CccDNA

cccDNA即共價閉合環狀DNA (covalently closed circular DNA)是一種有三級結構的雙鏈環狀DNA分子，最早是在噬菌體中發現的，而在自然界中被DNA病毒（如多瘤病毒科）感染的其它細胞中也有發現。^[1]^[2]

產生

cccDNA在一些病毒侵染宿主細胞核時產生並在隨後永久的存在於核內，其前體是一種線型的雙鏈DNA，通過DNA連接酶作用閉環而形成cccDNA。大部分情況下這些病毒的DNA只有在這樣的狀態下才能進行轉錄。cccDNA通常被認為是游離的DNA或是「微小基因組」 (minigenome)，但質體不一定是cccDNA。

存在形式

cccDNA的存在形式在逆轉錄病毒（或者說單鏈RNA逆轉錄病毒，如正反轉錄病毒亞科）和擬逆轉錄病毒（即雙鏈DNA逆轉錄病毒）的侵染過程中是相同的。然而前者DNA立刻整合到宿主基因組中，後者則是游離在核質中。

實例

cccDNA是花椰菜病毒科和肝病毒科（包括乙肝病毒 HBV）的典型特徵。其中乙肝病毒(HBV)的cccDNA的前體是與蛋白質外殼相連的「鬆弛環狀DNA」（rcDNA 或 relaxed circular DNA），其本質是含有可配對的黏性末端的線型DNA（因鹼基互補配對而成環狀）。^[3] 被乙肝病毒感染之後，通過治療可以使cccDNA不易被活化，因此cccDNA的活性是乙肝的治療中的一個指標。^[4]

結構

參見　DNA超螺旋

參見

資料來源

^ Mosevitskaia, T.V., E.B. Pavel'chuk, and N.V. Tomilin. Substrate of a UV-induced repair system providing for W-reactivation of lambda phage. Genetika. 1976, 12 (8): 131–138. PMID 1001892.
^ Kunisada, T. and H. Yamagishi. Sequence repetition and genomic distribution of small polydisperse circular DNA purified from HeLa cells. Gene. November 1984, 31 (1–3): 213–223. PMID 6098526. doi:10.1016/0378-1119(84)90212-9.
^ Guo H., D. Jiang, T. Zhou, A. Cuconati, T.M. Block, and J.T. Guo. Characterization of the intracellular deproteinized relaxed circular DNA of hepatitis B virus: an intermediate of covalently closed circular DNA formation. J Virol. November 2007, 81 (22): 12472–12484. PMC 2169032  . PMID 17804499. doi:10.1128/JVI.01123-07.
^ Bourne, E.J.; Dienstag, J.L.; Lopez, V.A.; et al. Quantitative analysis of HBV cccDNA from clinical specimens: correlation with clinical and virological response during antiviral therapy. Journal of Viral Hepatitis. January 2007, 14 (1): 56–63. doi:10.1111/j.1365-2893.2006.00775.x.

[Mosevitskaia1976-1] Mosevitskaia, T.V., E.B. Pavel'chuk, and N.V. Tomilin. Substrate of a UV-induced repair system providing for W-reactivation of lambda phage. Genetika. 1976, 12 (8): 131–138. PMID 1001892.

[Kunisada1984-2] Kunisada, T. and H. Yamagishi. Sequence repetition and genomic distribution of small polydisperse circular DNA purified from HeLa cells. Gene. November 1984, 31 (1–3): 213–223. PMID 6098526. doi:10.1016/0378-1119(84)90212-9.

[Guo2007-3] Guo H., D. Jiang, T. Zhou, A. Cuconati, T.M. Block, and J.T. Guo. Characterization of the intracellular deproteinized relaxed circular DNA of hepatitis B virus: an intermediate of covalently closed circular DNA formation. J Virol. November 2007, 81 (22): 12472–12484. PMC 2169032  . PMID 17804499. doi:10.1128/JVI.01123-07.

[Bourne2007-4] Bourne, E.J.; Dienstag, J.L.; Lopez, V.A.; et al. Quantitative analysis of HBV cccDNA from clinical specimens: correlation with clinical and virological response during antiviral therapy. Journal of Viral Hepatitis. January 2007, 14 (1): 56–63. doi:10.1111/j.1365-2893.2006.00775.x.

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