假基因

(重定向自偽基因

假基因(Pseudogenes,Pseudo-意为“假”)是一类染色体上的基因片段。假基因的序列通常与对应的基因相似,但至少是丧失了一部分功能,如基因不能表达编码蛋白质没有功能[3]

一些假基因形成的学说,左图表示从基因到蛋白质的过程,右上图表示假基因形成的传统学说:一个基因发生了复制,随后一个基因发生突变,成为假基因。一个新的学说认为,假基因是基因转录的RNA逆转录并整合到DNA上形成的[1][2]

一般认为,假基因最初是功能对生物生存并非必要的基因。随着突变的积累,出现编码区提前出现终止密码子移码突变英语Translational frameshift等情况,逐渐变为无功能的假基因。另外,拷贝数变异英语Copy-number variation(Copy-number variation, CNV)也可能产生假基因。在拷贝数变异中,1kb(千碱基对)以上的DNA片段会发生复制或删除[4]。一部分假基因既没有内含子,也没有启动子(这种启动子被认为是通过mRNA的逆转录转移到染色体上的,称为“加工”假基因(processed pseudogenes))[5],但部分假基因仍然拥有一些与正常基因相同的特征,比如拥有CpG岛等启动子、RNA剪接位点等。

假基因这一名词是由雅克(Jacq)等人于1977年最早提出的[6]。长期以来生物学家们认为假基因是没有功能的垃圾DNA,惟近年来的研究还表明假基因和其他非编码片段一样,拥有调控基因表达的功能。假基因的调控作用对维持生物体的生理活动有着重要意义,一部分假基因在某些疾病的发展中也扮演着重要角色[7]

在进化生物学研究中,这些因为演化而丧失功能的假基因,对他们进行序列分析意义则相对重大,一直是研究者获知生物进化历程的手段。假基因一般会拥有一些源基因的特征。按照进化论的观点,两个亲缘关系较近的物种拥有同一祖先。对假基因进行序列比对、分析,即可验证两物种是否拥有同一祖先,并能计算出两物种开始分离的时间(结果能精确到百万年)。

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: 经处理的假基因 (Processed)、未经处理的假基因 (Non-processed)、单套假基因 (Unitary pseudogenes)、假的假基因 (Pseudo-pseudogenes)。

细菌假基因 编辑

细菌基因组中也存在假基因[47]。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%[48]

参见 编辑

参考 编辑

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