A nucleotide of the formula: ##STR4## wherein X is hydrogen or --CO(CH2)n --CO--R3 ; wherein each of m and n is a natural number up to 20; andRo is selected from the group consisting of orthochlorophenyl, para-chlorophenyl, phenylthio and 5'-chloro-8-oxyquionolyl; R1 is hydrogen or a protective group for a 5'-end hydroxyl group selected from the group consisting of trityl, monomethoxytrityl, di-me