It was Friedreich Miescher (1868) who could for the first time isolate a phosphorus containing substance called ‘nuclein’ from the nuclei of the pus cells. He further found the nuclein to consist of an acid portion (known to¬day as DNA) and a basic portion (the protein called protamine). The ‘nuclein’ was renamed as nucleic acid by Ri¬chard Altmann (1889), a student of Miescher. The two nitrogenous bases¬ pyrimidines and purines were identi¬fied by Emil Fischer (1880), Their presence as cytosine, thymine (two pyrimidines) and adenine and guanine (the two purines, as well as 5-carbon sugar and phosphoric acid in ‘nuclein’ was made known by Albrecht Kossel. Soon it became clear that there are two kinds of nucleic acids- DNA (deoxyri¬bonucleic acid) and RNA (ribonucleic acid).

The successive nucleotides are joined together by covalent bonds, designated as phosphodiester bonds to form nucleic acid. This bond involves a phosphate group to establish link be¬tween 5’ carbon of pentose sugar of one nucleotide and 3’ carbon of the next¬ nucleotide. This provides enormous opportunity for elaboration of a polynucleotide chain having millions of nucleotide monomer units. The phosphodiester linkage also provides a sugar-phosphate backbone to the re¬sulting polymer. Further, this arrange¬ment not only provides a polarity to the resulting macromolecule but also a direction. If it starts from 5’C it would end in 3’C.

Therefore, while referring to a poly¬nucleotide or even a mononucleotide, the groups present at either end (5’C and 3’C) is indicated. Such a group could either be a phosphate group (in¬dicated as ‘P’) or a hydroxyl group (¬-OH). For instance, a polynucleotide or oligonucleotide with four nucleotides has a constitution 5’ AGCT 3’ or 5’ pApGpCpT. OH 3’.

It would mean that this tetranucleotide with only four nu¬cleotide units has phosphate group at the 5’ end and a hydroxyl group at the 3’ end. This is indicated in a short hand notation, which is a very con¬venient method to unite nucleotide se¬quence with 5’ terminus on the left and 3’ terminus to the right, since a DNA molecule consists of two complemen¬tary strands arranged in opposite di¬rection as per polarity (antiparallel) the same is shown as 5’3’ in one strand and 3’5’ in the ‘other strand.

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It should be borne in mind that the polynucleotide chain is always synthesised in 5’3’ direction (i.e., written and read from left to right), i.e., its ‘growth’ proceeds from a nucleotide having phosphate at 5’C carbon of its sugar to the 3’C of the sugar of the next nucleotide and the process is repeated in the same manner.
DNA and RNA though are nucleic acid, they have the basic differences.