucleicacidsAtaconcentrationof1mgml-1and1cmpathlength,double-strandedDNA（dsDNA）hasA260=20(or1OD260DNA=50µg/ml).RNAandsingle-strandedDNA（ssDNA）haveA260=25(or1OD260RNA/ssDNA=40µg/ml).ThevaluesforRNAandsingle-strandedDNAdependonbasecompositionandsecondarystructure.在波长260nm处，1mg/ml浓度的DNA经过1cm光路的吸光度值为20ODPuredsDNAhasanA260/A280of1.8,andpureRNAoneofaround2.0.Protein,withλmax=280nmhasaA260/A280ratioofless1(actuallyaround0.5).protein纯度分析，PurityofDNAThermalDenaturation

热变性IncreasedtemperaturecanbringaboutthedenaturationofDNAandRNA.RNAdenaturesgraduallyonheating,butdouble-strandedDNA‘melts’co-operativelytogivesinglestrandsatadefinedtemperature,Tm,whichisafunctionoftheG+CcontentoftheDNA.DenaturationmaybedetectedbythechangeinA260.Renaturation

复性DNArenaturesoncolding,butwillonlyformfullydouble-strandednativeDNAifthecoolingissufficientlyslowtoallowthecomplementaystrandsto

anneal（退火）.ThermaldenaturationandrenaturationHybridizationTherenaturationofregionsofcomplementaritybetweendifferentnucleicstrandsisknownashybridization.基因的功能

Genes:FunctionStoringInformationReplicationMutation遗传信息的流向DNARNAProtein转录翻译甲硫氨酸脯氨酸苯丙氨酸谷氨酸谷氨酸谷氨酰胺丝氨酸缬氨酸丙氨酸组氨酸丙氨酸甘氨酸谷氨酰胺DNARNA多肽链蛋白质转录逆转录翻译折叠组装成熟复制中心法则Exon1Exon2Exon3Exon4Intron1Intron2Intron3

5’3’DNAPremRNAmRNATranscriptionmRNAprocessing3’-untranslatedregion5’-untranslatedregionTranslationProteinTherolesofRNA储存遗传信息(病毒，viruses)结构分子(rRNAs)信息转移(mRNAs)信息翻译(tRNA)酶功能(核酶，ribozymes)TraditionalRNAsWhatisRNA?RibonucleicacidRibonucleotides(Ribose,base,&phosphate)TypesCoding:messengerRNA(mRNA)Non-coding:RibosomalRNA(rRNA)TransferRNA(tRNA)SmallnuclearRNA(snRNA)SmallnucleolarRNA(snoRNA)InterferenceRNA(RNAi)ShortinterferingRNA(siRNA)MicroRNA(miRNA)mRNAStructureCodingregionUntranslatedregions5’UTR7methyl-GcapBoundbycapbindingproteinsTranslationregulation3’UTRStabilityelementsSubcellularlocalization(zipcodes)poly(A)tailmRNA基因的命名

GeneNomenclature基因命名的基本原则人类基因命名委员会(HumanGeneNomenclatureCommittee)的基本指导方针：应随新技术的发展而发展，而不应受到历史的局限性。基因命名的基本原则：简明、独特、能够表达基因的特征或功能。基因符号的使用规则：独特、简短、仅用拉丁字母和阿拉伯数字，不应含有标点符号。GeneNomenclature基因命名：浏览伦敦大学学院网站http://www.gene.ucl.ac.uk/nomenclature/命名指南：浏览伦敦大学学院网站http://www.gene.ucl.ac.uk/nomenclature/guidelines.html人类基因组组织(HUGO)/Guidelinesforhumangenenomenclaturewerefirstpublishedin1979[1],whentheHumanGeneNomenclatureCommitteewasfirstgiventheauthoritytoapproveandimplementhumangenenamesandsymbols.Updatesoftheseguidelineswerepublishedin1987[2],1995[3],and1997[4].

Withtherecentpublicationsofthecompletehumangenomesequencethereisanestimatedtotalof26,000-40,000genes,assuggestedbytheInternationalHumanGenomeSequencingConsortium[5]andVenteretal.[6].

Thus,theguidelineshavebeenupdatedtoaccommodatetheirapplicationtothiswealthofinformation,althoughsymbolsarestillonlyassignedwhenrequiredforcommunication.TheseupdateswerederivedwithinputfromtheHUGOGeneNomenclatureCommittee(HGNC)InternationalAdvisoryCommitteeandattendeesoftheASHG01NWGeneNomenclatureWorkshop.

AllapprovedhumangenesymbolscanbefoundintheGenewdatabase[7]./cgi/content/full/30/1/1691.Criteriaforsymbol