Moieties chuỗi bao gồm các chu kỳ saccharide, các căn cứ được gọi là nucleoside và thymine trong trường hợp của DNA), và các este phosphoric các nucleoside là các nucleotide. Do đó, DNA có thể được coi là một copolymer thống kê từ bốn comonomers khác nhau (do đó nó được gọi là : | POLYMER CHAINS WITH REGULAR CONFORMATIONS 117 Cytosine C Guanine G Chain moieties comprising these saccharide cycles and the bases are called nucleosides adenosine guanosine cytidine and thymine in the case of DNA and the phosphoric esters of these nucleosides are the nucleotides adenylic acid guanylic acid cytidylic acid and thymidylic acid . Hence DNA can be regarded as a statistical copolymer between four different comonomers hence it is called quaterpolymer H O---H-N _ N- jugar H Guanine-cytosine Unlike ribonucleic acids which are single-strand polymers DNA form doublestrand helices a sort of twisted ladder consisting of two complementary chains. This complementarity occurs through intermolecular hydrogen bonding between two pairs of bases between the adenosine of one strand and the thymidine of another and likewise between cytidine and guanosine. This double helix was identified by Crick and Watson in 1954 the rungs of the twisted ladder correspond to the pair of bases. DNA is always formed by replication duplication upon separation of the double strands the intermediate single strands are the matrices for the generation of new DNA chains. After replication each double helix includes one old strand and a new one. Three successive nucleotides of DNA provide the code for one amino acid and the genetic code is determined by the sequence of these triplets. Each nucleus in a living cell contains long thread-like structures called chromosomes which carry bits of genes. Both chromosomes and genes are made of DNA which is often called the blueprint for life every living cell contains indeed a copy of the blueprint. Figure shows the complexity of such a structure underscoring the progress made by biology to unveil it. 118 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES Figure . Representation of the DNA double-helix organization showing A-T and C-G links. . CHAIN PACKING . Assembly of Random Coils Taken individually random coils commonly exhibit a .
