A nucleotide of the formula: ##STR3## 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 orthrochlorophenyl; para-chlorophenyl, phenylthio and 5'-chloro-8-oyquinoly; R1 is hydrogen or a protective group of a 5'-end hydroxyl group selected from the group consisting of trityl, monomethoxy-trityl, di-meth