Gibberellin & stem elongation (CIE A-level biology)

This lesson describes the role of gibberellin in stem elongation, including the role of the dominant allele, Le, and the recessive allele, le. The PowerPoint and accompanying resources have been designed to cover specification point 16.2 (7) of the CIE A-level biology specification (for assessment in 2025-27).

The lesson begins with the introduction of the Le gene loci, along with some initial details of this gene’s regulation of height in some plants. The students met gibberellin in topic 15, so the first of several prior knowledge checks is used to challenge them to recall the name of this plant hormone from a description about seed germination. Moving forwards, the students will learn that the enzyme encoded for by the Le gene is involved in the gibberellin synthesis pathway, where it catalyses the conversion of the inactive precursor GA20 to GA1, which is active gibberellin. As this lesson is part of topic 16, the genetics that underpins the biology is continually referenced, and again, an exam-style question is used to ensure that students recognise that homozygous dominant and heterozygous genotypes would be expressed as tall plants.

The rest of the lesson considers the recessive allele, and how the substitution of alanine with threonine affects the shape of the active site, rendering the enzyme as non-functional.

All answers to the knowledge checks are embedded into the PowerPoint to allow students to assess their progress.