Alevelbiology讲义6Geneticcontrol.docx

Alevelbiology讲义6Geneticcontrol.docx

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Alevelbiology讲义6Geneticcontrol

FGeneticcontrol

ⅠSyllabusContents

1StructureandreplicationofDNA

Structure:

doublehelix

Thebasicunitisnucleotide.Anucleotideismadeofa5-carbonsugar（deoxyribose）,aphophateandanitrogenbase.FournitrogenbasesinDNAare:

A（adenine）,C（cytosine）,G（guanine）andT（thymine）.

A=T（2hydrogenbonds）

C

G（3hydrogenbonds）

Semi-conservativereplication:

theDNAdoublehelixunzips,andeachstrandservesasatemplatefortheformationofanewstrand.Thenewmoleculeseachconsistofoneoldstrandandonenewstrand.

Direction:

from5'to3'end.

2RoleofDNAinproteinsynthesis

ⅡSamplesofA-LEVELbiologyexaminationforanalyzing

1describethestructureofRNAandDNAandexplaintheimportanceofbasepairingandthedifferenthydrogenbondingbetweenbases;

thestructureofDNAandRNA

DNA

RNA

Stand

Doublestrands

Singlestrand

Unit

Nucleotide

Nucleotide

Pentosesugar

Deoxyribose

Ribose

Phosphategroup

Yes

Yes

Nitrogen-containingbase

A,T,C,G

A,U,C,G

Basepairing

A=T（2hydrogenbonds）

C

G（3hydrogenbonds）

A=U（2hydrogenbonds）

C

G（3hydrogenbonds）

Complementarypairing

BasecomplementarypairingisveryimportantforDNAreplicationandtranscription.Itensurestheaccuracyofgeneticinformationtransmission.ThenumberoffourkindsofdifferentbasesandtheirsequenceinaDNAmoleculerepresentsomegeneticinformation.DuringDNAsemi-conservereplicationortranscription,oneoldDNAstrandactasatemplate,fourkindsofbasesarepairingbyApairwithTinDNAreplicationorApairwithUintranscriptionandCpairwithG.Therearelittleerroroccurredduringbasepairing.

Sample:

1.（9700_w11_qp_11,21）WhatmakestheexactcopyingofDNAmoleculespossible?

Abasepairing

Bhydrogenbondingbetweennucleotides

Csugar-phosphatebackbone

Dthedoublehelixshape

Key:

A

Interpretion:

BasecomplementarypairingisveryimportantforDNAreplication.Itensurestheaccuracyofgeneticinformationtransmission.ThenumberoffourkindsofdifferentbasesandtheirsequenceinaDNAmoleculerepresentsomegeneticinformation.DuringDNAsemi-conservereplication,oneoldDNAstrandactasatemplate,fourkindsofbasesarepairingbyApairwithTinDNAreplicationandCpairwithG.Therearelittleerroroccurredduringbasepairing.

2explainhowDNAreplicatessemi-conservativelyduringinterphase;

DNAmoleculesreplicateduringinterphase.Thehydrogenbondsbetweenthebasesbreak,allowingfreenucleotidestofallintopositionoppositetheircomplementaryonesoneachstrandoftheoriginalDNAmolecule.Adjacentnucleotidesarethenlinked,throughtheirphosphatesandsugars,toformnewstrands.Twocompletenewmoleculesarethusformedfromoneoldone,eachnewmoleculecontainingoneoldstrandandonenew.

Sample:

1.（9700_w11_qp_11,23）Bacteriaweregrowninamediumcontaining15N.Afterseveralgenerations,alloftheDNAcontained15N.Someofthesebacteriaweretransferredtoamediumcontainingthecommonisotopeofnitrogen,14N.Thebacteriawereallowedtodivideonce.TheDNAofsomeofthesebacteriawasextractedandanalysed.ThisDNAwasallhybridDNAcontainingequalamountsof14Nand15N.

Theremainingbacteriawereleftinthemediumwith14Nandallowedtodivideonemoretime.

TheDNAofsomeofthesebacteriawasextractedandanalysed.

WhatisthecompositionofthisDNA?

A25%hybridDNA

B50%hybridDNA

C75%hybridDNA

D100%hybridDNA

Key:

B

Interpretion:

InthefirstDNAreplication,oneoldDNAstrandcontains15NactastemplateandreplicatesanewDNAstrandcontain14N,sodoestheotheroldone.Asaresult,oneDNAmoleculecontain15NisdividedintotwoDNAmoleculescontainboth15Nand14N,namelyhybridDNA.

InthesecondDNAreplication,oneoldDNAstrandcontains15NactastemplateandreplicatesanewDNAstrandcontain14N,formedahybridDNA.However,theotheroldDNAstrandcontains14NandreplicatesanewDNAstrandalsocontain14N,formedanon-hybridDNA.

Inotherwords,only50%DNAproducedinthesecondDNAreplicationarehybridDNA.

3statethatageneisasequenceofnucleotidesaspartofaDNAmolecule,whichcodesforapolypeptideandstatethatamutationisachangeinthesequencethatmayresultinanalteredpolypeptide;

ThesequenceofbasesonaDNAmoleculescodesforthesequenceofaminoacidinaproteinorpolypeptide.Eachaminoacidiscodedforbythreebases.AlengthofDNAcodingforonecompleteproteinorpolypeptideisagene.

MutationcanbedefinedasanunpredictablechangeinthebasesequenceinaDNAmolecule（genemutation）orinthestructureornumberofchromosomes（chromosomemutation）.Newallelesarisebygenemutation.Genemutationincludebasesubstitution,deletionsoradditions.

Baseadditionsordeletionsusuallyhaveaverysignificanteffectonthestructure,andthereforethefunction,ofthepolypeptidethattheallelecodesfor.Baseadditionsordeletionsalwayshavelargeeffects,becausetheyaltereverysetofthreebasesthat'follows'themintheDNAmolecule.Theyaresaidtocauseframeshiftsinthecode.Often,theeffectsaresolargethattheproteinthatismadeistotallyuseless.Ortheymayintroducea'stop'tripletpartwaythroughagene,sothatacompleteproteinisnevermadeatall.

Basesubstitutions,ontheotherhand,oftenhavenoeffectatallbecausemanyaminoacidshavemorethanonetripletcode,soevenifonebaseischangedthesameaminoacidisstillcodedfor.However,basesubstitutionscanhaveverylargeeffects.If,forexample,thebasesequenceATGmutatedtoATT,thishasproduceda'stop'triplet,sothesynthesisoftheproteinwouldstopatthispoint.

Sample:

1.（9700_w11_qp_11,22）WhichmoleculehasitssynthesisdirectlycontrolledbyDNA?

Aamylase

Bcholesterol

Cglycogen

Dphospholipid

Key:

A

Interpretion:

amylaseisproteinandproteinsynthesisiscontrolledbyDNA.

4describethewayinwhichthenucleotidesequencecodesfortheaminoacidsequenceinapolypeptidewithreferencetothenucleotidesequenceforHbA（normal）andHbS（sicklecell）allelesofthegenefortheβ-haemoglobinpolypeptide;

Thehaemoglobinmolecule,whichismadeupoftwoαchainsandtwoβchains,isnearlyspherical.

Thegenewhichcodesfortheaminoacidsequenceintheβchainsisnotthesameineveryone.Inmostpeople,theβchainsbeginwiththeaminoacidsequence:

Val-His-Leu-Thr-Pro-Glu-Glu-Lys

Butinsomepeople,thebasesequenceCTTisreplacedbyCAT,andtheaminoacidsequencebecomes:

Val-His-Leu-Thr-Pro-Val-Glu-Lys

Thissmalldifferenceintheaminoacidsequencemakeslittledifferencetothehaemoglobinmoleculewhenitiscombinedwithoxygen.Butwhenitisnotcombinedwithoxygen,the'unusual'βchainsmakethehaemoglobinmoleculemuchlesssoluble.Themoleculestendtosticktoeachother,forminglongfibresinsidetheredbloodcells.Theredcellsarepulledoutofshape,intoahalf-moonorsickleshape.Sicklecellanaemiaisaseverehereditarydiseaseandmaybefatal.

HbA（normal）andHbS（sicklecell）

（leftisHbA,rightisHbS）

Sample:

1.（9700_s10_qp_11,23）WhatistheminimumnumberofbasesubstitutionsrequiredtochangethenucleotidesequenceoftheHbA（normal）alleletotheHbS（sicklecell）allele?

A1B2C3D4

Key:

A

Interpretion:

thebasesequenceCTTisreplacedbyCAT,andtheaminoacidGlubecomesVal.

5describehowtheinformationonDNAisusedduringtranscriptionandtranslationtoconstructpolypeptides,includingtheroleofmessengerRNA（mRNA）,transferRNA（tRNA）andtheribosomes;

Transcription:

Duringproteinsynthesis,acomplementarycopyofthebasesequenceonageneismade,bybuildingamoleculeofmRNAagainstoneDNAstrand.ThemRNAthenmovestoaribosomeinthecytoplasm.

Translation:

tRNAmoleculeswithcomplementarytripletsofbasestemporarilypairwiththebasetripletsonmRNA,bringappropriateaminoacids.Astwoaminoacidsareheldsidebyside,apeptidebondformsbetweenthem.TheribosomemovesalongthemRNAmolecule,sothatappropriateaminoacidsaregraduallylinkedtogether,followingthesequencelaiddownbythebasesequenceonthemRNA.

TypesandrolesofRNA

mRNA:

copythegeneticinformationstoredintheDNA.

rRNA:

componentsofribosomes.

Roleofribosomes:

siteofproteinorpolypeptidesynthesis.

tRNA:

transfertheaminoacidstotheribosomeinthetranslation.

Sample:

1.（9700_s11_qp_11,22）Thefollowingeventsoccurduringtranscription.

1Bondsbreakbetweencomplementarybases.

2Bondsformbetweencomplementarybases.

3Sugar-phosphatebondsform.

4Freenucleotidespairwithcomplementarynucleotides.

BeforethemRNAleavesthenucleus,whicheventswillhaveoccurredtwice?

A1and2onlyB1,3and4onlyC2,3and4onlyD1,2,3and4

Key:

A

Interpretion:

thefirstofbondsbreakbetweencomplementarybasesoccurswhenDNAdouble-strandunwind.ThesecondofbondsbreakbetweencomplementarybasesoccurswhenmRNAseparatesfromoneDNAstrand.ThefirstofbondsformbetweencomplementarybasesoccurswhenmRNAisproducedintranscription.ThesecondofbondsformbetweencomplementarybasesoccurswhenDNArecoverstowindaftermRNAseparatesfromoneDNAstrand.

ⅢPractices

1.（9700_w11_qp_11,20）Inageneticengineeringexperimentapieceofdouble-strandedDNAcontaining6000nucleotidescodingforaspecificpolypeptideistranscribedandtranslated.

Whatisthetotalnumberofaminoacidsinthispolypeptide?

A500B1000C2000D3000

2.（9700_s11_qp_11,21）ThemechanismofactionoffourdrugsthatinhibitDNAreplicationisstatedbelow.

●AphidicholineinhibitsDNApolymerase.