§1.3  Semi-discontinuous Replication（半不连续）

★Replication direction(exhibits polarity)

   The daughter strand is synthesized continuously in the 5’-3’ direction.  

No enzyme capable of  synthesizing DNA in the 3’ to 5’ direction

Experiment of Okazaki

★前导链(leading strand): the daughter strand is synthesized continuously in the 5’-3’ direction.  

★冈崎片段(Okazaki fragments): Many DNA fragments are synthesized sequentially on the DNA template strand having the 5´- end. They are 1000 – 2000 nt long for prokaryotes and 100-200 nt long for eukaryotes. 

★后随链(lagging strand):The daughter strand consisting  of Okazaki fragments. 

Replication of a circular chromosome in prokaryotes(原核生物) 

★The replication process starts from the origin, and proceeds in two opposite directions.  It is named θ replication(θ复制). 

Replication of eukaryotes(真核生物) 

★Eukaryotes have multiple origins-多复制起始点. 

★The space between two adjacent origins is called the replicon-复制子, a functional unit of replication. 

Electron micrograph of replicating Drosophila DNA showing multiple “eyes”(bubbles). DNA复制的电镜照片