AQA A-Level Chemistry: Cisplatin as an Anticancer Drug — mark scheme explained

The question

Explain how cisplatin prevents DNA replication in cancer cells. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the structure of cisplatin.

• S2

  Cisplatin has the formula [Pt(NH 3 ) 2 Cl 2 ] and is a platinum(II) complex with two ammonia ligands and two chloride ligands.

• S3

  Step 2: Describe the mechanism of action.

• S4

  When cisplatin enters a cancer cell, it undergoes a ligand replacement reaction where the chloride ligands are replaced by nitrogen atoms from guanine bases in the DNA.

• S5

  Step 3: Explain the formation of covalent bonds.

• S6

  This process forms covalent bonds between platinum and the nitrogen atoms of the guanine bases, leading to the formation of intrastrand cross-links.

• S7

  Step 4: Describe the impact on DNA replication.

• S8

  These cross-links distort the DNA double helix, making it difficult for the replication machinery to read and copy the genetic information. As a result, DNA replication is halted, which ultimately leads to cell death through apoptosis.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the structure of cisplatin.

2. S2

   Cisplatin has the formula [Pt(NH 3 ) 2 Cl 2 ] and is a platinum(II) complex with two ammonia ligands and two chloride ligands.

3. S3

   Step 2: Describe the mechanism of action.

4. S4

   When cisplatin enters a cancer cell, it undergoes a ligand replacement reaction where the chloride ligands are replaced by nitrogen atoms from guanine bases in the DNA.

5. S5

   Step 3: Explain the formation of covalent bonds.

6. S6

   This process forms covalent bonds between platinum and the nitrogen atoms of the guanine bases, leading to the formation of intrastrand cross-links.

7. S7

   Step 4: Describe the impact on DNA replication.

8. S8

   These cross-links distort the DNA double helix, making it difficult for the replication machinery to read and copy the genetic information. As a result, DNA replication is halted, which ultimately leads to cell death through apoptosis.

9. Final answer: Cisplatin prevents DNA replication by forming covalent bonds with nitrogen atoms on guanine bases, leading to intrastrand cross-links that distort the DNA double helix and halt replication.

Common mistakes

• Misunderstanding the mechanism of DNA cross-linking by cisplatin. — Review the ligand replacement reaction and the formation of covalent bonds in detail. Understand that these bonds distort the DNA double helix and halt replication.
• Failing to recognize the importance of rapidly dividing cells. — Emphasize that rapidly dividing cells are more dependent on DNA replication. Cisplatin's interference with this process is particularly effective in these cells.
• Not fully explaining the adverse effects of cisplatin. — Practice listing and explaining common adverse effects such as nephrotoxicity, neurotoxicity, hematotoxicity, and gastrointestinal issues. Provide clear and concise explanations for each effect.
• Overlooking the balance between benefits and risks in treatment decisions. — Discuss how healthcare providers weigh the benefits and risks to determine the most appropriate treatment plan for each patient. Consider factors such as cancer type, stage, overall health, and ability to tolerate side effects.
• Confusing cisplatin with other platinum-based drugs. — Review the specific structure and mechanism of cisplatin. Understand that while other platinum-based drugs like carboplatin have similar mechanisms, they differ in their structures and side effect profiles.
• Failing to explain the role of ligand replacement reactions. — Practice explaining the ligand replacement reaction step-by-step. Emphasize that this process is crucial for cisplatin's mechanism of action.

Where the marks go

• Full worked solution (all marking points)5 marks

Related questions

• AQA A-Level Chemistry: Mass Number, Atomic Number, Isotopes, and Mass Spectrometry — mark scheme explained →
• AQA A-Level Chemistry: Relative Atomic Mass and Relative Molecular Mass — mark scheme explained →