Home / A level Biology 5.2 Chromosome behaviour in mitosis – Exam style question – Paper 2

A level Biology 5.2 Chromosome behaviour in mitosis – Exam style question – Paper 2

A level Biology Paper 2 - All Chapters
Question

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.

(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).

(b) Abacavir is an analogue drug used in the treatment of some viral diseases. It enters a cell infected by a virus and is metabolised to the analogue carbovir triphosphate. Fig. 5.1 shows the molecular structure of abacavir and carbovir triphosphate.

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.

(i) With reference to Fig. 5.1, explain whether carbovir triphosphate will replace a purine or a pyrimidine nucleotide in the elongating polynucleotide chain.

(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.

(iii) Suggest and explain how carbovir triphosphate interferes with the action of DNA polymerase and how this may prevent the synthesis of viral DNA.

▶️ Answer/Explanation
Solution

(a)

Explanation: The table is completed as follows: Adenine (A) and Guanine (G) are purines present in both RNA and DNA. Cytosine (C) is a pyrimidine present in both, while Thymine (T) is a pyrimidine found only in DNA. Uracil (U) is a pyrimidine exclusive to RNA.

(b)(i) purine because it is a, double ring structure / has two rings ;

Explanation: Carbovir triphosphate replaces a purine nucleotide (adenine/guanine) because it has a double-ring structure, matching the purine base structure in DNA/RNA.

(b)(ii) any two from:

  • carbovir triphosphate is (similar to / same as), an activated nucleotide / a nucleotide / not a nucleoside ;
  • carbovir triphosphate, has (three) phosphates / is activated / is phosphorylated ;
  • detail of, (DNA) polymerase action / (activated DNA) nucleotides ;
  • ref. to triangle / triangular cycloalkane, no longer present / not on DNA nucleotides ;

Explanation: Conversion to carbovir triphosphate makes it resemble activated nucleotides (with three phosphates), enabling DNA polymerase to incorporate it into the growing chain. The absence of the triangular cycloalkane group further enhances compatibility.

(b)(iii) any four from:

  • prevents polymerase from adding DNA nucleotide to growing chain / AW ;
  • similar shape to, substrate / (activated / phosphorylated) nucleotide ;
  • acts as an inhibitor ;
  • fits into / binds to, active site of, enzyme / DNA polymerase ;
  • (DNA polymerase) may not form phosphodiester bonds ;
  • ref. to proofreading mechanism ;

Explanation: Carbovir triphosphate acts as a competitive inhibitor, binding to DNA polymerase’s active site and blocking further nucleotide addition. Its structural similarity allows incorporation but disrupts phosphodiester bond formation, halting viral DNA synthesis.

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