Nucleotide and nucleoside analogues are therapeutic drugs that have a similar structure to nucleotides or nucleosides of RNA and DNA. A nucleoside is composed of a nitrogenous organic base (base) and a pentose sugar.

1. (a)The names of the bases present in RNA and DNA nucleotides can be abbreviated using a single letter. These are shown in Table 5.1. Complete Table. 5.1 by stating: • the name of each base • whether the base is a purine or pyrimidine • whether the base is present – only in an RNA molecule (write RNA in the table) – only in a DNA molecule (write DNA in the table) – in RNA and in DNA molecules (write the word both in the table). [Table 5.1][4]
2. (b)(i)Carbovir triphosphate can be inserted into an elongating polynucleotide chain instead of a nucleotide. This interferes with the action of DNA polymerase during the synthesis of viral DNA. With reference to Fig. 5.1, explain whether carbovir triphosphate will replace a purine or a pyrimidine nucleotide in the elongating polynucleotide chain.[1]
3. (b)(ii)With reference to Fig. 5.1 and the action of DNA polymerase, suggest why the conversion of abacavir to carbovir triphosphate increases the chance of the analogue being added to the viral polynucleotide chain.[2]
4. (b)(iii)Suggest and explain how carbovir triphosphate interferes with the action of DNA polymerase and how this may prevent the synthesis of viral DNA.[4]

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