Chapter3NucleicAcid1.PhysicalandchemicalstructureofDNA•Double-strandedhelix•Majorgrooveandminorgroove•Basepairing•Thetwostrandsareantiparallel•G+Ccontent(percentG+C)•SatelliteDNASatelliteDNAconsistsofhighlyrepetitiveDNAandissocalledbecauserepetitionsofashortDNAsequencetendtoproduceadifferentfrequencyofthenucleotidesadenine,cytosine,guanineandthymine,andthushaveadifferentdensityfrombulkDNA-suchthattheyformasecondor'satellite'bandwhengenomicDNAisseparatedonadensitygradient.2.AlternateDNAstructureTwobaseshavebeenextrudedfrombasestackingatthejunction.Thewhitelinegoesfromphosphatetophosphatealongthechain.Oisshownred,Nblue,PyellowandCgrey.3.CircularandsuperhelicalDNADNAcanalsoformadouble-stranded,covalently-closedcircle.Thesecircularmoleculesareoftencoiledintoasuperhelix,theformationofwhichiscatalyzedbyenzymescalledtopoisomerases.4.DenaturationofDNADenaturation:AtransitionfromthenativetothedenaturedstateDNAdenaturation:alsocalledDNAmelting,istheprocessbywhichdouble-strandedDNAunwindsandseparatesintosingle-strandedstrandsthroughthebreakingofhydrogenbondingbetweenthebases.Hyperchromicity/Hyperchromiceffect:thestrikingincreaseinabsorbanceofDNA(A260)causedbythedenaturationofthedouble-strandedDNAmoleculeMeltingtemperature(Tm):thetemperatureatwhichhalfoftheDNAstrandsareinthedouble-helicalstateandhalfaredenatured.Themeltingtemperaturedependsonboththelengthofthemolecule,andthespecificnucleotidesequencecompositionofthatmolecule.FactorsAffectingTm•G-Ccontentofsample•reagentsthatincreasethesolubilityofthebases(anythingthatdisruptsH-bondsorbasestacking)•Saltconcentration•pH•Length5.RenaturationStrandscanbeinducedtorenature(anneal)underproperconditions.Factorstoconsider:•Temperature•Saltconcentration•DNAconcentration•TimeRepetitiveSequences•Unique：SingleCopyGenes•Slightlyrepetitive(2-10copies)•Middlerepetitive(10-hundreds)--Clustered--Dispersed•Highlyrepetitive(hundredstomillions)--ShortsequencesinsatelliteDNA--SequencesofnormallengthincertaingenesthatexistinverylargenumbersC-valueParadoxThereisapparentlyalackofassociationbetweenC-value(theamountofDNApresentinthehaploidgenomeofdifferentorganisms)andthedegreeoforganismalcomplexityofvariousmulti-cellularorganisms.In1971,Thomasnamedthisphenomenon,C-valueParadox.在每一种生物中其单倍体基因组的DNA总量是特异的，被称为C值(CValue)。C值和生物结构或组成的复杂性不一致的现象称为C值悖论(C-valueparadox)。6.HybridizationHybridization:thetechniquewhereinrenaturedDNAisformedfromseparatesingle-strandedsamples.Heteroduplexing:renaturationcombinedwithelectronmicroscopyinaprocedureallowsthelocalizationofcommon,distinct,andmissingsequencesinDNA.DNA-RNAhybridization(Northernhybridization):theuseoffilterhybridizationtodetectsequencecomplementaritybetweenasinglestrandofDNAandanRNAmolecule.7.ThestructureofRNATypes:mRNA,tRNA,rRNADistinctions:-ribosereplacesdeoxyribose;-UreplacesT;-Single-strandedConformation:stem-looporhairpin8.HydrolysisofnucleicacidThephosphodiesterbondsofbothDNAandRNAcanbebrokenbyhydrolysiseitherchemicallyorenzymatically.Ribozymes:theRNAenzymes,areabletocleaveandformspecificphosphodiesterbondsinamanneranalogoustoproteinenzymes.Chapter6ThegeneticmaterialThePathtotheWatsonandCrickModel1928,Griffith,transformationinpneumococci(肺炎球菌)19,Avery,GriffithstransformingprinciplewasDNA1950,Chargaff,apatternintheamountsofthefourbases19...
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