假基因

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一些假基因形成的學說,左圖表示從基因到蛋白質的過程,右上圖表示假基因形成的傳統學說:一個基因發生了複製,隨後一個基因發生突變,成爲假基因。一個新的學說認爲,假基因是基因轉錄的RNA逆轉錄並整合到DNA上形成的[1][2]

假基因(Pseudogenes,Pseudo-意爲「假」)是一類染色體上的基因片段。假基因的序列通常與對應的基因相似,但至少是喪失了一部分功能,如基因不能表達編碼蛋白質沒有功能[3]

一般認爲,假基因最初是功能對生物生存並非必要的基因。隨着突變的積累,出現編碼區提前出現終止密碼子移碼突變英语Translational frameshift等情況,逐漸變爲無功能的假基因。另外,拷貝數變異英语Copy-number variation(Copy-number variation, CNV)也可能產生假基因。在拷貝數變異中,1kb(千鹼基對)以上的DNA片段會發生複製或刪除[4]。一部分假基因既沒有內含子,也沒有啓動子(這種啓動子被認爲是通過mRNA的逆轉錄轉移到染色體上的,稱爲「加工」假基因(processed pseudogenes))[5],但部分假基因仍然擁有一些與正常基因相同的特徵,比如擁有CpG島等啓動子、RNA剪接位點等。

假基因這一名詞是由雅克(Jacq)等人於1977年最早提出的[6]。長期以來生物學家們認爲假基因是沒有功能的垃圾DNA,惟近年來的研究還表明假基因和其他非編碼片段一樣,擁有調控基因表達的功能。假基因的調控作用對維持生物體的生理活動有着重要意義,一部分假基因在某些疾病的發展中也扮演着重要角色[7]

在進化生物學研究中,假基因序列分析一直是研究者獲知生物進化歷程的手段。假基因一般會擁有一些源基因的特徵。按照進化論的觀點,兩個親緣關係較近的物種擁有同一祖先。對假基因進行序列比對、分析,即可驗證兩物種是否擁有同一祖先,並能計算出兩物種開始分離的時間(結果能精確到百萬年)。

特性[编辑]

类型及成因[编辑]

根据不同的起源机制和特点,假基因可大致分为如下四类:

细菌假基因[编辑]

细菌基因组中也存在假基因[48]。这些拥有假基因的细菌通常为共生细胞内寄生,因此它们不需要一些生活在外界复杂环境中的细菌所必须的基因。一个极端的例子是麻风病的病原体--麻风杆菌Mycobacterium leprae)的基因组,已报道有1,133个假基因约占其转录组的50%[49]

參見[编辑]

參考[编辑]

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