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DAN含胸腺嘧啶而不含尿嘧啶,是由于脱氧尿苷三磷酸酶(dUTPase)及尿嘧啶-DNA糖苷酶等活性的存在,阻止了尿嘧啶参入DNA。在缺乏这些酶的大肠杆菌T4噬菌体双重突变型dut~-ung~-的DNA中,尿嘧啶部分地置换胸腺嘧啶而参入DNA,并且,这种参有尿嘧啶的DNA仍有正常功能,可以持续好几个世代,对于噬菌体的复制和转录影响不大。这说明,尿嘧啶参入DNA是一种普通现象。与病毒比较,细胞大概只有较小的尿嘧啶置换胸啶嘧啶的耐量。
DNA containing thymine without uracil is due to the presence of deoxyuridine triphosphatase (dUTPase) and uracil-DNA glycosidase activity, preventing the incorporation of uracil into DNA. In DNA of the dut ~ -ung ~ - double mutant of T4 bacteriophage lacking these enzymes, uracil partially substitutes thymine for incorporation into DNA, and this uracil-containing DNA still has normal function and can sustain Several generations have little effect on phage replication and transcription. This shows that uracil into DNA is a common phenomenon. Compared with the virus, cells probably only smaller uracil thiophene displacement tolerance.