<prompt> The term modified base refers toPurine or pyrimidine attached to deoxyriboseA purine or pyrimidine that has been alteredA nitrogen containing ring other than a purine or pyrimidine that is part of a nucleotideA purine or pyrimidine attached to a sugar by O-glycoside linkage  <response>: A modified base is one that has been modified after nucleic acid chain has been formed. These bases are modified by methylation, reduction, acetylation or glycosylation.In t-RNA there are many modified bases, e.g. dihydrouridine, ribothymidine, pseudouridine, and 7-methylguanosine.In DNA, the most common modified base is 5-methylcytosine.Other important modified bases are acetylcytosine and dimethyladenine. 