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maturation of the viral mRNA molecules, involving guanylylation and methylationto produce a ‘cap’ structure containing 7-methylguanosine 5’-phosphate in a5’- 5’-pyrophosphate structure at the 5’- end of the mRNA, must occur. Oncethe synthesis of the components needed to make fresh viral particles has beencompleted, maturatian occurs, in which the nucleic acid and protein componentsare assmbled to from the complete viral particle or virion, and finally this isreleased from the cell as a new , infective virus. RNA viruses require that theirnucleic acid be replicated also, in order that new viral particles can form. SomeRNA viruses code for an enzyme ‘transcriptase’ or ‘replicase’ (RNA-directedRNA polymerase), which replicates RNA strands directly without involvingDNA 278 while others encode an enzyme ‘reverse transcriptase’ (RNA-directedDNA polymerase), which transcribes the RNA sequence into DNA-the reverse ofthe usual sequence which can subsequently become integrated into the host cellchromosome and is transcribed to from copies of the viral RNA. 279 Purine andpyrimidine antimetabolites which are active against viruses are thus most likelyto be effective by inhibiting specifically the viral enzymes which are required toreplicate the viral nucleic acids, or inhibiting the viral enzymes responsible fortranscription and capping of mRNA, without inhibiting the correspondingenzymes of the host cell.Alternatively, incorporation of an antimetabolite into a viral nucleic acidcausing the cessation of strand synthesis, or otherwise disturbing the normalfunction of the nucleic acid in replication or in directing protein synthesis, offersanother way of preventing the replication of functional virus. 280 Again, it isimportant that the integrity of the nuclear material of the host cell should not becompromised.One therefore seeks to exploit the differences between the viralenzymes to the discomfiture of the virus. 5’-Amino- 5- iodo- 2’,5’-dideoxyuridine(Aminoidoxuridine;28) displays good antiherpes activity and isless cytotoxic than (27). 281,282 The Phosphorylation of Aminoidoxuridine(28) is catalyzed specifically by virus-induced thymidine kinase, and thus57Pyrimidine…..