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Structurally DNA is a polynucleotide. A formal between polynucleotides and proteins may therefore be perceived. Ploynucleotides are the products of nucleotide condensation, just as proteins are produced by the polymerization of amino acids. This similarity of structures is an important element which facilitantes the transfer of genetic information between these two distinct classes of macromolecules. The structure of nuclcotides and their constituent purine and pyrimidine bases are examined in the text.
The base composition of DNA varies considerably among species, particularly prokaryotes, which have a range of 25% -75% in adenine-thymine content. This range narrows with evolution, reaching limiting vatues of 45%-53% in mammals.
In addition to the four common bases, adenine, guanine, thymine, and cytosine, which occur in DNA from all sources, DNA isolated from many plant and animal tissues contains small amounts of the base 5-methylcytosine. Methylated derivatives of the bases are also present in all DNA molecules examined to date. In addition, the DNA of certain bacteriophages contains 5-hydroxymethylcytosine in place of cytosine, and thes derivative occurs in a glucosylated form. Even uracil, a base constituent of RNA, has been found in certain Bacillus subtilis phages, instead of thymine.

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Answer 1

DNA在结构上是一种多核苷酸。因此，它会作为一种介于多核苷酸和蛋白质之间的物质被感知到。多核苷酸是核苷酸凝结的产物。就像蛋白质是由氨基酸聚合而成的一样。这种类似的结构是一种非常重要的元素，它可以促进两个级别界限明显的大分子之间的遗传信息传输。核苷酸的结构、构成它们的嘌呤和嘧啶的主要成分是本文讲解的重点。
DNA的碱基组成在生物体身上呈螺旋状结构，尤其是原核生物，占到了腺嘌呤——胸腺嘧啶中25%-75%的水平。这个范围不断扩展，在动物体内到达45%-53%。除了来自所有源头并且出现在DNA中的四个碱基腺嘌呤，鸟嘌呤，胸腺嘧啶和胞嘧啶之外，DNA被与许多以少量的5-甲基胞嘧啶为基础的植物和动物的组织隔离开来。迄今为止，这个基础的甲基化派生物也出现在所有的DNA大分子中。除此以外，某些包含5—甲基胞嘧啶的噬菌体的DNA取代了胞嘧啶。并且这些派生物以葡萄糖基的形势出现。即使是构成DNA碱基之一尿嘧啶，也在枯草杆菌的噬菌体中被发现，而不是胸腺嘧啶。