Lesson Plan

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Lesson Plan
Grade: Date: 25/02/2026
Subject: Biology
Lesson Topic: state that the bases adenine and guanine are purines with a double ring structure, and that the bases cytosine, thymine and uracil are pyrimidines with a single ring structure (structural formulae for bases are not expected)
Learning Objective/s:
  • Describe the structural difference between purine and pyrimidine bases.
  • Identify which DNA/RNA bases are purines and which are pyrimidines.
  • Explain how base pairing depends on the purine‑pyrimidine complementarity.
  • Summarise why this distinction maintains a uniform DNA helix width.
Materials Needed:
  • Projector or interactive whiteboard
  • Slide deck showing base classifications and pairing
  • Handout worksheet with a blank base table
  • Ball‑and‑stick molecular model kits (optional)
  • Exit‑ticket cards
 Introduction:
Begin with a quick image of a DNA double helix to spark curiosity. Ask students what they already know about the shapes of the nitrogenous bases. State that today they will be able to classify each base and explain why this matters for DNA structure and function.
Lesson Structure:
  1. Do‑now (5'): Complete a blank table labeling each base as purine or pyrimidine.
  2. Mini‑lecture (10'): Present definitions of purine and pyrimidine and show the classification table.
  3. Guided practice (10'): In pairs, fill a worksheet that matches bases to their ring structure and explains pairing rules.
  4. Check for understanding (5'): Whole‑class quick quiz (Kahoot or show of hands).
  5. Application activity (10'): Build a short DNA segment with model kits, highlighting purine‑pyrimidine pairing.
  6. Recap & exit ticket (5'): Students write one sentence describing why the purine‑pyrimidine distinction is essential for DNA stability.
Conclusion:
Review the key points: purines have double rings, pyrimidines have single rings, and their pairing keeps the helix uniform. Collect exit tickets to gauge understanding, and assign a brief homework: draw a table of the five bases with their categories and one example of a hydrogen‑bonding pair.