文档介绍：DNA 为什么没有 U 在生物体中, 胞嘧啶以一个非常低的速率脱氨基形成尿嘧啶。这是一种潜在的灾难性变化, 因为脱氨基产生尿嘧啶, 它与腺嘌呤配对而不与鸟嘌呤配对,产生两个结果: 1 、当包含脱氨基的胞嘧啶(即尿嘧啶)的 DNA 转录时,在 RNA 中出现一个错误的碱基, 即错误的尿嘧啶指导一个腺嘌呤加入到 RNA 中; 2 、腺嘌呤而不是鸟嘌呤,会类似地存在于以脱氨基的 DNA 链为模板合成、新复制的 DNA 链中。为了阻止这种结果的发生, 在细胞内存在一种 DNA 修复系统, 它可以将尿嘧啶从 DNA 中去除,取代以胞嘧啶。去除由胞嘧啶脱氨基形成的尿嘧啶的必要性, 解释了 DNA 中为什么不存在尿嘧啶。如果尿嘧啶是 DNA 的一个碱基, 将不能区分“正确的”尿嘧啶和由胞嘧啶脱氨基形成的尿嘧啶。 Sources of deoxyuridine in DNA include the presence of dUTP because dUMP (the substrate for thymidylate synthase) or dUDP (from ribonucleotide reductase action on UDP) are phosphorylated to the triphosphate. DNA polymerase recognizes pounds as substrates. Another source is the deamination of deoxycytidine in DNA, promoted bya variety pounds. If deoxyuridine is ona template strand of the DNA, it will direct the incorporation of anA in the newly made strand of DNA. This will convert a G-C pair to an A-T pair. Cells have evolved ways of preventing mutations caused by either mechanism. First, cells contain an enzyme, dUTPase,which hydro-lyses dUTP to dUMP. Thus the triphosphate substrate is taken away from DNA polymerase before it can serve asa substrate. Secondly, U residues in DNA from whatever source cause the gap repair system to be activated so that these alterations are removed before they can be replicated. The base-pairing in DNA of dU and dT are identical; each forms two hydrogen bonds with dA on the opposite strand. However,