In a nucleotide the base is joined to 1' carbon of pentose by an N-β- glycosyl bond and a phosphate is esterified to 5' carbon. Phosphate of 5' carbon reacts with −OH group attached to 3' ribose sugar carbon. During this bond formation a water molecule is removed. This process is continued to make a polynucleotide with one end 3' sugar carbon attached -OH free (called 3' end) and at other end 5' sugar carbon attached phosphate free (called 5' end).