Nucleic acids are polymers of nucleotides, which ones have three characteristic components: a nitrogenous base, a pentose and a phosphate. The successive nucleotides of both DNA and RNA are covalently linked through phosphate-group “bridges”. Specifically, the 5’-hydroxyl group of one nucleotide unit is joined to the 3’-hydroxyl group of the next nucleotide by a phosphodiester-linkage.

DNA and RNA both contain two major purine bases, adenine (A) and guanine (G). DNA and RNA also contain two major pyrimidines in both type of nucleic acid, one of these is cytosine (C). The single important difference between the bases of DNA and those of RNA is the nature of the second major pyrimidine: thymine (T) in DNA and uracil (U) in RNA.

DNA

RNA

Note that the green color and the blue one indicates that the strand is in 5’?3’ direction and in dark red it is seen the phosphodiester-linkage.