DNA 復制過會頭尾短兩截, 怎樣可令 DNA 變成一個圓鏈, 不會短兩截?

DNA 復制過會頭尾短兩截, 只係講緊一條棒型, 好似人類染色體咁有 [頭] 有 [尾] 既DNA.
圓鏈DNA只出現在prokaryote [例如細菌], 由一個origin [開始點] 向前後兩個方向同時進行複製 [bidirectional], 以致複製中既圓形DNA睇落有d似希臘字母 [θ] 參考維基文章:
http://en.wikipedia.org/wiki/Theta_structure
A Theta structure is an intermediate structure formed during the replication of a circular DNA molecule (prokaryote DNA), two replication forks can proceed independently around the DNA ring and when viewed from above it resembles the Greek letter "theta" (θ). Originally discovered by Cairns, it lead to the understanding that (in this case) bidirectional DNA replication could take place. Proof of the bidirectional nature came from providing replicating cells with a pulse of tritiated thymidine, quenching rapidly and then autoradiographing. Results showed that the radioactive thymidine was incorporated into both forks fo the theta structure, not just one, indicating synthesis at both forks, in opposite directions around the loop.
最後, 當左右開[工]的enzyme係圓形DNA上 origin既對面相遇, θ就變成 8 字型, 上半係新DNA, 下半係舊DNA. 再係 8 字中心點解開兩條DNA,成為兩個 o 字. 完成!
咁樣複製係唔會少咁d野架.

2008-06-19 10:54:17 補充：
人既 DNA太長, 不可以用一個大圓形DNA來裝, 一定要分成幾十條 染色體, 因為細胞分裂時, 要排好d染色體[sister chromatids], 準確咁分開入兩個細胞, 保證兩個細胞入面個46條冇重覆又冇缺少. 採用圓形DNA就做唔到. 唯有犧牲左 [圓形DNA複製不變短] 既好處, 來換取 [大數量基因準確分配到細胞中]既更重要功能.

所以只會係prokaryote呢d基因數目好少既生物會用圓型DNA.