英语翻译

英语翻译
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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