因能量的需要，DNA的5”端必须带有PPP游离dNTP具有pPPOH3OH5'pppppp中5'pPPOH3pp在0.2MNacl的生理环境中，磷酸基团间的电负性，使dNTP难以聚合到需要其他机制以解脱DNA的5”端，双链DNA的5”端碱基配对困难
A T C G + 5’ ppp OH 3’ G ppp OH G 5’ ppp 游离dNTP具有ppp 因能量的需要，DNA的5’端必须带有PPP ppp OH 3’ A T C G 在0.2M Nacl 的生理环境中，磷酸基团间的电负性，使dNTP难以聚合到 DNA的5’端，双链DNA的5’端碱基配对困难 需要其他机制以解脱
碱基发生错配后的校正TCGOHpppOHpppppOH → pPp>OH费时、费能、增加脱磷酸、加磷酸的能量消耗
碱基发生错配后的校正. 费时、费能、增加脱磷酸、加磷酸的能量消耗 A T C G ppp OH ppp + A p OH T C G pp p OH T C G T C G ppp OH
3.3 DNA的半保留复制 (Semi-Conservation Replication)
Firstgeneration(a)-+1Conservative:DDDDLLThreeExpecteddensityDLgradient results:replicationDDLLDDFirstSecondhypothesesgenerationgeneration(b)Semiconservative:-1/+11+1+1DDDL→ExpecteddensityDgradientresults:DDDLLLDLFirstSecondgenerationgeneration(c)+1II+I1 + I1+ 1RandomdispersiveDDMixedMixedAllmixedExpecteddensitygradientresults:DLDD25%D;75%L
Three replication hypotheses
(a)GeSemi-Conservation ReplicationoN150.3DNA0.7N141.01.1N141.51.9N142.53.04.10and1.9mixedM.Meselson andF.W.Stahl,Sciences44:675,19580and4.1.mixed
(CsCl gradient centrifuge) N15 N14 DNA Semi-Conservation Replication M. Meselson and F. W. Stahl, Sciences 44:675, 1958. N14 N14